5.16.1) Plant hormones and growth in plants

Key information required for this part of the Plant responses topic for OCR A Level Biology

What are plant hormones

Chemical messengers that are produced in one part of the plant and act in another to give a widespread and long lasting effect

What is the process for a seed to germinate once in soil

Absorption of water ruptures seed coat, activating the embryo to make gibberellins. This stimulates production of enzymes which break down food stores (endosperm in monocots, cotyledons in dicots) by hydrolysing starch into maltose and then glucose. This produces ATP for growth.

What are the two pieces of experimental evidence used to support the role of gibberellins in germination

Breeding mutant variants of seeds without genes to make gibberellins results in no germination. Adding gibberellins externally stimulates germination; adding gibberellin biosynthesis inhibitors prevents germination. Removal of these or adding gibberellins stimulates germination

What do auxins do, where are they produced and how do they move around the plant

Stimulate growth; cells in tip of stem, roots and in meristem; through transport tissue and between cells

What is the process for auxin causing cell elongation

Auxin binds to cell receptor in plasma membrane, causing H+ ions to move from cytoplasm through plasma membrane to cell wall, making the pH fall to around 5 in cell wall. This is the optimal pH for enzymes to break down cellulose, making it flexible and stretchy. Water moves into cells, causing them to swell and elongate. Maturation of cells causes auxin to be destroyed.

What is the result while auxin is at low concentration, medium concentration, high concentration and very high concentration

Root growth is stimulated; root growth is inhibited, lateral growth is stimulated; root and lateral growth are inhibited, apical growth is stimulated; root, lateral and apical growth are inhibited

What is apical dominance and what is the experimental evidence for it being caused by auxin

Growth in the main shoot is stimulated while lateral shoot growth is inhibited, particularly further up the plant. This results in cone-shaped growth; removal of apical shoot stimulates lateral shoot growth, and reintroducing auxin artificially reinserts apical dominance

How does concentration of gibberellins affect plant growth

Low concentrations result in the plant having short stems, whereas high concentrations result in the plant becoming tall and thin

What is synergism and antagonism in plant hormones

Hormones work together to produce a greater response; hormones have opposite, so the concentration of each determines the effect and strength of the effect