Plant Hormones and Economic Botany: Notes and MCQs

A plant has no nervous system, so it coordinates growth with chemicals. Five groups of these chemical messengers, called phytohormones or plant growth regulators, decide how a plant grows towards light, how tall it becomes, when its seeds sleep and wake, when its fruit ripens and when its leaves fall. The same molecules are used in farming and horticulture, which is why the topic appears in both the biology and the agriculture sections of question papers. The second half of this chapter is economic botany, the plants that feed, clothe, flavour and cure us, and the one fact examiners want from each: which part of the plant the product is.

The five plant hormones

Auxin was the first to be found. Charles Darwin and his son noticed that a grass seedling bends towards light only if its tip is intact, and F. W. Went later isolated the substance from oat seedling tips. The natural auxin is indole-3-acetic acid. Auxin causes cell elongation, keeps the main shoot dominant over the side buds, a habit called apical dominance, starts the roots on a cutting, prevents the early fall of leaves and fruits and can make seedless fruit, which is called parthenocarpy. Synthetic auxins are used in the field: naphthalene acetic acid and indole butyric acid in rooting, and 2,4-dichlorophenoxyacetic acid, or 2,4-D, as a weedkiller that destroys broad-leaved dicot weeds in a crop of grasses such as wheat.

Gibberellin came out of a plant disease. Japanese rice plants suffering from the foolish seedling disease grew abnormally tall, and the cause was found to be a chemical from the fungus Gibberella fujikuroi; the best known of the group is gibberellic acid. Gibberellin lengthens stems, which is used to make grapes larger and to produce bolting in sugar beet, breaks seed and bud dormancy, hastens the malting of barley in the brewing industry and delays the ageing of leaves.

Cytokinin was discovered as kinetin from the degraded DNA of herring sperm, and the natural form, zeatin, was obtained from maize grains. Its chief work is to promote cell division, which is why it is indispensable in plant tissue culture, where a high auxin to cytokinin ratio makes roots and a high cytokinin to auxin ratio makes shoots. Cytokinin also delays leaf fall and encourages side branches by countering apical dominance.

Abscisic acid is the stress hormone of plants. It closes the stomata when water is short, so it is called an antitranspirant, puts seeds and buds into dormancy and generally inhibits growth, acting as the antagonist of gibberellin. Ethylene is the only gaseous hormone. It ripens fruit, which is why one ripe banana ripens the rest of the bunch and why ethephon is sprayed to ripen tomatoes and mangoes uniformly; it also causes leaves and fruits to fall, speeds up ageing, breaks dormancy in potato buds and induces flowering in pineapple.

Economically important plants

The questions here are of one kind: name the plant or name the part. Among fibres, cotton comes from the surface hairs on the seed of Gossypium, jute from the stem of Corchorus, coir from the husk, that is the mesocarp, of the coconut, and linen from the stem of flax. Among beverages, tea is made from the young leaves of Camellia sinensis, coffee from the roasted seeds of Coffea and cocoa from the seeds of Theobroma cacao. Sugar comes from the stem of sugarcane and natural rubber from the latex of Hevea brasiliensis. Tobacco is the dried leaf of Nicotiana tabacum.

Medicinal plants and the part used

Quinine, the old specific against malaria, comes from the bark of Cinchona. Reserpine, used for high blood pressure, comes from the root of Rauvolfia serpentina, the sarpagandha. Opium and the morphine in it come from the latex of the unripe capsule of Papaver somniferum. Atropine comes from Atropa belladonna, digitalis from the foxglove leaf, and the anti-cancer alkaloids vincristine and vinblastine from Catharanthus roseus, the periwinkle. Neem, Azadirachta indica, gives azadirachtin, used as a biopesticide.

Which part do we eat

Examiners also ask what a vegetable actually is. The potato is a stem tuber and the sweet potato a root; the onion is a bulb of fleshy leaves and garlic a cluster of such cloves; ginger and turmeric are rhizomes, that is underground stems; carrot, radish and beetroot are roots; cauliflower is an inflorescence and cabbage a tight bud of leaves; the edible part of the coconut is the endosperm; and the apple is a false fruit, because the fleshy part grows from the thalamus and not from the ovary.