Plant Biology

1. Plant Structure and Tissues

Meristems

Definition. Meristems are regions of actively dividing, undifferentiated cells that provide new cells for plant growth.

Meristem type	Location	Function
Apical meristem	Tips of shoots and roots	Primary growth: elongation of shoots and roots.
Lateral meristem (cambium and cork cambium)	In the vascular bundles and cortex of stems and roots	Secondary growth: thickening of stems and roots (woody plants).

Plant Tissues

Tissue	Cell type / structure	Function
Epidermis	Single layer of cells, often with cuticle	Protection; prevention of water loss (cuticle is waxy).
Parenchyma	Thin-walled, living cells with large vacuoles	Photosynthesis (mesophyll), storage, secretion.
Collenchyma	Elongated cells with unevenly thickened cellulose walls (no lignin)	Flexible structural support (e.g., in leaf stalks and young stems).
Sclerenchyma	Dead cells with thick, lignified secondary walls	Rigid structural support (e.g., fibres in vascular tissue, sclereids in nut shells).
Xylem	Vessels (dead, hollow, lignified) and tracheids (dead, lignified, tapered)	Water and mineral transport from roots to shoots; structural support.
Phloem	Sieve tube elements (living, no nucleus) and companion cells	Transports organic compounds (mainly sucrose) from source to sink.

Leaf Structure

Structure	Function
Cuticle	Waxy layer reducing water loss by transpiration.
Upper epidermis	Transparent, allows light through; protective layer.
Palisade mesophyll	Closely packed, elongated cells with many chloroplasts; primary site of photosynthesis.
Spongy mesophyll	Loosely packed cells with air spaces; facilitates gas exchange ($\mathrm{CO}_2$ and $\mathrm{O}_2$ diffusion).
Lower epidermis	Contains stomata (pores) surrounded by guard cells; controls gas exchange and transpiration.
Vascular bundles	Xylem (water transport) and phloem (sugar transport) running through the leaf.

Stomata and Guard Cells

Each stoma is a pore bounded by two guard cells. Guard cells regulate the opening and closing of stomata:

• Open: when guard cells are turgid (water enters by osmosis), they bend outward due to unevenly thickened cell walls (thicker on the inner side), opening the stomatal pore. This occurs in light (photosynthesis in guard cells produces ATP, driving $\mathrm{K}^+$ uptake and subsequent water entry).
• Closed: when guard cells are flaccid (water leaves by osmosis), they become less turgid and the pore closes. This occurs in darkness, under water stress, or at high $\mathrm{CO}_2$ concentrations.

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2. Transport in Plants

Water Uptake by Roots

Water is absorbed by root hair cells in the zone of maturation of the root.

Adaptations of root hair cells:

• Long, thin projections greatly increase surface area for absorption.
• Thin cell walls minimise diffusion distance.
• Large vacuole and high solute concentration maintain a steep water potential gradient for osmosis.

Pathways of water movement across the root cortex:

Pathway	Description
Apoplast	Water moves through the cell walls and intercellular spaces (does not cross any membranes). Blocked by the Casparian strip (waxy, suberised band in the endodermal cell walls).
Symplast	Water moves from cell to cell through the cytoplasm via plasmodesmata (channels connecting adjacent cells).
Vacuolar	Water moves across membranes and through vacuoles (slowest pathway).

At the endodermis, the Casparian strip blocks the apoplast pathway, forcing water and dissolved minerals to enter the symplast (cross cell membranes). This provides selective control over mineral uptake.

Water Transport via Xylem

Cohesion-tension theory: the primary mechanism for water movement up the xylem.

1. Transpiration pull: water evaporates from the spongy mesophyll cell walls into the air spaces and exits through stomata (transpiration). This creates a negative pressure (tension) in the xylem.
2. Cohesion: hydrogen bonding between water molecules allows the column of water to be pulled upward as a continuous chain.
3. Adhesion: water molecules adhere to the hydrophilic walls of xylem vessels, aiding upward movement.
4. The tension created by transpiration at the leaves is transmitted down the xylem to the roots, drawing water upward.

Root pressure contributes minimally: active transport of ions into the xylem lowers the water potential, causing water to enter by osmosis, pushing water upward. This is significant only at night (low transpiration) or in small plants.

Factors Affecting Transpiration Rate

Factor	Effect on transpiration rate	Explanation
Temperature	Increase	Higher temperature increases kinetic energy of water molecules and evaporation rate.
Humidity	Decrease	Higher humidity reduces the water potential gradient between leaf and air.
Wind speed	Increase	Wind removes the boundary layer of moist air near the leaf, maintaining the gradient.
Light intensity	Increase	Light causes guard cells to open stomata, increasing the area for water loss.

Translocation in Phloem

Definition. Translocation is the transport of organic compounds (mainly sucrose) from source to sink in the phloem.

• Source: a region where organic compounds are produced or released (e.g., photosynthesising leaves, storage organs during germination).
• Sink: a region where organic compounds are used or stored (e.g., growing tips, roots, developing fruits, storage organs at the end of the growing season).

The Pressure Flow Hypothesis (Mass Flow)

1. At the source, sucrose is actively loaded into the phloem sieve tubes by companion cells (active transport using ATP). This lowers the water potential inside the sieve tube.
2. Water enters the sieve tube from the xylem by osmosis, creating high hydrostatic pressure at the source end.
3. At the sink, sucrose is unloaded (removed from the phloem) by diffusion or active transport. This raises the water potential inside the sieve tube.
4. Water leaves the sieve tube by osmosis, reducing hydrostatic pressure at the sink end.
5. The pressure gradient drives bulk flow of the sucrose solution from source to sink.

Evidence for the pressure flow hypothesis:

• Aphids feed on phloem sap; analysis of sap from severed stylets shows high sucrose concentration.
• Ringing experiments (removing a ring of bark from a tree trunk) cause sugar accumulation above the ring and stunting below it.
• Sieve tube pressure is higher at the source than at the sink.

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3. Plant Reproduction

Flower Structure

A typical flower contains:

Structure	Description	Function
Sepals	Leaf-like, often green	Protect the flower bud.
Petals	Often brightly coloured	Attract pollinators.
Stamens (androecium)	Anther (produces pollen) + filament (supports anther)	Male reproductive organs.
Carpels (gynoecium)	Stigma (receives pollen), style (connects stigma to ovary), ovary (contains ovules)	Female reproductive organs.
Nectaries	Glands producing nectar	Reward for pollinators.

Pollination

Definition. Pollination is the transfer of pollen from an anther to a stigma.

• Self-pollination: pollen from the same plant (or the same flower) fertilises the ovule. Produces offspring genetically identical (or very similar) to the parent; less genetic diversity.
• Cross-pollination: pollen from one plant is transferred to the stigma of a different plant of the same species. Promotes genetic diversity; often relies on external agents (wind, insects, birds, bats, water).

Insect-pollinated flowers: large, brightly coloured petals; scent; nectar; sticky pollen; stigma inside the flower. Wind-pollinated flowers: small, inconspicuous petals; large, feathery stigmas to catch pollen; large quantities of lightweight, smooth pollen; anthers hang outside the flower.

Fertilisation

After pollination, the pollen grain germinates on the stigma, producing a pollen tube that grows down the style toward the ovule. The pollen tube carries two male gametes.

Double fertilisation (unique to angiosperms):

1. The pollen tube enters the ovule through the micropyle.
2. One male gamete fuses with the egg cell ($n$) to form the diploid zygote ($2n$). This is fertilisation.
3. The second male gamete fuses with the two polar nuclei ($n + n$) to form the triploid endosperm ($3n$), a nutritive tissue for the developing embryo.

Seed Structure

Part	Origin	Function
Seed coat (testa)	Integuments of the ovule	Protection.
Embryo	Zygote	Develops into the new plant.
Radicle	Part of the embryo	Develops into the root.
Plumule	Part of the embryo	Develops into the shoot.
Cotyledon(s)	Part of the embryo	Store nutrients (monocots: one cotyledon; dicots: two).
Endosperm	Triploid tissue from double fertilisation	Stores starch, proteins, and lipids for the embryo.

Fruit Development

After fertilisation, the ovule develops into the seed and the ovary develops into the fruit. The fruit protects the seeds and aids in their dispersal.

Seed dispersal mechanisms:

• Wind: seeds with wings or parachutes (e.g., dandelion, maple).
• Animal ingestion: fleshy fruits eaten; seeds pass through the digestive tract and are deposited with fertiliser (e.g., berries).
• Animal attachment: hooks or barbs that cling to fur (e.g., burdock).
• Explosive mechanism: pod dries and splits, ejecting seeds (e.g., pea pods).
• Water: seeds with air-filled cavities that float (e.g., coconut).

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4. Plant Growth

Germination

Definition. Germination is the process by which a dormant seed resumes growth and develops into a new plant.

Conditions for germination:

Condition	Role
Water	Rehydrates tissues; activates metabolic enzymes; causes the seed to swell, rupturing the testa.
Oxygen	Required for aerobic respiration to produce ATP for growth.
Suitable temperature	Enzymes have optimal temperature ranges; too low and reactions are too slow; too high and enzymes denature.

Some seeds also require specific conditions such as:

• Light (photodormancy broken by light exposure).
• Fire (smoke and heat break dormancy in some species, e.g., certain Australian plants).
• Cold stratification (prolonged exposure to low temperatures, e.g., many temperate species).
• Scarification (physical damage to the seed coat, e.g., passing through an animal's digestive tract).

Stages of germination:

1. Water is absorbed (imbibition); the seed swells.
2. Gibberellin (a plant hormone) is produced by the embryo.
3. Gibberellin stimulates the production of amylase in the aleurone layer (in cereals).
4. Amylase hydrolyses starch in the endosperm into maltose, which is converted to glucose for cellular respiration.
5. The radicle emerges first, growing downward (positive gravitropism).
6. The plumule emerges, growing upward (negative gravitropism, positive phototropism).
7. The seedling transitions to photosynthesis once the leaves are exposed to light.

Plant Hormones

Plant hormones (phytohormones) are chemical messengers that regulate growth, development, and responses to stimuli.

Hormone	Site of production	Primary functions
Auxin (IAA)	Shoot tips (apical meristem)	Cell elongation (acid growth hypothesis); apical dominance; root initiation; phototropism; gravitropism.
Gibberellin	Young leaves, roots, embryos	Stem elongation; seed germination (stimulates amylase production); flowering; fruit development.
Cytokinin	Root tips	Cell division (cytokinesis); delay leaf senescence; promote shoot growth.
Abscisic acid (ABA)	Leaves, stems, root caps	Inhibits growth; closes stomata (antagonistic to auxin); maintains seed dormancy.
Ethylene	Ripening fruits, senescing tissues	Fruit ripening; leaf abscission (dropping); senescence.

Phototropism

Definition. Phototropism is the growth response of a plant toward (positive) or away from (negative) light.

Mechanism (auxin-mediated):

1. Auxin (IAA) is produced in the shoot tip (apical meristem).
2. Auxin is transported down the shoot. On the side of the shoot exposed to light, auxin transport is inhibited, causing auxin to accumulate on the shaded side.
3. Higher auxin concentration on the shaded side promotes cell elongation (by activating proton pumps, lowering cell wall pH, and activating expansin proteins that loosen the cell wall).
4. The shaded side elongates more than the illuminated side, causing the shoot to bend toward the light.

Evidence: Darwin's experiments with coleoptiles (1880) demonstrated that the tip of the coleoptile is sensitive to light; Went (1928) extracted the chemical messenger (auxin) from coleoptile tips.

Gravitropism

Definition. Gravitropism is the growth response of a plant to gravity.

• Shoots: negative gravitropism (grow upward, away from gravity).
• Roots: positive gravitropism (grow downward, toward gravity).

Mechanism (statolith hypothesis):

1. Specialised cells in the root cap contain amyloplasts (statoliths): starch-filled organelles that sediment to the bottom of the cell under gravity.
2. This sedimentation triggers redistribution of auxin in the root.
3. In roots, auxin inhibits cell elongation. Higher auxin concentration on the lower side of the root inhibits elongation there, while the upper side elongates more, causing the root to bend downward.
4. In shoots, auxin promotes elongation; the lower side elongates more, causing the shoot to bend upward.

Phytochromes

Definition. Phytochromes are photoreceptor pigments in plants that detect red ($660\;\mathrm{nm}$) and far-red ($730\;\mathrm{nm}$) light.

Phytochromes exist in two interconvertible forms:

• $\mathrm{P}_{\mathrm{r}}$ ($660\;\mathrm{nm}$ absorbing form): inactive; converts to $\mathrm{P}_{\mathrm{fr}}$ when it absorbs red light.
• $\mathrm{P}_{\mathrm{fr}}$ ($730\;\mathrm{nm}$ absorbing form): biologically active; converts back to $\mathrm{P}_{\mathrm{r}}$ when it absorbs far-red light, or slowly reverts in darkness.

Functions of phytochromes:

• Seed germination: many seeds require red light ($\mathrm{P}_{\mathrm{fr}}$) to germinate; exposure to far-red light ($\mathrm{P}_{\mathrm{r}}$) inhibits germination. This detects whether seeds are buried under leaf canopy (far-red rich) or in open sunlight (red rich).
• Flowering: phytochromes detect day length (photoperiodism), allowing plants to flower at the correct season.
  • Long-day plants: flower when the night length is shorter than a critical duration (i.e., days are long). $\mathrm{P}_{\mathrm{fr}}$ accumulates.
  • Short-day plants: flower when the night length exceeds a critical duration (i.e., days are short). $\mathrm{P}_{\mathrm{fr}}$ levels drop sufficiently during a long night.

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Common Pitfalls

• Confusing xylem and phloem: xylem transports water and minerals upward (dead cells, one-way); phloem transports sugars bidirectionally (living cells).
• Stating that "plants breathe in $\mathrm{CO}_2$ at night": plants respire $24$ hours a day; at night (no photosynthesis), net $\mathrm{CO}_2$ release occurs because photosynthesis has ceased but respiration continues.
• Confusing pollination and fertilisation: pollination is the transfer of pollen to a stigma; fertilisation is the fusion of gametes inside the ovule.
• Misunderstanding double fertilisation: angiosperms are unique in having two fertilisation events --- one produces the zygote, the other produces the endosperm.
• Confusing the effects of auxin in roots and shoots: auxin promotes cell elongation in shoots but inhibits cell elongation in roots.

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Practice Problems

Question 1: Transpiration Rate and Environmental Factors

A plant is moved from a cool, humid, still environment to a warm, dry, windy environment. Describe and explain the effect on the transpiration rate.

Answer

The transpiration rate will increase significantly. Three factors change simultaneously:

1. Increased temperature: raises the kinetic energy of water molecules, increasing the rate of evaporation from the spongy mesophyll cell walls.
2. Decreased humidity: increases the water potential gradient between the moist air inside the leaf and the drier air outside, driving faster diffusion of water vapour out through the stomata.
3. Increased wind speed: removes the layer of saturated air (boundary layer) near the leaf surface, maintaining a steep water potential gradient.

All three factors increase the rate of water loss, so the plant must increase water uptake through the roots to maintain turgor. If water loss exceeds uptake, the plant may wilt.

Question 2: Pressure Flow Hypothesis

Explain why sucrose must be actively loaded into the phloem at the source, and describe the consequence if active loading were inhibited (e.g., by a metabolic poison that blocks ATP production).

Answer

Sucrose is loaded into the phloem sieve tubes against its concentration gradient (from low concentration in mesophyll cells to high concentration in sieve tubes). This requires active transport (via companion cells), consuming ATP. Active loading lowers the water potential inside the sieve tube, causing water to enter by osmosis from the xylem, generating the high hydrostatic pressure at the source that drives mass flow.

If ATP production is blocked (e.g., by a metabolic poison), active loading of sucrose cannot occur. Sucrose concentration in the sieve tube remains low, so water does not enter by osmosis, and no pressure gradient is established. Translocation would stop, and sugars would accumulate in the source tissues (leaves). This would also inhibit photosynthesis (product accumulation causes feedback inhibition) and starve sink tissues (growing tips, roots, fruits) of carbohydrates.

Question 3: Double Fertilisation

A flowering plant has a diploid chromosome number of $2n = 24$. State the chromosome number of: (a) the egg cell, (b) the polar nuclei (each), (c) the zygote, and (d) the endosperm.

Answer

The haploid number is $n = 12$.

(a) Egg cell: $n = 12$ (produced by meiosis). (b) Each polar nucleus: $n = 12$ (produced by meiosis). (c) Zygote: $2n = 24$ (fusion of egg $n = 12$ + male gamete $n = 12$). (d) Endosperm: $3n = 36$ (fusion of two polar nuclei $n + n = 24$ + male gamete $n = 12$).

Question 4: Phototropism Experiment

A coleoptile is illuminated from one side. A researcher places an impermeable barrier (mica sheet) on the illuminated side of the coleoptile tip. Predict and explain the result.

Answer

The coleoptile will not bend (it will grow straight). Normally, auxin accumulates on the shaded side, promoting differential cell elongation and causing the coleoptile to bend toward light. In this experiment, the impermeable barrier on the illuminated side blocks the lateral redistribution of auxin from the illuminated side to the shaded side. As a result, auxin concentration remains equal on both sides, cell elongation is uniform, and the coleoptile grows straight.

Note: if the barrier were placed on the shaded side, auxin would accumulate on that side (it cannot move past the barrier), and the coleoptile would still bend toward the light (or potentially bend more, as auxin is trapped on the shaded side).

Question 5: Phytochrome and Seed Germination

A batch of lettuce seeds is exposed to a brief pulse of red light ($660\;\mathrm{nm}$), followed immediately by a brief pulse of far-red light ($730\;\mathrm{nm}$). The seeds are then placed in darkness. Predict whether the seeds will germinate, and explain the role of phytochrome in this response.

Answer

The seeds will not germinate (or will have a very low germination rate).

Phytochrome exists in two interconvertible forms:

• Red light converts $\mathrm{P}_{\mathrm{r}}$ (inactive) to $\mathrm{P}_{\mathrm{fr}}$ (active, promotes germination).
• Far-red light converts $\mathrm{P}_{\mathrm{fr}}$ back to $\mathrm{P}_{\mathrm{r}}$.

The red light pulse converts phytochrome to the active $\mathrm{P}_{\mathrm{fr}}$ form, which would promote germination. However, the subsequent far-red pulse immediately reconverts $\mathrm{P}_{\mathrm{fr}}$ back to $\mathrm{P}_{\mathrm{r}}$. Since the final exposure is far-red light, the seeds are left predominantly in the $\mathrm{P}_{\mathrm{r}}$ (inactive) state. In darkness, $\mathrm{P}_{\mathrm{fr}}$ slowly reverts to $\mathrm{P}_{\mathrm{r}}$ anyway. With insufficient $\mathrm{P}_{\mathrm{fr}}$, the biochemical pathways triggering germination (e.g., amylase production) are not activated.

This demonstrates that the last light exposure determines the phytochrome state and hence the germination response --- a property known as phytochrome reversibility.

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Worked Examples

Worked Example: Transpiration Rate Calculation Using a Potometer

A potometer containing a leafy shoot records water uptake of $2.4\;\mathrm{mL}$ over $30$ minutes. The total leaf area of the shoot is $80\;\mathrm{cm}^2$. Calculate the transpiration rate in $\mathrm{mL/cm^2/hour}$ and convert to $\mathrm{mmol\;H_2O/m^2/s}$.

Solution

Transpiration rate: $\frac{2.4\;\mathrm{mL}}{80\;\mathrm{cm}^2 \times 0.5\;\mathrm{hours}} = \frac{2.4}{40} = 0.060\;\mathrm{mL/(cm^2 \cdot h)}$

Convert to $\mathrm{mmol\;H_2O/m^2/s}$: $0.060\;\mathrm{mL/(cm^2 \cdot h)} = 0.060 \times 10^{-2}\;\mathrm{L/(cm^2 \cdot h)} = 0.060 \times 10^{-2} \times 10^4\;\mathrm{L/(m^2 \cdot h)} = 0.60\;\mathrm{L/(m^2 \cdot h)}$

Molar mass of water $= 18\;\mathrm{g/mol}$, density $= 1\;\mathrm{g/mL}$. Moles per hour: $\frac{600\;\mathrm{mL/h}}{18\;\mathrm{g/mol}} = 33.3\;\mathrm{mol/h}$

Convert to $\mathrm{mmol/s}$: $\frac{33.3 \times 10^3\;\mathrm{mmol}}{3600\;\mathrm{s}} = 9.3\;\mathrm{mmol/(m^2 \cdot s)}$

A typical transpiration rate for a mesophytic plant in daylight is $1$--$10\;\mathrm{mmol/(m^2 \cdot s)}$, so this value is within the expected physiological range. The potometer measures water uptake as a proxy for transpiration, assuming that water uptake approximates water loss (valid under steady-state conditions when the plant is not accumulating or depleting internal water reserves).

Worked Example: Xylem Water Transport and Cohesion-Tension Theory

A $10\;\mathrm{m}$ tall tree has a xylem vessel radius of $50\;\mathrm{\mu m}$. The surface tension of water at $20^\circ\mathrm{C}$ is $0.073\;\mathrm{N/m}$ and the contact angle between water and the xylem wall is approximately $0^\circ$. Using the capillary rise equation $h = \frac{2\gamma \cos\theta}{\rho g r}$, calculate whether capillary action alone can account for water reaching the top of the tree.

Solution

$h = \frac{2 \times 0.073 \times \cos(0^\circ)}{1000 \times 9.81 \times 50 \times 10^{-6}}$ $= \frac{0.146}{0.4905} = 0.298\;\mathrm{m}$

Capillary rise is approximately $0.30\;\mathrm{m}$, which is far less than the $10\;\mathrm{m}$ height of the tree. This demonstrates that capillary action alone is insufficient to explain water transport in tall plants. The primary mechanism is the cohesion-tension theory: transpiration at the leaves creates a negative pressure (tension) of approximately $-1$ to $-3\;\mathrm{MPa}$ in the xylem, and the cohesive forces between water molecules (hydrogen bonds) allow this tension to pull a continuous water column upward from the roots to the crown. Root pressure (typically $0.1$--$0.5\;\mathrm{MPa}$) provides a minor additional contribution.

Worked Example: Phloem Translocation Rate

Using an aphid stylet technique, researchers collect phloem sap from a sieve tube at a rate of $0.5\;\mathrm{\mu L/h}$. The sucrose concentration in the sap is $250\;\mathrm{mmol/L}$. Calculate the mass transfer rate of sucrose in $\mathrm{mg/h}$.

Solution

Sucrose molar mass: $\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} = 342\;\mathrm{g/mol}$

Moles of sucrose per hour: $250 \times 10^{-3}\;\mathrm{mol/L} \times 0.5 \times 10^{-6}\;\mathrm{L/h} = 1.25 \times 10^{-7}\;\mathrm{mol/h}$

Mass transfer rate: $1.25 \times 10^{-7}\;\mathrm{mol/h} \times 342\;\mathrm{g/mol} = 4.28 \times 10^{-5}\;\mathrm{g/h} = 0.0428\;\mathrm{mg/h}$

Per single sieve tube, this is a modest amount, but a tree trunk may contain thousands of sieve tubes operating simultaneously. For a mature tree with $10000$ active sieve tubes, the total sucrose transport would be approximately $428\;\mathrm{mg/h}$ or $10.3\;\mathrm{g/day}$. This is consistent with the observation that photosynthesising leaves can export $50$--$80\%$ of their daily carbon gain as sucrose via phloem translocation.

Worked Example: Seed Germination and Enzyme Activity

Seeds of barley ($2n = 14$) are germinated in the presence and absence of gibberellin. The amount of maltose released from the endosperm is measured over time:

Time (hours)	Maltose without gibberellin (mg)	Maltose with gibberellin (mg)
0	0	0
6	2	15
12	5	42
18	8	78
24	10	105

Explain the difference between the two conditions and identify the biochemical pathway involved.

Solution

The seeds treated with gibberellin release significantly more maltose ($105\;\mathrm{mg}$ vs $10\;\mathrm{mg}$ at $24$ hours) -- approximately a $10$-fold increase. This demonstrates the essential role of gibberellin in stimulating seed germination.

Biochemical pathway:

1. The embryo synthesises and releases gibberellin upon imbibition (water uptake).
2. Gibberellin diffuses to the aleurone layer (a protein-rich tissue surrounding the endosperm in cereal seeds).
3. Gibberellin binds to its receptor in aleurone cells, triggering a signal transduction cascade that activates gene expression.
4. The aleurone cells synthesise and secrete alpha-amylase (and other hydrolytic enzymes).
5. Alpha-amylase hydrolyses starch in the endosperm into maltose, which is then converted to glucose for the growing embryo.

The control (no added gibberellin) shows a small amount of maltose release ($10\;\mathrm{mg}$), likely from endogenous gibberellin produced by the embryo or from low basal amylase activity. The much larger response with added gibberellin confirms that the aleurone's amylase production is gibberellin-dependent.

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Common Pitfalls (Expanded)

• Confusing xylem and phloem: xylem transports water and minerals upward (dead cells, unidirectional); phloem transports organic compounds (mainly sucrose) bidirectionally (living cells).
• Stating that "plants breathe in CO2 at night": plants respire $24$ hours a day. At night, net $\mathrm{CO}_2$ release occurs because photosynthesis has ceased but respiration continues.
• Confusing pollination and fertilisation: pollination is the transfer of pollen to a stigma; fertilisation is the fusion of gametes inside the ovule. A significant time delay (hours to months) can separate these events.
• Misunderstanding double fertilisation: angiosperms are unique in having two fertilisation events -- one produces the diploid zygote, the other produces the triploid endosperm. Only one male gamete is involved in producing the embryo.
• Confusing the effects of auxin in roots and shoots: auxin promotes cell elongation in shoots but inhibits cell elongation in roots. This is a common source of error in phototropism and gravitropism questions.
• Describing transpiration as "water loss" without context: transpiration is an inevitable consequence of gas exchange (stomata must be open for $\mathrm{CO}_2$ uptake), not merely wasteful water loss. The cooling effect of transpiration is also physiologically important.
• Confusing apical dominance with apical dominance removal: removing the shoot tip (apical bud) removes the auxin source, relieving lateral bud inhibition. This is the principle behind pruning in horticulture.

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Exam-Style Problems

Problem 1: Data Analysis -- Potometer Experiment

A student uses a potometer to investigate the effect of wind speed on transpiration rate. The following data are collected at constant temperature ($25^\circ\mathrm{C}$) and humidity ($50\%$):

Wind speed (m/s)	Water uptake (mL/10 min)
0 (still air)	0.8
1.0	1.6
2.0	2.4
3.0	2.9
4.0	3.1
5.0	3.2

(a) Plot the data and describe the relationship between wind speed and transpiration rate. (b) Explain the biological mechanism responsible for this relationship. (c) Explain why the relationship is not linear at higher wind speeds (plateau effect).

Problem 2: Extended Response -- Adaptations of Xerophytes and Hydrophytes

Compare and contrast the anatomical and physiological adaptations of xerophytes (e.g., cacti) and hydrophytes (e.g., water lilies) to their respective environments. In your response, address: (a) leaf morphology and surface area, (b) stomatal distribution and regulation, (c) vascular tissue modifications, and (d) support structures. Explain how each adaptation relates to the availability of water in the environment.

Problem 3: Quantitative -- Seed Germination and Chi-Squared Test

A researcher tests the effect of different pre-treatments on seed germination. $100$ seeds are divided into four groups of $25$: Group A (control, water only), Group B (scarified), Group C (stratified at $4^\circ\mathrm{C}$ for $2$ weeks), Group D (gibberellin solution). After $7$ days, the number of germinated seeds in each group is: A = 8, B = 18, C = 20, D = 23. (a) Calculate the germination percentage for each group. (b) If the expected germination under the null hypothesis is $50\%$ for all groups, perform a chi-squared test to determine whether the treatments significantly affect germination ($p = 0.05$, critical value $= 7.82$ for $3$ degrees of freedom).

Problem 4: Extended Response -- Auxin and Tropisms

A researcher places a plant shoot horizontally in darkness. After $2$ hours, the shoot bends upward. When the same experiment is repeated with an agar block containing auxin applied asymmetrically to one side of a de-tipped shoot, the shoot bends away from the auxin source. (a) Explain the gravitropic response in terms of auxin redistribution and differential cell elongation. (b) Explain why auxin inhibits root elongation but promotes shoot elongation. (c) Describe the statolith hypothesis and explain how amyloplasts function as gravity sensors.

Problem 5: Data Analysis -- Ringing Experiment

A ringing experiment is performed on a tree by removing a $1\;\mathrm{cm}$ wide ring of bark (including phloem) from the trunk at a height of $1\;\mathrm{m}$. After $4$ weeks, the following observations are made: (i) a swelling appears immediately above the ring, (ii) the bark below the ring becomes dry and cracked, (iii) roots below the ring show reduced growth. (a) Explain each observation in terms of phloem translocation. (b) Why does removing xylem (a separate experiment) produce a different result (wilting above the ring)? (c) Explain why the xylem continues to function despite the bark removal.

Problem 6: Extended Response -- Phytochrome and Flowering

A long-day plant (e.g., spinach) and a short-day plant (e.g., chrysanthemum) are grown under identical conditions with a $16$-hour photoperiod. (a) Predict which plant will flower and explain why, referring to the role of phytochrome and critical night length. (b) If a flash of red light ($660\;\mathrm{nm}$) is given in the middle of the dark period, predict the effect on each species and explain the mechanism. (c) If the red light flash is immediately followed by a far-red flash ($730\;\mathrm{nm}$), predict the outcome and explain.

Problem 7: Extended Response -- Abscisic Acid and Stomatal Regulation

During a period of drought, a plant closes its stomata to reduce water loss. (a) Describe the biochemical pathway by which abscisic acid (ABA) causes stomatal closure, including the role of guard cell ion channels, water potential, and turgor pressure. (b) Explain the trade-off between reducing transpiration and maintaining $\mathrm{CO}_2$ uptake for photosynthesis. (c) Discuss why this trade-off may limit plant growth more in hot, arid environments than in cool, humid environments.

Problem 8: Quantitative -- Fertilisation and Chromosome Numbers

A flowering plant species has $2n = 16$. (a) State the chromosome number in the following cells: microspore mother cell, microspore, generative cell, sperm cell, megaspore mother cell, megaspore, egg cell, polar nucleus, zygote, and endosperm. (b) Explain why the endosperm is triploid and describe its function in seed development. (c) If a mutation causes meiosis to fail in the megaspore mother cell, producing a diploid egg cell instead of a haploid one, predict the chromosome number of the resulting zygote and endosperm after fertilisation.

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If You Get These Wrong, Revise:

• Cell structure and membrane transport --> Review ./cell-biology
• Photosynthesis and biochemistry --> Review ./molecular-biology
• Genetics and meiosis --> Review ./genetics
• Human physiology -- gas exchange --> Review ./human-physiology
• Ecosystems and energy flow --> Review ./ecology

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5. Translocation in Detail (Extended)

Evidence for the Pressure Flow Hypothesis

Aphid stylet technique: aphids feed by inserting their stylet (mouthpart) into a sieve tube. When the stylet is severed, phloem sap continues to exude, demonstrating positive pressure inside the sieve tube. Analysis of the sap shows high sugar concentration ($10$--$30\%$ sucrose by mass), consistent with active loading at the source.

Radioactive tracer experiments: $^{14}\mathrm{C}$-labelled $\mathrm{CO}_2$ is supplied to a source leaf. The label appears in the phloem within minutes and is detected in sink tissues (roots, growing tips) within hours. This confirms bidirectional transport.

Ringing experiments: removing a ring of bark (including phloem) from a tree trunk causes sugar accumulation above the ring (swelling) and stunted growth below it, confirming that phloem is responsible for sugar transport.

Loading Mechanism at the Source

Sucrose is actively loaded into companion cells via sucrose-proton symport: the companion cell uses a $\mathrm{H}^+$-ATPase to pump $\mathrm{H}^+$ into the apoplast (cell wall space), creating a proton gradient. Sucrose enters the companion cell via a sucrose-$\mathrm{H}^+$ co-transporter (SUT1 or SUT2), moving against its concentration gradient using the energy of the proton gradient.

Sucrose then passes to the sieve tube element through plasmodesmata.

Unloading at the Sink

Sucrose is unloaded from the sieve tube by:

• Simple diffusion (when sink sucrose concentration is low).
• Active transport (into growing storage tissues).
• Apoplastic pathway: sucrose exits the sieve tube and is taken up by sink cells.

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6. Mineral Ion Uptake

Mechanisms of Ion Uptake

Roots absorb mineral ions ($\mathrm{NO}_3^-$, $\mathrm{NH}_4^+$, $\mathrm{K}^+$, $\mathrm{Ca}^{2+}$, $\mathrm{Mg}^{2+}$, $\mathrm{PO}_4^{3-}$, $\mathrm{SO}_4^{2-}$) from the soil solution.

Active transport: most mineral ions are absorbed against their concentration gradient by:

• Proton pumps ($\mathrm{H}^+$-ATPases) in the root cell membrane pump $\mathrm{H}^+$ into the soil, creating an electrochemical gradient (negative charge inside the cell; proton gradient).
• Ion channels and carriers use this gradient for secondary active transport (e.g., $\mathrm{NO}_3^-$-$\mathrm{H}^+$ symport; $\mathrm{K}^+$ channels).

Mycorrhizae: symbiotic associations between plant roots and fungi that greatly increase the absorptive surface area. The fungal hyphae extend into the soil beyond the root's depletion zone, accessing nutrients (especially phosphorus) that roots alone cannot reach. The fungus receives organic carbon (sugars) from the plant in return.

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7. Plant Hormones (Extended)

Auxin (IAA) -- Detailed Mechanism

Biosynthesis: tryptophan $\to$ (tryptophan aminotransferase) $\to$ indole-3-pyruvic acid $\to$ (indole-3-acetic acid synthase) $\to$ IAA.

Acid growth hypothesis:

1. Auxin activates proton pumps ($\mathrm{H}^+$-ATPases) in the cell wall, pumping $\mathrm{H}^+$ from the cytoplasm into the cell wall space.
2. The lowered pH ($\approx 5.0$) activates expansins (wall-loosening proteins) that break cross-links between cellulose microfibrils.
3. The cell wall becomes more flexible and extends (elongates) as turgor pressure drives expansion.

Gibberellins -- Extended

Gibberellin biosynthesis pathway (in plants and fungi):

GGDP (geranylgeranyl diphosphate) $\xrightarrow{\text{ent-kaurene synthase}}$ ent-kaurene $\xrightarrow{\text{ent-kaurene oxidase}}$ $\to$ GA$_{12} \to \cdots \to$ GA$_1$ (the bioactive form).

Gibberellin in stem elongation:

• Stimulates cell division and elongation in the subapical region of shoots.
• Dwarf (mutant) varieties of pea and maize have defective gibberellin biosynthesis (e.g., Mendel's tall/dwarf pea experiment is explained by a mutation affecting GA synthesis).
• Application of GA to dwarf varieties restores normal height.

Gibberellin in seed germination:

• Embryo releases gibberellin upon imbibition.
• GA diffuses to the aleurone layer, where it binds to a receptor and activates gene expression.
• The aleurone cells synthesise and secrete $\alpha$-amylase, which hydrolyses starch in the endosperm to maltose for the growing embryo.

Cytokinin: Cell Division

• Produced primarily in root tips.
• Promotes cytokinesis (cell plate formation via activation of the phragmoplast).
• Delays leaf senescence (counteracts the effect of ethylene and ABA).
• Promotes shoot growth when applied in combination with auxin (cytokinin:auxin ratio determines shoot vs root differentiation in callus tissue culture --- high cytokinin favours shoot formation, high auxin favours root formation).

Ethylene: The Gaseous Hormone

• Produced by ripening fruits, senescing tissues, and stressed plants.
• Fruit ripening: ethylene stimulates the conversion of starch to sugars, degradation of chlorophyll (loss of green colour), softening of cell walls (pectin degradation), and production of volatile aroma compounds.
• Leaf abscission: ethylene promotes the formation of the abscission zone at the base of the petiole. Cellulase and pectinase enzymes break down cell walls in this zone, causing leaf fall.
• Triple response in seedlings: ethylene causes stem thickening (reduces elongation), horizontal growth, and formation of an apical hook (protects the growing tip as it pushes through soil).
• Commercial application: "one rotten apple spoils the bunch" --- ethylene released by a ripening fruit accelerates ripening of nearby fruits. Fruit can be delayed by storing in low-temperature, low-ethylene environments or accelerated by exposure to exogenous ethylene.

Abscisic Acid (ABA) -- Extended

Role in stomatal closure:

1. Under water stress, roots synthesise ABA and transport it to leaves via the xylem.
2. ABA binds to receptors on guard cell membranes.
3. ABA activates anion channels (especially SLAC1, slow-type anion channel) and inhibits $\mathrm{K}^+$ inward channels.
4. $\mathrm{Cl}^-$ and malate$^{2-}$ exit the guard cell, followed by $\mathrm{K}^+$.
5. The loss of solutes lowers the guard cell water potential; water exits by osmosis.
6. Guard cells become flaccid, and the stomatal pore closes.

Seed dormancy: high ABA levels maintain dormancy by inhibiting gibberellin synthesis. Dormancy is broken when ABA levels decline (e.g., through cold stratification or leaching by rain).

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8. Adaptations of Plants to Extreme Environments

Xerophytes (Dry Environments)

Adaptation	Function
Thick cuticle	Reduces water loss through the epidermis.
Sunken stomata	Stomata in pits, reducing transpiration by creating a humid microenvironment.
Rolled leaves	Reduced surface area exposed to air; inner surface is protected.
Hairy/trichomes	Trap a layer of still humid air, reducing the transpiration gradient.
CAM photosynthesis	Stomata open at night (reducing transpiration); store $\mathrm{CO}_2$ as malic acid.
Deep root systems	Access water deep underground.
Succulent water storage tissues	Store water in vacuoles; available during drought.
Reduced leaf area	Fewer leaves = less surface area for transpiration.

Hydrophytes (Aquatic Environments)

Adaptation	Function
Aerenchyma	Air-filled tissue providing buoyancy and gas transport to submerged roots.
Stomata on upper leaf surface	Access to air; not submerged.
Thin/no cuticle	No need for water conservation; maximises gas exchange.
Large air spaces in spongy mesophyll	Maximises buoyancy and gas exchange.
Flexible petioles	Leaves float on water surface.
Reduced vascular tissue	Less support needed; water is abundant.

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9. Photoperiodism in Detail

Long-Day Plants (LDP)

Flower when the night length is shorter than a critical duration (i.e., days are long, nights are short).

Examples: spinach, lettuce, radish, wheat, clover.

Mechanism:

• During the day, $\mathrm{P}_{\mathrm{fr}}$ accumulates (red light converts $\mathrm{P}_{\mathrm{r}}$ to $\mathrm{P}_{\mathrm{fr}}$).
• During a short night, not enough $\mathrm{P}_{\mathrm{fr}}$ reverts to $\mathrm{P}_{\mathrm{r}}$ to drop below the critical level.
• High $\mathrm{P}_{\mathrm{fr}}$ promotes the synthesis of a flowering hormone (florigen).

Short-Day Plants (SDP)

Flower when the night length exceeds a critical duration (i.e., days are short, nights are long).

Examples: chrysanthemum, poinsettia, soybean, strawberry.

Mechanism:

• During a long night, $\mathrm{P}_{\mathrm{fr}}$ reverts to $\mathrm{P}_{\mathrm{r}}$ extensively.
• Low $\mathrm{P}_{\mathrm{fr}}$ removes the inhibition of the flowering gene and promotes florigen synthesis.

Phytochrome Reversibility Experiments

Classic experiment (Borthwick and Hendricks, 1952):

• Short-day plants (chrysanthemums) exposed to a long night with a brief flash of red light ($660\;\mathrm{nm}$) at the midpoint: flowering is inhibited (red light creates $\mathrm{P}_{\mathrm{fr}}$, shortening the perceived night length).
• If the red flash is immediately followed by a far-red flash ($730\;\mathrm{nm}$): $\mathrm{P}_{\mathrm{fr}}$ is converted back to $\mathrm{P}_{\mathrm{r}}$, and flowering proceeds (the far-red flash "cancels" the red flash).

This demonstrated that the last light exposure determines the phytochrome state.

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10. Crop Yield and Plant Biotechnology

Increasing Crop Yield

Genetic approaches:

• Selective breeding: choosing parents with desirable traits (yield, disease resistance) and crossing them over many generations.
• Green Revolution varieties: dwarf wheat and rice varieties (e.g., IR8 rice, Norin 10 wheat) with short stems that resist lodging (falling over in wind); combined with high fertiliser application, these varieties dramatically increased yields in the 1960s--1970s.
• GM crops: Bt cotton (insect resistance), Golden Rice ($\beta$-carotene enrichment), herbicide-tolerant soybeans (Roundup Ready).

Agricultural approaches:

• Fertiliser application (providing N, P, K).
• Irrigation.
• Pest control (integrated pest management).
• Crop rotation to maintain soil fertility and reduce pest buildup.
• Greenhouse/glasshouse cultivation for climate control and extended growing seasons.

Photoperiodism in Agriculture

• Photoperiod manipulation: controlling day length in greenhouses to induce or delay flowering (e.g., chrysanthemums are short-day plants; in commercial production, black cloth is used to artificially extend the night, inducing flowering for the holiday market).
• Day-neutral plants: tomato, rice, maize (flower regardless of day length; useful for cultivation across latitudes).

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Exam-Style Problems (Extended)

Problem 9: Extended Response -- Water Transport and Cohesion-Tension

Describe the cohesion-tension theory of water transport in xylem, explaining how transpiration pull, cohesion, and adhesion contribute to the upward movement of water. Use Fick's law to explain why the rate of transpiration increases with wind speed. Explain why very tall trees (e.g., coast redwoods, $>100\;\mathrm{m}$) pose a challenge to the cohesion-tension theory and discuss how root pressure and capillary action contribute. Calculate whether capillary action alone can account for water transport in a $100\;\mathrm{m}$ tall tree given a xylem vessel radius of $25\;\mathrm{\mu m}$, surface tension $0.073\;\mathrm{N/m}$, and contact angle $0^\circ$.

Problem 10: Data Analysis -- Plant Hormone Experiment

A researcher treats coleoptile sections with different concentrations of IAA and measures the increase in length after 24 hours in darkness:

IAA concentration ($\mathrm{mg/L}$)	Length increase (mm)
0 (control)	2
0.01	5
0.1	12
1.0	18
10.0	20
100.0	19

(a) Plot the data and describe the relationship. (b) Explain the biological basis for the plateau at high IAA concentrations. (c) If the same experiment is repeated in the presence of an auxin transport inhibitor, predict the result and explain why.

Problem 11: Extended Response -- Phloem Translocation

Compare and contrast xylem and phloem transport with respect to: (a) the substance transported, (b) the direction of transport, (c) the driving force, (d) the living vs dead status of conducting cells, and (e) the role of ATP. Describe the pressure flow hypothesis for phloem translocation, explaining how active loading at the source and unloading at the sink create the pressure gradient. Discuss two pieces of experimental evidence that support this hypothesis.

Problem 12: Quantitative -- Transpiration Rate Calculation

A potometer experiment measures water uptake by a leafy shoot. The following data are collected at $25^\circ\mathrm{C}$ and $50\%$ humidity:

Wind speed (m/s)	Water uptake (mL/h)
0	8.0
1.0	12.0
2.0	16.0
3.0	19.0
4.0	21.0
5.0	22.0
6.0	22.5

The total leaf area is $120\;\mathrm{cm}^2$. (a) Calculate the transpiration rate at each wind speed in $\mathrm{mmol\;H_2O/m^2/s}$. (molar mass of water $= 18\;\mathrm{g/mol}$; density $= 1\;\mathrm{g/mL}$.) (b) Explain why the relationship between wind speed and transpiration rate is not linear at higher speeds (plateau). (c) Predict how the results would differ at $35^\circ\mathrm{C}$ and $70\%$ humidity.

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Additional Worked Examples

Worked Example: Phloem Transport and Mass Flow Hypothesis

A source leaf produces sucrose at a rate of $5\;\mathrm{\mu mol/h}$ and loads it into the phloem. The sieve tube has a radius of $15\;\mathrm{\mu m}$ and the concentration difference between source and sink is $500\;\mathrm{mmol/L}$ ($0.5\;\mathrm{mol/L}$). (a) Explain the mass flow (pressure flow) hypothesis of phloem transport. (b) Calculate the osmotic pressure difference using the van't Hoff equation ($\Pi = iCRT$) at $25^\circ\mathrm{C}$. (c) Explain why phloem transport is bidirectional, unlike xylem transport.

Solution

(a) Mass flow (pressure flow) hypothesis (Munch, 1930):

1. Loading: sucrose is actively transported (loaded) into the sieve tube at the source (e.g., a photosynthetic leaf) by companion cells. This increases the solute concentration inside the sieve tube.
2. Water uptake: the high solute concentration lowers the water potential, causing water to enter the sieve tube from the xylem by osmosis. This creates high hydrostatic pressure at the source.
3. Flow: the pressure gradient drives bulk flow of the sap (sucrose solution) through the sieve tubes from source to sink (region of lower pressure).
4. Unloading: at the sink (e.g., a growing root or fruit), sucrose is actively removed from the sieve tube by sink cells. This increases the water potential inside the sieve tube, causing water to leave by osmosis and reducing the hydrostatic pressure at the sink.
5. The pressure difference between source (high) and sink (low) maintains the flow.

(b) $\Pi = iCRT$, where $i = 1$ (sucrose does not ionise), $C = 0.5\;\mathrm{mol/L}$, $R = 8.314\;\mathrm{J/(mol \cdot K)}$, $T = 298\;\mathrm{K}$.

$\Pi = 1 \times 0.5 \times 8.314 \times 298 = 1238\;\mathrm{kPa} = 12.4\;\mathrm{atm}$.

This osmotic pressure difference drives water uptake at the source and contributes to the pressure gradient for mass flow.

(c) Phloem transport can be bidirectional because different sieve tubes (or different sieve elements within the same sieve tube) can transport sap in different directions simultaneously. A leaf can be a source (exporting sucrose to roots) and a sink (importing sucrose for its own growth) at different times or for different compounds. Xylem transport is always unidirectional (upward, from roots to leaves) because it is driven by transpiration pull (a negative pressure) and root pressure, both of which only operate in one direction.

Worked Example: Mineral Ion Uptake and Active Transport

A plant root absorbs nitrate ($\mathrm{NO}_3^-$) from a soil solution containing $2.0\;\mathrm{mmol/L}$ $\mathrm{NO}_3^-$. The root cell cytoplasm has a $\mathrm{NO}_3^-$ concentration of $50\;\mathrm{mmol/L}$. The membrane potential is $-120\;\mathrm{mV}$ (inside negative). (a) Calculate the equilibrium potential for $\mathrm{NO}_3^-$ at $25^\circ\mathrm{C}$. (b) Calculate the electrochemical driving force. (c) In which direction does the driving force act? (d) What type of transport is required for nitrate uptake?

Solution

(a) $E_{\mathrm{NO}_3} = \frac{RT}{zF}\ln\frac{[\mathrm{NO}_3^-]_{out}}{[\mathrm{NO}_3^-]_{in}}$ $= \frac{0.0267}{-1}\ln\frac{2.0}{50.0}$ $= -0.0267 \times \ln(0.04)$ $= -0.0267 \times (-3.219)$ $= +85.9\;\mathrm{mV}$

(b) Driving force $= V_m - E_{\mathrm{ion}} = -120 - 85.9 = -205.9\;\mathrm{mV}$.

(c) The negative driving force means the electrochemical gradient favours $\mathrm{NO}_3^-$ uptake (into the cell, against the concentration gradient but aided by the membrane potential). Wait -- the concentration gradient favours efflux ($50$ inside vs $2$ outside), but the electrical gradient (attracting the negative $\mathrm{NO}_3^-$ into the negatively charged cell) is very strong.

Let me recalculate: the net electrochemical potential for $\mathrm{NO}_3^-$ inside the cell is $\Delta\mu = zF(V_m - E_{\mathrm{ion}}) = (-1) \times F \times (-205.9\;\mathrm{mV}) = +205.9\;\mathrm{mV}$. This positive value means the ion is more stable inside; uptake is favoured by the electrical gradient but opposed by the concentration gradient. The net effect: the electrical gradient ($-120\;\mathrm{mV}$ attracting $\mathrm{NO}_3^-$) outweighs the concentration gradient (favouring efflux).

(d) Since nitrate is being accumulated against its concentration gradient (from $2$ to $50\;\mathrm{mmol/L}$), active transport is required. Specifically, nitrate is taken up by nitrate transporters (NRT1 and NRT2 families) that use the proton gradient ($\mathrm{H}^+$ co-transport): nitrate enters the cell together with protons (symport), driven by the proton electrochemical gradient maintained by the plasma membrane $\mathrm{H}^+$-ATPase (primary active transport).

Worked Example: Photoperiodism and Flowering

A short-day plant (SDP) flowers when the night length exceeds a critical dark period of $12$ hours. A long-day plant (LDP) flowers when the night length is less than a critical dark period of $10$ hours. (a) On June 21 (summer solstice, day length $= 15$ hours at a temperate latitude), will each plant flower? (b) On December 21 (winter solstice, day length $= 9$ hours), will each plant flower? (c) A SDP is given a $15$-hour night with a 10-minute flash of red light ($660\;\mathrm{nm}$) in the middle of the dark period. Will it flower? (d) If the red flash is followed immediately by far-red light ($730\;\mathrm{nm}$), will it flower?

Solution

(a) June 21: day $= 15\;\mathrm{h}$, night $= 9\;\mathrm{h}$.

• SDP: critical dark period $= 12\;\mathrm{h}$. Night $= 9\;\mathrm{h} < 12\;\mathrm{h}$. No flowering.
• LDP: critical dark period $= 10\;\mathrm{h}$. Night $= 9\;\mathrm{h} < 10\;\mathrm{h}$. Flowering.

(b) December 21: day $= 9\;\mathrm{h}$, night $= 15\;\mathrm{h}$.

• SDP: night $= 15\;\mathrm{h} > 12\;\mathrm{h}$. Flowering.
• LDP: night $= 15\;\mathrm{h} > 10\;\mathrm{h}$. No flowering.

(c) The red flash in the middle of the dark period converts $\mathrm{P_{fr}}$ (the active form of phytochrome) from $\mathrm{P_r}$. This interrupts the dark period -- the plant perceives the $15$-hour night as two shorter nights ($7.5\;\mathrm{h}$ each), both below the critical dark period. No flowering in the SDP.

(d) Far-red light converts $\mathrm{P_{fr}}$ back to $\mathrm{P_r}$. If the red flash is immediately followed by far-red, the net effect is as if no flash occurred (the phytochrome system is reversed). The plant flowers (the dark period is perceived as continuous).

This is the classic red/far-red reversible phytochrome response, discovered by Borthwick and Hendricks (1952), which demonstrated that phytochrome is the photoreceptor for photoperiodism.

Worked Example: Translocation of Auxin and Apical Dominance

A researcher applies radioactive IAA ($^{14}\mathrm{C}$-labelled auxin) to the tip of a shoot and measures its distribution after $24$ hours. The results show that most of the radioactivity is concentrated in the stem below the apex, with very little in the lateral buds. (a) Explain the polar (basipetal) transport of auxin. (b) Explain the mechanism of apical dominance. (c) If the apical bud is removed, predict the effect on lateral bud growth. (d) If IAA is applied to the decapitated stump, predict the effect.

Solution

(a) Polar auxin transport: auxin (IAA) is transported directionally from the shoot apex toward the base (basipetal transport). This is mediated by PIN proteins (auxin efflux carriers) that are localised on the basal (bottom) side of cells. The mechanism:

1. IAA enters the cell from the apical side via diffusion or AUX1 influx carriers.
2. PIN efflux carriers on the basal membrane pump IAA into the cell wall space below.
3. IAA enters the next cell from its apical side, and the process repeats.
4. The net result is unidirectional (top-to-bottom) transport.

(b) Apical dominance: the apical bud produces auxin, which is transported basipetally through the stem. High auxin concentrations in the stem inhibit the growth of lateral buds. Two proposed mechanisms:

1. Direct inhibition: auxin is transported into lateral buds and directly suppresses their growth at high concentrations.
2. Indirect (nutrient diversion) hypothesis: auxin promotes stem elongation, and the actively growing stem is a stronger sink for nutrients (sugars, minerals) than the lateral buds. The lateral buds are starved of nutrients and remain dormant.

Current evidence supports a combination of both mechanisms, with auxin-regulated genes (including those encoding transcription factors like BRANCHED1) playing a key role.

(c) Apical bud removal (decapitation): the source of auxin is removed. Auxin levels in the stem drop, releasing the lateral buds from inhibition. The lateral buds begin to grow, producing branches. This is widely used in horticulture (pruning to promote bushier growth).

(d) IAA applied to decapitated stump: the exogenous auxin replaces the apical bud's auxin source. Auxin levels in the stem are restored, and the lateral buds remain inhibited. No branching occurs. This experiment confirms that auxin is the signal responsible for apical dominance.

Worked Example: Seed Germination and Gibberellins

A seed of barley (Hordeum vulgare) is germinating. The aleurone layer produces amylase in response to gibberellic acid ($\mathrm{GA}_3$) from the embryo. (a) Describe the signal transduction pathway from $\mathrm{GA}_3$ perception to amylase gene expression. (b) Calculate the rate of starch breakdown if the amylase produces $0.5\;\mathrm{mg}$ of maltose per minute from a starch substrate. How long would it take to break down $500\;\mathrm{mg}$ of starch? (c) Explain why this mechanism is important in brewing.

Solution

(a) GA signal transduction in barley aleurone cells:

1. $\mathrm{GA}_3$ diffuses from the embryo to the aleurone layer.
2. $\mathrm{GA}_3$ binds to its receptor (GID1) in the aleurone cell cytoplasm.
3. The GA-GID1 complex interacts with DELLA repressor proteins, promoting their ubiquitination by the SCF ubiquitin ligase complex and subsequent degradation by the $26\mathrm{S}$ proteasome.
4. DELLA proteins normally repress the transcription factor GAMYB. Their degradation releases GAMYB.
5. GAMYB binds to the promoter of amylase genes (e.g., high-pI and low-pI $\alpha$-amylase) and activates their transcription.
6. Amylase mRNA is translated, and amylase is secreted into the endosperm, where it hydrolyses starch to maltose (a disaccharide).

(b) Rate $= 0.5\;\mathrm{mg/min}$. Time for $500\;\mathrm{mg}$: $500 / 0.5 = 1000\;\mathrm{min} = 16.7\;\mathrm{h}$.

Note: in practice, amylase activity is not linear indefinitely. As the starch is depleted, the rate slows (substrate limitation), and product inhibition (maltose inhibits amylase) further reduces the rate.

(c) In brewing:

1. Barley grains are germinated ("malting"), during which the embryo produces GA, stimulating the aleurone to produce amylase.
2. Amylase breaks down the endosperm starch into fermentable sugars (maltose, glucose).
3. The malted barley is dried and crushed. Hot water is added ("mashing"), reactivating the amylase.
4. The resulting sugar solution ("wort") is fermented by yeast to produce alcohol.
5. Understanding GA and amylase is important for optimising malting conditions (temperature, moisture, duration) to maximise sugar yield.

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Additional Common Pitfalls

• Confusing xylem and phloem transport direction: xylem transports water and minerals upward (unidirectional, from roots to leaves); phloem transports organic nutrients (sucrose, amino acids) bidirectionally (source to sink).
• Stating that transpiration is "wasteful": while transpiration does cause water loss, it also provides the driving force for water uptake and mineral transport, cools the leaf, and maintains turgor pressure.
• Confusing phototropism and photoperiodism: phototropism is directional growth toward light (controlled by auxin); photoperiodism is the response to day/night length that controls flowering (controlled by phytochrome).
• Assuming all plant hormones act independently: plant hormones interact in complex ways. For example, auxin and cytokinin interact to control organogenesis (high auxin:cytokinin ratio promotes roots; low ratio promotes shoots).
• Confusing $\mathrm{P}_r$ and $\mathrm{P}_{fr}$: $\mathrm{P}_r$ (red-absorbing) is the inactive form; $\mathrm{P}_{fr}$ (far-red-absorbing) is the biologically active form. Red light converts $\mathrm{P}_r$ to $\mathrm{P}_{fr}$; far-red light converts $\mathrm{P}_{fr}$ back to $\mathrm{P}_r$.
• Forgetting that transpiration rate depends on the vapour pressure deficit, not just temperature: humidity is equally important. High humidity (low vapour pressure deficit) reduces transpiration even at high temperatures.

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Additional Exam-Style Problems with Full Solutions

Problem 13: Extended Response -- Xylem Structure and Water Transport

Describe the structure of xylem vessels and explain how their structure is adapted for long-distance water transport. Include: (a) the role of lignin, (b) the importance of vessel diameter, (c) the formation of bordered pits, (d) the role of capillary action, and (e) why the cohesion-tension theory is the accepted explanation for water movement in plants.

Answer 13

(a) Lignin: xylem vessels are heavily lignified (impregnated with lignin, a complex polymer of phenolic compounds). Lignin provides:

• Mechanical strength and rigidity, preventing collapse of the vessel under the large negative tension (up to $-2\;\mathrm{MPa}$) generated by transpiration pull.
• Waterproofing, making the vessel walls impermeable to water (so water moves through the lumen, not the walls).
• Protection against pathogens and decay (lignin is resistant to enzymatic degradation).

(b) Vessel diameter: wider vessels offer less resistance to flow (Poiseuille's law: flow rate is proportional to $r^4$). However, wider vessels are more vulnerable to cavitation (air bubble formation) and collapse. Plants balance efficiency (wide vessels) and safety (narrow vessels, tracheids). Vessels in tropical vines can be $>500\;\mathrm{\mu m}$ wide; in cold climates, vessels are narrower to reduce freezing-induced cavitation.

(c) Bordered pits: thin areas in the vessel wall where lignification is reduced. They allow lateral water movement between adjacent vessels. The pit membrane (a modified primary cell wall) is porous but strong. If an air bubble (embolism) enters one vessel, the pit membrane prevents the air from spreading to adjacent vessels, containing the damage. This is called air-seeding prevention.

(d) Capillary action: the narrow diameter of xylem vessels and tracheids creates a meniscus at the air-water interface. Adhesion of water to the hydrophilic vessel walls and cohesion between water molecules generates a capillary force that can raise water by approximately $0.5$--$1\;\mathrm{m}$ (depending on vessel diameter). While capillary action contributes to water rise in small plants, it is insufficient to explain water transport in tall trees ($>10\;\mathrm{m}$). The main driving force is transpiration pull.

(e) Cohesion-tension theory (Dixon and Joly, 1894):

1. Water evaporates from mesophyll cell walls in the leaf (transpiration), creating a negative water potential at the leaf surface.
2. This negative potential pulls water from the xylem, generating tension (negative pressure) in the water column.
3. Water molecules are held together by hydrogen bonds (cohesion) and adhere to the hydrophilic xylem walls (adhesion), transmitting the tension down the entire water column from leaf to root.
4. Water enters the roots from the soil by osmosis (driven by the lower water potential in the root xylem).
5. The result is a continuous column of water under tension, pulled upward by transpiration.

Evidence: xylem sap is under negative pressure (measured with pressure chambers); cutting a stem causes air to be drawn in (demonstrating tension); the system can transport water to the top of the tallest trees ($>100\;\mathrm{m}$).

Problem 14: Data Analysis -- Mineral Deficiency Symptoms

A hydroponics experiment grows tomato plants in nutrient solutions lacking specific mineral ions. The following symptoms are observed:

Missing ion	Symptom
Nitrogen	Yellowing of older leaves, stunted growth
Magnesium	Interveinal chlorosis (yellowing between veins) in older leaves
Iron	Interveinal chlorosis in young leaves
Calcium	Deformed, necrotic growing tips; blossom end rot of fruit
Phosphorus	Dark green leaves with purple pigmentation; poor root growth

(a) Explain the biochemical role of each ion that accounts for the observed symptom. (b) Explain why nitrogen and magnesium deficiency symptoms appear in older leaves first, while iron deficiency appears in younger leaves first. (c) A plant shows both interveinal chlorosis and necrotic leaf tips. Suggest which two ions might be deficient and explain how you would confirm this.

Answer 14

(a) Nitrogen: nitrogen is a component of amino acids, proteins, nucleotides, and chlorophyll. Deficiency causes reduced chlorophyll production (chlorosis), reduced protein synthesis (stunted growth), and the plant mobilises nitrogen from older leaves (where it has been incorporated into proteins that are broken down) and transports it to younger leaves. Hence older leaves yellow first.

Magnesium: magnesium is the central atom of the chlorophyll molecule (essential for chlorophyll structure and function). Deficiency reduces chlorophyll production, causing chlorosis. Magnesium is mobile in the plant and is redistributed from older to younger leaves.

Iron: iron is required for chlorophyll synthesis (as a cofactor for enzymes in the chlorophyll biosynthetic pathway), though it is not part of the chlorophyll molecule itself. Iron is immobile in the plant (it precipitates as insoluble compounds in older tissues and cannot be easily remobilised). New leaves develop chlorosis because they cannot obtain iron from older leaves.

Calcium: calcium is a component of the middle lamella (calcium pectate, which cements cell walls together). It is also a signalling ion (second messenger). Calcium is immobile in the phloem (it cannot be remobilised from older tissues). Growing tips and young leaves are affected first because they cannot receive calcium from older parts. Blossom end rot results from insufficient calcium delivery to the fruit.

Phosphorus: phosphorus is a component of ATP, nucleic acids (DNA, RNA), and phospholipids (cell membranes). Deficiency causes poor root growth (low ATP for active transport), dark green leaves (accumulation of chlorophyll relative to growth), and anthocyanin (purple pigment) accumulation (a stress response).

(b) Mobile nutrients (N, P, K, Mg) can be remobilised from older leaves and transported to growing regions via the phloem. Deficiency symptoms therefore appear first in older leaves.

Immobile nutrients (Ca, Fe, B, Zn, Mn) cannot be remobilised. Once deposited in older tissues, they remain there. Deficiency symptoms appear first in younger leaves and growing tips, which receive insufficient supply from the roots.

(c) The combination of interveinal chlorosis and necrotic leaf tips suggests:

• Interveinal chlorosis: magnesium (older leaves) or iron (younger leaves) deficiency.
• Necrotic tips: potassium deficiency (marginal necrosis is characteristic) or calcium deficiency (growing tip necrosis).

To confirm: perform tissue analysis (measure ion concentrations in the affected and unaffected leaves) and compare to known deficiency thresholds. Alternatively, add back suspected missing ions one at a time to the hydroponic solution and observe symptom resolution.

Problem 15: Extended Response -- Plant Responses to Abiotic Stress

Describe the adaptations of plants to three abiotic stresses: (a) water stress (drought), (b) salinity, and (c) extreme temperatures. For each stress, describe the physiological and biochemical adaptations, including the role of specific hormones and osmoprotectants.

Answer 15

(a) Water stress (drought):

• Stomatal closure: ABA (abscisic acid) is produced in roots (in response to low soil water potential) and leaves (in response to low turgor). ABA binds to receptors on guard cells, causing:
  • Efflux of $\mathrm{K}^+$ and $\mathrm{Cl}^-$ from guard cells (through ion channels).
  • Loss of water by osmosis from guard cells.
  • Guard cells become flaccid, closing the stomatal pore. This reduces transpiration and water loss but also reduces $\mathrm{CO}_2$ uptake and photosynthesis.
• Root-to-shoot ratio increase: drought stimulates root growth over shoot growth, improving water uptake capacity.
• Osmotic adjustment: accumulation of compatible solutes (proline, glycine betaine, trehalose, soluble sugars) that lower the osmotic potential of cells, maintaining water uptake and turgor without interfering with enzyme function.
• Leaf adaptations: smaller leaves, thicker cuticle, sunken stomata, leaf rolling (reduces exposed surface area), leaf abscission (in severe drought).

(b) Salinity:

• Problem: high $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ concentrations in soil lower the water potential, making water uptake difficult (physiological drought). $\mathrm{Na}^+$ is also toxic to enzymes at high concentrations.
• Ion exclusion: roots actively exclude $\mathrm{Na}^+$ by the SOS1 $\mathrm{Na}^+/H^+$ antiporter on the plasma membrane, pumping $\mathrm{Na}^+$ out of root cells.
• Ion compartmentalisation: $\mathrm{Na}^+$ is sequestered in vacuoles by the tonoplast $\mathrm{Na}^+/H^+$ antiporter (NHX), keeping it away from cytoplasmic enzymes.
• Salt glands: some halophytes (salt-tolerant plants) have specialised salt glands that excrete excess salt onto the leaf surface, where it crystallises.
• Compatible solutes: accumulation of proline, glycine betaine, and polyols for osmotic adjustment.
• Morphological adaptations: succulence (water storage in fleshy leaves), reduced leaf area.

(c) Extreme temperatures:

• Heat stress:
  • Heat shock proteins (HSPs): produced in response to rapid temperature increase. HSPs act as molecular chaperones, preventing protein denaturation and assisting in refolding damaged proteins.
  • Membrane fluidity: membranes become too fluid at high temperatures. Plants adjust lipid composition (increase saturated fatty acids) to maintain optimal membrane fluidity.
  • Evaporative cooling: transpiration cools the leaf (approximately $2^\circ\mathrm{C}$ below air temperature).
• Cold stress:
  • Membrane fluidity: membranes become too rigid at low temperatures. Plants increase the proportion of unsaturated fatty acids (more double bonds, creating kinks) to maintain fluidity.
  • Antifreeze proteins: some plants produce proteins that inhibit ice crystal growth.
  • Supercooling: some cells can remain liquid below $0^\circ\mathrm{C}$ by accumulating solutes that depress the freezing point.
  • Cold acclimation (hardening): exposure to gradually decreasing temperatures triggers changes in gene expression (via CBF/DREB transcription factors), increasing tolerance to subsequent freezing. This involves accumulation of soluble sugars, proline, and antifreeze proteins.
  • ABA: involved in both cold and drought responses, promoting stomatal closure and stress-responsive gene expression.

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Cross-References to Related Topics

• Cell membrane and transport: Review ./cell-biology for membrane structure, osmosis, and active transport.
• Enzymes and protein structure: Review ./molecular-biology for enzyme kinetics and protein folding.
• Photosynthesis and light reactions: Review ./metabolism-cell-biology for light absorption, electron transport, and the Calvin cycle.
• Genetics and gene expression: Review ./genetics-advanced for transcription factors and gene regulation in plant development.
• Ecology and biomes: Review ./ecology for plant adaptations to different biomes and ecosystems.

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Supplementary: Plant Hormones in Detail (HL Extension)

Overview of Plant Hormones (Phytohormones)

Plant hormones are signalling molecules produced in small quantities that regulate growth, development, and responses to environmental stimuli. Unlike animal hormones, they are not produced in specialised glands; instead, they are synthesised in various tissues and can act locally or be transported to distant sites.

Auxin (IAA -- Indole-3-Acetic Acid)

Sites of synthesis: shoot apical meristem, young leaves, developing seeds.

Transport: polar (basipetal) transport via PIN efflux carriers. Auxin moves from the shoot apex toward the base. This directional transport is essential for its role in tropisms and apical dominance.

Functions:

• Cell elongation: auxin stimulates cell elongation in the shoot by activating proton pumps ($\mathrm{H}^+$-ATPases) on the plasma membrane. The resulting acidification of the cell wall activates expansins (proteins that loosen cross-links between cellulose microfibrils), allowing the wall to stretch under turgor pressure (the acid growth hypothesis).
• Apical dominance: auxin produced by the apical bud suppresses lateral bud growth (see worked examples above).
• Root initiation: auxin promotes the formation of lateral roots and adventitious roots. This is used commercially in rooting powders.
• Fruit development: auxin promotes fruit set (prevents fruit abscission after fertilisation). Synthetic auxins (e.g., 2,4-D) are used as herbicides (selective for broadleaf plants because they are more sensitive to auxin than grasses).
• Tropisms: differential auxin distribution causes bending toward (phototropism) or away from (gravitropism) stimuli.

Phototropism mechanism: light is perceived by phototropins (blue-light receptors) on the shoot tip. Auxin is redistributed to the shaded side of the shoot. Cells on the shaded side elongate more than cells on the lit side, causing the shoot to bend toward light.

Gibberellins (GAs)

Sites of synthesis: young leaves, roots, developing seeds.

Transport: not polar; moves through the phloem and xylem.

Functions:

• Stem elongation: gibberellins stimulate cell division and elongation in the internodes. Application of GA to dwarf varieties (e.g., dwarf peas, dwarf maize) causes them to grow to normal height (dwarfism in these varieties is caused by GA deficiency or insensitivity).
• Seed germination: GA produced by the embryo diffuses to the aleurone layer, stimulating amylase production (see worked example above). Amylase breaks down starch in the endosperm, providing sugars for the growing embryo.
• Bolting: GA promotes flowering in long-day plants and rosette plants (biennials that flower in the second year after exposure to cold, vernalisation, followed by long days).
• Fruit development: GA promotes fruit set in some species (e.g., grapes, where GA application produces larger, seedless fruit).

Cytokinins

Sites of synthesis: root apical meristem (primary site), also shoot tips.

Transport: moves through the xylem (upward from roots to shoots).

Functions:

• Cell division: cytokinins promote cytokinesis (cell division), working in conjunction with auxin. The auxin:cytokinin ratio determines organogenesis in tissue culture:
  • High auxin : low cytokinin $\to$ roots
  • Low auxin : high cytokinin $\to$ shoots
  • Balanced ratio $\to$ callus (undifferentiated tissue)
• Delay senescence: cytokinins slow leaf ageing (senescence) by maintaining protein synthesis and chlorophyll content. This is used commercially (cytokinin sprays to extend shelf life of cut flowers and vegetables).
• Apical bud growth: cytokinins counteract auxin's inhibition of lateral buds, promoting lateral bud outgrowth.
• Shoot apical dominance: the balance between auxin (from the shoot apex) and cytokinins (from the roots) determines the degree of apical dominance.

Abscisic Acid (ABA)

Site of synthesis: leaves, stems, root caps (especially in response to stress).

Functions:

• Stomatal closure: ABA is the primary hormone responsible for closing stomata during water stress. It binds to receptors on guard cell membranes, triggering $\mathrm{K}^+$ efflux, loss of turgor, and stomatal closure.
• Seed dormancy: ABA maintains seed dormancy by inhibiting germination. High ABA levels during seed development prevent premature germination. Dormancy is broken when ABA levels decline (e.g., after stratification -- cold treatment -- or imbibition).
• Stress responses: ABA mediates responses to drought, salinity, and cold stress by activating stress-responsive genes (via AREB/ABF transcription factors).

Ethylene ($\mathrm{C}_2\mathrm{H}_4$)

Site of synthesis: ripening fruits, senescing tissues, stressed tissues.

Unique property: ethylene is a gas, so it can diffuse between cells and between plants (acting as a signalling molecule between individuals).

Functions:

• Fruit ripening: ethylene triggers the conversion of starch to sugars, cell wall breakdown (softening), chlorophyll degradation (colour change from green to red/yellow), and volatile production (aroma). This is used commercially: unripe fruit is shipped and then exposed to ethylene to induce ripening on arrival ("climacteric" fruits like bananas, tomatoes, apples).
• Leaf abscission: ethylene promotes the breakdown of the abscission zone (cell wall degradation by cellulase and pectinase), causing leaves, flowers, and fruit to fall.
• Triple response: in dark-grown seedlings, ethylene causes: inhibition of stem elongation, thickening of the stem, and horizontal growth (apical hook). This is a diagnostic test for ethylene sensitivity.
• Senescence: ethylene accelerates ageing in leaves and flowers.

Interactions Between Hormones

Plant hormones rarely act in isolation; their effects are modulated by interactions:

• Auxin and cytokinin: antagonistic in apical dominance (auxin inhibits, cytokinin promotes lateral bud growth); synergistic in cell division (both required).
• Auxin and ethylene: auxin stimulates ethylene production (auxin upregulates ACC synthase, the rate-limiting enzyme in ethylene biosynthesis). High auxin in root tips causes ethylene production, which inhibits root elongation.
• GA and ABA: antagonistic in seed germination (GA promotes, ABA inhibits). The GA:ABA ratio determines whether a seed germinates or remains dormant.
• Cytokinin and ABA: cytokinin delays senescence; ABA promotes stress responses that may lead to senescence.

Worked Example: Hormone Interaction in Apical Dominance

A plant is decapitated (apical bud removed). In one treatment, cytokinin is applied to the cut surface; in another, auxin is applied. In a third, both are applied. (a) Predict the effect on lateral bud growth in each treatment. (b) Explain the mechanism.

Solution

(a) Decapitation only: lateral buds grow (apical dominance is released). Decapitation + cytokinin: lateral buds grow more vigorously than decapitation alone (cytokinin promotes lateral bud outgrowth). Decapitation + auxin: lateral buds remain inhibited (auxin restores apical dominance). Decapitation + auxin + cytokinin: intermediate effect; the outcome depends on the relative concentrations (cytokinin partially overrides auxin's inhibitory effect).

(b) Mechanism: the apical bud is the primary source of auxin, which is transported basipetally through the stem. High auxin in the stem inhibits lateral bud growth (directly, by suppressing cell division in lateral bud meristems, and indirectly, by maintaining the stem as a strong nutrient sink). Cytokinin, produced primarily in the roots and transported upward, promotes lateral bud growth by stimulating cell division. The balance between auxin (inhibitory) and cytokinin (promotory) determines whether lateral buds remain dormant or grow. Removing the apical bud eliminates the auxin source; applying auxin restores it; applying cytokinin provides an additional growth stimulus.