Chapter 7: Control of Growth and Development

Auxin, first identified as the hormone behind phototropism (stem bending toward light), promotes cell elongation and is responsible for apical dominance, where the stem tip suppresses lateral branching. Gibberellins regulate stem internode elongation, seed germination, and flowering, especially under low-light conditions. Cytokinins stimulate cell division and delay senescence, while abscisic acid enforces dormancy and stress responses. Ethylene, a gaseous hormone, accelerates fruit ripening, leaf abscission, and senescence. Tropisms are central to plant adaptation: phototropism guides stems toward light; geotropism (gravitropism) directs roots downward and shoots upward; and thigmotropism enables tendrils to coil around supports. Beyond tropisms, plants display nastic movements such as the seismonastic folding of Mimosa pudica leaves or the closing of Venus flytraps, demonstrating rapid growth or turgor responses independent of directional stimuli. Environmental regulation also shapes development: temperature cues trigger dormancy and flowering cycles, while vernalization (cold treatment) is essential for biennials like cabbage and carrots to flower in their second year. Perhaps most influential is photoperiodism, the plant’s ability to measure day length (or more precisely, night length) to time flowering. Short-day, long-day, and day-neutral plants rely on critical photoperiods to ensure reproductive success before environmental stress sets in. Although the elusive flowering hormone “florigen” has not been fully isolated, experiments show it moves from induced leaves to apical meristems, switching growth from vegetative to reproductive. The chapter concludes with practical applications: horticulturists use synthetic auxins to stimulate root growth in cuttings, gibberellins to promote fruit size, ethylene to induce flowering in pineapples, and growth retardants to produce compact ornamentals. Herbicides such as 2,4-D exploit auxinlike activity to selectively kill weeds. Together, these insights reveal how plants integrate hormonal signals and environmental rhythms to coordinate growth, survival, and reproduction.