RESPONDING TO THE ENVIRONMENT (PLANTS)

• stimulate growth • cause apical dominance • cause tropism in stems and roots • stimulate the development of adventitious roots in stem cuttings • stimulate the development of fruit • control the abscission of leaves and roots

• stem elongation • root growth • the germination of seeds • promotes lowering

• is an inhibitor of growth • causes plants to become dormant in winter • abscission in leaves and fruit • lack of water stimulates the production of abscisic acid • causes the closing of stomata when the plant wilts

PLANT UPRIGHT POSITION: * auxin produced at the tip of the root * this even distribution brings about equal growth on all sides of the root * root grows downward and the shoot grows upwards PLANT HORIZONTAL POSITION: * the auxin concentration will be high on the underside of the root * more growth occurs on the upper side of the root because auxins on the lower side inhibit growth * root bends downwards and the shoot bends upwards

multi-directional light (from all sides): * auxins move downward evenly * this even distribution brings about equal growth on all sides of the stem * as a result, the stem grows upward unilateral light (light from one side only): * the auxin concentration will be high on the dark side – light destroys auxins * more growth occurs on the dark side because auxins stimulate growth on the dark side * stem bends upwards towards the light

• some synthetic herbicides contain high concentrations of auxins that accelerate the metabolism of broad-leaved dicotyledonous weeds and therefore stimulate growth • these weeds grow so fast that their water absorption and food production are insufficient • the plants weaken and die • higher concentration of auxins are needed to kill monocotyledons

MECHANICAL: * thorns discourage herbivores from feeding on plants * presence of thorns slows the feeding rate of herbivores and wear down their molars over time CHEMICAL: * acts as repellents or influence the digestibility of plant material