Plant Physiology and Developmental Botany

(b) Two photosystems operating simultaneously

The red drop (quantum yield falls beyond about 680 nm) is removed by adding shorter-wavelength light, which is Emerson's enhancement effect. This showed that two light reactions, PS I and PS II, work together in photosynthesis.

(b) Lumen of thylakoids

Water splitting and plastoquinone proton pumping release protons into the thylakoid lumen, creating the proton gradient. ATP synthase uses this gradient to make ATP on the stroma side, so the lumen has the highest proton concentration.

(c) IAA

The Avena (oat) coleoptile curvature test, by Went, measures auxin: agar blocks containing IAA placed asymmetrically cause bending in decapitated coleoptiles. The bending angle is proportional to IAA concentration. GA3 is assayed by dwarf pea or maize tests, ABA and ethylene by other tests.

(b) $\mathrm{C_4}$

The listed features are the hallmarks of C$_4$ plants: Kranz anatomy and PEP carboxylase concentrate CO$_2$ around RuBisCO, so photorespiration is avoided. They also have better water use efficiency and keep high photosynthetic rates at high temperature. C$_3$ plants lose carbon through photorespiration, and CAM plants are adapted to arid conditions with slow growth.

(b) Both processes can happen together because the diffusion coefficient of water and $\mathrm{CO_2}$ is different.

Water vapour moves out and CO$_2$ moves in along their own concentration gradients through the same open stomata, and the two gases diffuse independently. They differ in diffusion coefficient and gradient, so both go on simultaneously during the day. Stomata are open mainly in the day, so it is wrong to say they happen only at night or at separate times.

(b) Active absorption

Root pressure develops from active uptake of ions by root cells into the xylem, which lowers its water potential so water enters by osmosis and pushes up. Increased transpiration causes suction (negative pressure), not root pressure, and passive absorption is driven by transpiration pull.

(b) Manganese and Chlorine

Photolysis of water at photosystem II needs Mn (in the oxygen-evolving complex) and Cl$^-$ as cofactors to release O$_2$, protons and electrons. Mg is in chlorophyll, and K and Mo are not directly involved in water splitting.

(b) Leghaemoglobin

Nitrogenase is highly oxygen-sensitive. Leghaemoglobin in root nodules is an oxygen scavenger that keeps the free oxygen level low. Cytochromes, xanthophyll and carotene are not oxygen scavengers in nodules.

(b) Avena coleoptile curvature

The Avena coleoptile curvature test is the classic bioassay for auxin (IAA). Lettuce hypocotyl elongation is used for gibberellins, while hydroponics and the potometer are not bioassays.

(c) Pistia

Pistia is a floating aquatic plant that absorbs water through its whole surface, so roots play only an insignificant role in water absorption. Wheat, sunflower and pea are terrestrial plants that depend on roots for water.

(c) Tensile strength of water

Water columns in xylem do not break because of cohesion and adhesion, which give water high tensile strength. Root pressure is too weak for tall trees, dissolved sugars do not hold the column together, and lignification gives wall strength but not column strength.

(a) Stromal matrix

The light-independent reactions (Calvin cycle) occur in the stroma of the chloroplast, using ATP and NADPH made in the light reactions. The thylakoid lumen and photosystems I and II are on the thylakoid membrane and are sites of the light reactions.

(b) Etiolated

Seedlings grown in the dark become pale and spindly because there is no chlorophyll or light-regulated growth. This is etiolation. Embolism is an air bubble in xylem, defoliation is leaf loss, and mutation is a genetic change.

(d) Abscisic acid

ABA is called the stress hormone because its level rises under drought, salinity and cold, closing stomata and helping the plant tolerate stress. Ethylene is the fruit-ripening hormone, GA$_3$ promotes stem elongation and IAA is the main auxin for growth.

(a) It is the basis for quantitative determination of small amounts of growth-promoting substances.

Went's agar block experiment (the Avena curvature test) showed that the degree of bending of the stump is proportional to the amount of auxin in the agar, giving a bioassay for tiny quantities of auxin. It did not identify IAA chemically or prove polar transport, so (b), (c) and (d) are not what the experiment is significant for.

(d) Gibberellin

During germination gibberellins from the embryo induce alpha-amylase and other hydrolases in the aleurone layer, which mobilise stored food in the endosperm. ABA and ethylene do not do this, and ABA inhibits germination.

(d) Uphill transport

Facilitated diffusion needs membrane proteins, is selective and saturates, but it moves molecules down their concentration gradient without energy. Uphill transport needs ATP, so it is active transport.

(b) Ammonia

Nitrogenase reduces $N_2$ to $NH_3$, which is the first stable product in root nodules ($N_2 + 8e^- + 8H^+ + 16ATP \to 2NH_3 + H_2 + 16ADP + 16P_i$). Glutamate forms later by reductive amination of ammonia with alpha-ketoglutarate.

(c) Radial orientation of cellulose microfibrils in the cell wall of guard cells

Cellulose microfibrils in guard cell walls are arranged radially, so the cells cannot widen. When they become turgid they lengthen and bend outward, pulling the stomatal pore open. Decreased turgidity closes the stoma. Longitudinal microfibrils would not give this response.

(c) C$_3$ plants respond to higher temperatures with enhanced photosynthesis while C$_4$ plants have much lower temperature optimum

This statement is not correct because it is reversed. C$_4$ plants respond to higher temperatures (optimum about 30-45 °C), while C$_3$ plants have a lower optimum (about 20-25 °C). The other statements are correct: light saturates at about 10% of full sunlight, CO$_2$ up to 0.05% increases fixation, and tomato is grown in CO$_2$-enriched greenhouses.

(c) Auxins

Auxins delay abscission of young leaves and fruits and so prevent early drop. Ethylene promotes abscission. Cytokinins delay senescence and gibberellins promote elongation, but neither is the standard answer for preventing fruit and leaf drop.

(b) C$_4$ plants

In C$_4$ plants the primary CO$_2$ acceptor is phosphoenolpyruvate (PEP) in the mesophyll cells, with PEP carboxylase fixing CO$_2$ into oxaloacetate. In C$_3$ plants the acceptor is RuBP, fixed by RuBisCO.

(a) Zero

Water potential of pure water at standard temperature and atmospheric pressure is taken as zero. Adding solute lowers it, so the water potential of solutions is negative.

(c) $\mathrm{O_2}$ concentration

Guard cells respond to light, temperature and $\mathrm{CO_2}$ level inside the leaf (low $\mathrm{CO_2}$ opens stomata, high $\mathrm{CO_2}$ closes them), as well as water status and ABA. Ambient $\mathrm{O_2}$ concentration has no significant role in stomatal movement.

(b) NADH

The light reaction produces ATP (photophosphorylation), NADPH (reduction of $\mathrm{NADP^+}$ at PS I) and $\mathrm{O_2}$ (photolysis of water at PS II). NADH is a product of respiration, not photosynthesis; the light reaction makes NADPH.

(a) Green sulphur bacteria

Green sulphur bacteria carry out anoxygenic photosynthesis, using $\mathrm{H_2S}$ rather than water as the electron donor, so no $\mathrm{O_2}$ is released. Nostoc (cyanobacterium), Cycas and Chara (green alga) all use water and release oxygen.

(c) Auxin and Ethylene

Auxins (e.g. NAA) and ethylene promote flowering in pineapple and make flowering synchronous, so it can be induced throughout the year. Gibberellin promotes bolting and cytokinin promotes cell division, while ABA is inhibitory.

(b) Leaves

Photoperiod is perceived by the leaves, which then send a floral stimulus (florigen) through the phloem to the shoot apex. The shoot apex is where flowering takes place, not where the light signal is sensed. Experiments in which only a leaf is exposed to the right photoperiod still induce flowering.

(b) Water, mineral salts, some organic nitrogen and hormones

Xylem mainly carries water and mineral salts. It also carries some organic nitrogen (amino acids, amides) and hormones such as cytokinins made in the roots. Options (c) and (d) leave these out. Option (a) leaves out the hormones.

(b) Bi-directional

Sugars move in phloem from source to sink, and the direction depends on where the source and sink are. Leaves can send sugar down to roots, or up to growing buds and fruits, and a sink can be above or below the source. Movement in phloem is therefore bi-directional, unlike the one-way upward flow in xylem.

(c) PS-II to Cytb6f complex

Plastoquinone carries electrons from PS-II to the Cytb6f complex, and it also carries protons into the lumen. Plastocyanin then carries electrons from Cytb6f to PS-I. Ferredoxin and NADP reductase take them from PS-I to $\mathrm{NADP^+}$, so options (a), (b) and (d) are wrong.

(d) Gibberellin

Gibberellins cause elongation of internodes. Spraying sugarcane with gibberellin increases stem length and so the yield of sugar. Ethylene promotes ripening, abscisic acid is a growth inhibitor, and cytokinin promotes cell division.

(c) Log phase

The log (exponential) phase is the period of fastest growth, when the growth rate is greatest. The lag phase is slow growth, and in senescence growth ends. Dormancy is a resting state with no growth.

(d) 1 molecule of 3-C compound

When RuBisCO acts as an oxygenase, RuBP (5-C) is split into one 3-phosphoglycerate (3-C) and one phosphoglycolate (2-C). NCERT-style keys count the 3-C PGA, which enters the Calvin cycle, as the answer. Option (c), two 3-C molecules, is the carboxylation product, and no 6-C compound forms in oxygenation.

(c) Gibberellic acid

Gibberellic acid promotes germination and breaks seed dormancy, so it is not an inhibitor. Abscisic acid, phenolic acids and para-ascorbic acid are inhibitory substances that maintain dormancy.

(d) Root pressure

Loss of water as liquid droplets from grass blade tips at night and early morning is guttation, caused by positive root pressure when transpiration is low and soil moisture is high. Transpiration is loss as vapour, while imbibition and plasmolysis do not move water out as droplets.

(b) Ethylene

Deep water rice responds to submergence by accumulating ethylene, which promotes rapid elongation of internodes and petioles so the leaves reach the surface for air. Kinetin and 2,4-D do not drive this response, and GA3 acts downstream in the stem elongation but ethylene is the primary trigger.

(c) Active Transport

Moving ions against their concentration gradient needs energy (ATP) and carrier proteins, which is active transport. Facilitated diffusion, passive transport and osmosis all move substances down a gradient without energy expenditure.

(a) Dedifferentiation

Mature, differentiated mesophyll cells regain the capacity to divide and form an undifferentiated mass of cells (callus). This reversal is dedifferentiation; later, callus cells mature again by redifferentiation.

(a) 18 ATP and 12 NADPH2

One CO2 fixed needs 3 ATP and 2 NADPH. Glucose has 6 carbons, so 6 CO2 are fixed: $6\times3=18$ ATP and $6\times2=12$ NADPH.

(d) Both Statement I and Statement II are correct

Transpiration pull generates forces able to lift a xylem water column over 130 m, and evaporative cooling can lower leaf temperature by 10 to 15 degrees. Both statements are standard textbook facts.

(a) Gibberellic Acid

Spraying GA (gibberellins) on juvenile conifers hastens maturity and leads to early seed production. Zeatin promotes cell division, ABA is a stress hormone, and IBA is an auxin used for rooting.

(d) 680 nm

The reaction centre of PS II (P680) absorbs at 680 nm, whereas PS I (P700) absorbs at 700 nm. The other values are not reaction centre maxima.

(d) Manganese

Manganese is required for the water-splitting (oxygen-evolving) complex of PS II. Magnesium is part of chlorophyll but is a macronutrient, and molybdenum and copper act elsewhere (nitrogen metabolism, plastocyanin).

(d) A-II, B-IV, C-I, D-III

Cohesion is mutual attraction among water molecules, adhesion is attraction towards polar surfaces, surface tension arises from more attraction in the liquid phase, and guttation is water loss in liquid phase.