Plant Growth and Development

Auxins induce parthenocarpy (A is true). Phytohormones act as promoters or inhibitors (B is true). Differentiated parenchymal cells must dedifferentiate into meristems before redifferentiating (C is true). ABA is an inhibitor (D is false). Apical dominance inhibits lateral buds (E is false).

Synthetic auxins such as 2,4-D are used as weedicides to kill dicotyledonous weeds. They do not affect mature monocotyledonous plants such as grasses, so they are used to prepare weed-free lawns.

Ethylene promotes rapid internode/petiole elongation in deep water rice plants.

The response of plants to the relative lengths of day and night, especially with regard to flowering, is called photoperiodism.
It was first shown by W.W. Garner and H.A. Allard in 1920.
They worked with a mutant variety of tobacco called Maryland Mammoth, which kept growing vegetatively in summer and flowered only in winter in the greenhouse.
They found that it flowered only when the day length was shorter than a critical value; it is a short day plant.
They made similar observations in the Biloxi variety of soybean.
So the importance of day length in flowering was first shown in tobacco.

Photoperiodism is the response of plants to the relative lengths of day and night. What really matters is the length of the uninterrupted dark period.
Short day plants (long night plants) flower only when they get a continuous dark period longer than a critical length.
If the long night is interrupted even by a brief flash of light, the dark period is broken into two short nights, and a short day plant does not flower.
Here the plant flowered with a 12 hour uninterrupted night, but failed to flower when the night was interrupted by a flash of light. This is the typical behaviour of a short day plant.
A long day plant would be promoted (not inhibited) by a night break, and a day neutral plant would not respond to it at all.
So the plant is a short day plant.

The seeds of many temperate plants, such as apple, peach and cherry, do not germinate until they have been exposed to a period of chilling.
Stratification is the treatment in which seeds are kept at a low temperature (about 0 to $10^\circ C$) under moist conditions for some weeks.
During this treatment the level of inhibitors such as abscisic acid falls and that of gibberellins rises, so dormancy is broken.
Scarification is the weakening or breaking of a hard seed coat by mechanical or chemical means.
Vernalisation is a low temperature treatment that promotes flowering, not the breaking of seed dormancy.
Chelation is the binding of metal ions by organic molecules.
So the treatment is called stratification.

Abscisic acid helps seeds withstand desiccation (A-II); Ethylene promotes female flowers in cucumber (B-I); Gibberellin promotes bolting in beet, cabbage etc. (C-IV); Cytokinin helps in nutrient mobilisation (D-III).

F. Skoog observed that internodal segments of tobacco stem proliferated into callus only when the medium (supplied with auxin) also contained one of: extract of vascular tissue, yeast extract, or coconut milk. Abscisic acid was not one of the substances found to induce this effect.

All gibberellins (GAs) are acidic in nature (A is correct). Zeatin (a cytokinin) was first isolated from coconut milk (C is correct). Ethylene induces flowering in mango (D is correct). Auxins are not antagonists to GAs, and it is ABA that is antagonistic to GA (B is incorrect); auxins, not ABA, induce parthenocarpy in fruits like tomatoes (E is incorrect). So the correct set is A, C, D.

Seed dormancy caused by inhibitory substances can typically be overcome by chilling, treatment with gibberellic acid, or nitrate treatment. Ascorbic acid is not a standard method used to remove inhibitory substances/break dormancy in seeds.

Fruit ripener is Ethephon (a-iv); Herbicide is 2,4-D (b-iii); Bolting agent is $GA_3$ (c-ii); Stress hormone is Abscisic acid (d-i).

The term 'Kinetin' (a cytokinin) was coined by Skoog and Miller, who discovered it while studying factors promoting cell division in tobacco pith culture.

Auxin was discovered through experiments (by Darwin and later Went) on phototropism, the bending of coleoptiles towards light, which is influenced by the differential distribution of auxin.

Auxin promotes flowering in pineapple (A-IV); Gibberellin increases the length of grape stalks (B-III); Cytokinin helps in overcoming apical dominance (C-II); Ethylene promotes female flower formation in cucumber (D-I).

Plant growth is generally indeterminate (continuing throughout the life of the plant due to the presence of meristems), not determinate — so this statement is not correct.

Went's Avena curvature test showed that the degree of bending is proportional to the amount of auxin diffused into the agar.
It is therefore the basis for the quantitative estimation (bioassay) of small amounts of growth-promoting substances.

Indole-3-acetic acid (IAA) is a natural auxin.
In tea plantations the shoot tips are regularly removed; this removes apical dominance, so the lateral buds grow into branches and the bush becomes dense. This phenomenon, controlled by auxin, is widely applied in tea plantations and hedge-making.
Auxins are also used to initiate rooting in stem cuttings, which is how tea is commonly propagated.
Ethylene is used for fruit ripening, abscisic acid is a growth inhibitor, and zeatin is a cytokinin.
Hence the hormone commonly used in tea plantations is indole-3-acetic acid.

A tropism is a directional growth movement of a plant part in response to a unidirectional external stimulus.
When the stimulus is touch or contact, the movement is thigmotropism: the side of the tendril in contact with the support grows more slowly than the opposite side, so the tendril coils around the support.
A nastic movement (thigmonasty) is non-directional, e.g., the folding of $Mimosa$ leaves on touch.
Taxis is the movement of a whole organism or cell towards or away from a stimulus, which does not apply to a tendril.
Hence the coiling of pea tendrils is an example of thigmotropism.

Plant movements are autonomic (spontaneous, caused by internal factors) or paratonic (induced by external stimuli).
Movements are further of two kinds: growth movements, caused by unequal growth, and variation movements, caused by reversible changes in turgor.
The opening of a floral bud is caused by more growth on the upper (inner) surface of the sepals and petals than on the lower (outer) surface. This is called epinasty.
Epinasty is an irreversible growth movement, and it is controlled internally (by auxin distribution), so it is autonomic.
It is not a movement of locomotion, because the plant or organ does not move from one place to another.
It is not a movement of variation, because it is not due to turgor changes.
So the opening of floral buds into flowers is an autonomic movement of growth.

Senescence is the period between reproductive maturity and death, in which degenerative changes take place in cells, organs or the whole plant.
It is not a passive wearing out; it is an active, genetically programmed and hormonally controlled developmental process.
Leaf abscission is the best example: before a leaf falls, chlorophyll and proteins are broken down and the nutrients are withdrawn into the plant body.
An abscission layer is then actively formed at the base of the petiole, where enzymes such as cellulase and pectinase dissolve the cell walls and middle lamella.
This process is promoted by abscisic acid and ethylene and delayed by auxins and cytokinins, which shows that it is an active, regulated cellular process.
So senescence as an active developmental process is indicated in leaf abscission, as given in the answer key.

Phytochrome is the pigment that controls photoperiodic responses in plants. It exists in two interconvertible forms, $P_r$ and $P_{fr}$.
The $P_r$ form absorbs red light between 660 and 680 nm, with a peak at about 666 nm. It is the form synthesised in dark-grown seedlings.
When $P_r$ absorbs red light, it is converted into the $P_{fr}$ form.
The $P_{fr}$ form absorbs far-red light of about 730 nm and is converted back into $P_r$; it also reverts slowly in darkness.
$P_{fr}$ is the physiologically active form.
Among the given values, 680 nm lies in the red region absorbed by $P_r$, while 720 nm is in the far-red region absorbed by $P_{fr}$.
So the wavelength absorbed by the $P_r$ form is 680 nm.

The endosperm of a barley grain stores food mainly in the form of starch, which the embryo cannot use directly.
When the seed imbibes water, the embryo produces gibberellic acid (GA).
GA diffuses to the aleurone layer and induces it to synthesise and secrete hydrolytic enzymes, the most important being $\alpha$-amylase.
$\alpha$-amylase hydrolyses the starch of the endosperm into maltose and glucose.
These soluble sugars are absorbed by the scutellum and supplied to the growing embryo, so germination proceeds.
This is used in the malting of barley in the brewing industry.
So the enzyme that can stimulate the germination of barley seeds is $\alpha$-amylase.