Plant responses to the Environment Life Processes in Plants J Gerber and J Goliath 1

Plant hormones ◊General functions of the following plant hormones: Auxins Gibberellins Abscisic acid Investigate geotropism and phototropism by controlling variables ◊Role of auxins in geotropism through the regulation of differential growth ◊Role of auxins in phototropism through the regulation of differential growth ◊Uses of plant hormones in agriculture: Auxins as herbicides Auxins to control apical dominance Auxins and gibberellins to increase/stimulate fruit development J Gerber and J Goliath 2

Plant defense mechanisms ◊ The role of the following as plant defense mechanisms: Chemicals (protection) against infection from pathogenic micro- organisms and from being eaten by herbivores) Thorns (protection from being eaten by animals/to reduce water loss J Gerber and J Goliath 3

Activity 1- Tropism Instructions Explanation of activity on phototropism and geotropism. In pairs/groups discuss the information and answer the questions that follow. After 15 minutes each pair/group give feedback. J Gerber and J Goliath 4

Plant hormones Performs a specific function, which controls further growth and development of the plant. Stimulation – a process is turned on or activated. Inhibition – a process is turned off or stopped. J Gerber and J Goliath 5

Auxins Group of plant hormones that are mainly produced in the apical meristem of the stems and roots. Indole acetic acid is the most common natural auxin. Auxins move down in the phloem to cells lower down in the plant. Small amounts of auxins are produced in the growing points of roots. J Gerber and J Goliath 6

Functions of auxins Stimulate cell division. Stimulate cell elongation. Promotes apical dominance (agriculture). Develops adventitious roots. Cause tropic movement in stems and roots. Stimulates the development of fruit. J Gerber and J Goliath 7

Functions of gibberellins Stimulate stem elongation. Stimulate root growth. Promote flowering. Promote the growth of lateral buds. Stimulate the germination of seeds. J Gerber and J Goliath 8

Functions of abscisic acid Inhibits cell division and growth in stems and roots. Prevents bud and seed growth in winter. Promotes the abscission of leaves and frui.t Controls the stomata opening and closing. Speeds up fruit ripening. J Gerber and J Goliath 9

Role of auxins in phototropism Phototropism is the growth movement of a plant or part of a plant in response to unilateral light stimulus. A high auxin concentration builds up on the side of the stem furthest of the light source. Cell division and elongation are stimulated and grow faster. The stem curves in the direction of the light and positively phototropic. Negative phototropism occurs in roots – grow away from a light source. J Gerber and J Goliath 10

Role of auxins in geotropism Young seedling is placed horizontally – roots grow downwards and stem upwards. Auxins accumulate on the lower side of the root. Higher auxin concentration on the lower side of the root inhibits cell division and elongation. Cells on the upper side of the root grow faster – lower auxin concentration. Root curves downwards – positively geotropism. Stem curves upwards – negatively geotropism. J Gerber and J Goliath 11

Uses of plant hormones in agriculture Root formation in cuttings. Production of seedless fruit. Ripening of fruit. Increase in fruit size. Stimulation of germination of seeds. Weed control. Apical dominance. J Gerber and J Goliath 12

Auxins as herbicides Synthetic herbicides (weed killers) contain high concentrations of auxins. Accelerate the metabolism of broad-leaved dicotyledonous weeds and stimulate growth. Weeds grow so fast that water absorption is insufficient. Plants weaken and die. J Gerber and J Goliath 13

Apical dominance Apical dominance occurs when the apical bud of the main shoot grows more strongly than lateral bud below. Removing the apical bud from the top of the plant, auxin is also removed. More lateral grow below the apical bud. Pruning promotes the grow of lateral branches. The value of pruning – plant grows bushier and fruit formation can be controlled. J Gerber and J Goliath 14

Plant defense mechanisms- thorns Sharp thorns protect the plants against herbivores. Less damage to plants by herbivores and plants continue to grow. Reduce water loss. J Gerber and J Goliath 15

Plant defense mechanisms- chemicals Many plants produce chemicals and play a role in the defence against herbivores and pathogenic- organisms. Chemicals are poisonous or influence the digestibility of plant material. J Gerber and J Goliath 16