Plants respond to stimuli through growth responses: tropism and nastic responses. (p.98)
Tropism responses are growth responses of plant parts towards or away from a stimulus. (p.98)
Types of tropism: thigmotropism, geotropism, hydrotropism, phototropism and chemotropism. (p.98)
Tropism is slow, permanent, influenced by plant hormones, and its direction depends on the direction of the stimulus. (p.99)
Positive tropism is growth towards the stimulus; negative tropism is growth away from it. (p.99)
Thigmotropism is the response to touch; tendrils coil around objects. (p.99)
Geotropism is the response to gravity: shoots show negative geotropism and roots show positive geotropism. (p.99)
Hydrotropism is the response to water; roots show positive hydrotropism. (p.100)
Phototropism is the response to light; shoots show positive phototropism. (p.100)
Chemotropism is the response to chemicals; roots grow towards mineral salts and away from poisons. (p.100)
Nastic responses are quicker and more apparent than tropism, are not always growth responses, and are independent of stimulus direction. (p.101)
Types of nastic response: photonasty, seismonasty, nyctinasty, thermonasty and thigmonasty. (p.101)
Photonasty is a response to light, e.g. Japanese roses open in maximum sunlight and close at night. (p.101)
Seismonasty is a response to mechanical stimulus such as touch, shock or wind, e.g. mimosa leaves fold when touched. (p.101)
Nyctinasty is a circadian response to darkness, e.g. leaves of river tamarind close at night. (p.102)
Thermonasty is a response to temperature change, e.g. tulips open when warm and close when cold. (p.102)
Thigmonasty is a response to vibration, e.g. the Venus flytrap clamps shut on an insect. (p.102)
5.2 Phytohormone
Phytohormones are chemicals that stimulate and coordinate plant responses at low concentrations; they are synthesised in one organ and transported to target organs through the phloem. (p.103)
Auxin: controls phototropism and geotropism, apical dominance, root and shoot elongation, adventitious roots, and fruit abscission inhibition. (p.103)
Gibberellin: stimulates stem elongation, development of leaves, flowers and fruits, and seed germination. (p.103)
Cytokinin: stimulates cell division and elongation when auxin is present, seed germination, and lateral bud growth; delays leaf senescence. (p.103)
Abscisic acid: inhibits growth, stimulates abscission, induces seed dormancy and stomatal closing during drought. (p.103)
Ethylene: stimulates fruit ripening, senescence and abscission of leaves and fruits. (p.103)
Coleoptile experiments show that auxin produced at the coleoptile tip stimulates cell elongation and that uneven auxin distribution causes bending. (p.104)
High auxin concentration stimulates cell elongation in shoots but inhibits it in roots. (p.104)
In phototropism, auxin moves to the shaded side, cells there elongate more, and the shoot bends towards light. (p.105)
In geotropism, auxin accumulates on the lower side: high auxin stimulates shoot cell elongation so shoots bend upwards, and inhibits root cell elongation so roots bend downwards. (p.106)
Uneven auxin distribution in tendrils allows them to coil around objects (thigmotropism). (p.106)
5.3 Application of Phytohormones in Agriculture
Auxin is used as a weed killer, to promote root growth on stem cuttings, and to produce seedless fruit by parthenocarpy. (p.107)
Gibberellin is used in tissue culture to promote cell division and differentiation, and to delay leaf senescence in cut flowers. (p.107)
Cytokinin is used in tissue culture together with auxin to form roots and stems. (p.107)
Abscisic acid is used commercially to ripen fruit quickly and evenly and to promote simultaneous flowering. (p.107)
Ethylene is used to ripen fruit; bananas on trees are wrapped in straw sacks to trap ethylene. (p.107)
Avoid plastic bags when ripening fruit because trapped moisture can speed spoilage. (p.108)