Plant Physiology: Plant Hormones and Growth

Phytohormones (plant growth regulators)

Phytohormones are organic substances synthesised in one part of a plant, in very small (trace) amounts, and transported to another part where they regulate growth, development or physiological response, generally without being used as a nutrient. Five major groups are recognised: auxins, gibberellins, cytokinins, ethylene and abscisic acid (ABA). The first three, along with ethylene, are mainly growth promoting; ABA is mainly inhibitory.

Auxins

Discovered through Charles and Francis Darwin's experiments on the phototropic bending of grass coleoptiles, and later isolated and characterised by F. W. Went. The main natural auxin is indole 3 acetic acid (IAA), synthesised chiefly in shoot tips and young leaves; synthetic auxins used in agriculture include naphthalene acetic acid (NAA) and 2,4 dichlorophenoxyacetic acid (2,4 D, also used as a selective weedicide against broad leaved weeds).

Physiological roles:

• Promotes cell elongation just behind the shoot apical meristem, the basis of phototropic and geotropic curvature through unequal lateral distribution of auxin.
• Induces adventitious root formation on stem cuttings, used commercially in rooting powders.
• Maintains apical dominance: auxin from the terminal bud suppresses growth of lateral (axillary) buds; removing the terminal bud, as in pruning, releases the lateral buds to grow.
• Induces parthenocarpy (seedless fruit development without fertilization).
• Promotes flowering in some plants, for example pineapple.
• Used commercially to prevent premature fruit and leaf drop, and to inhibit sprouting of stored potatoes.

Gibberellins

Isolated in 1926 from the fungus Gibberella fujikuroi, which causes the "foolish seedling" (bakanae) disease of rice by making infected seedlings grow abnormally tall and spindly, the observation that first drew attention to gibberellins. They are synthesised mainly in young leaves, shoot tips and developing seeds, and move through both xylem and phloem.

Physiological roles:

• Promote stem/internode elongation and leaf expansion.
• Reverse genetic dwarfism when applied to dwarf varieties, restoring normal height.
• Break seed and bud dormancy.
• Induce parthenocarpy, used to produce seedless grapes and to increase fruit/berry size.
• Delay senescence, keeping fruit and leaves on the plant longer.
• Can substitute for the long day or cold requirement of some plants and induce bolting/flowering.