V.C.E. Biology Unit 2

Plant Tropisms and Hormonal control

Hormonal Systems

Hormones are protein based chemicals produced by cells in one part of an organism that cause an effect in another part of the organism.

Plant hormones are less specific than animal hormones. Some hormones work together to produce an effect.

Hormones are slow to act as they need to travel via the phloem.

The effect of hormones are long lasting.

Environmental Cues

Plant hormones react to a number of environmental conditions:

- Light – plants respond to the amount of light, the quality of light and the length of day, influencing when they flower, set fruit and disperse seeds. Growing toward a source of light is called phototropism.

- Temperature – seed dormancy and bud dormancy are influenced by the temperature.

Environmental Cues

- Gravity – plant roots grow toward the source of gravity, the centre of the earth – this is called positive geotropism.

- Touch – plants that send out tendrils, such as climbing peas, respond to the touch of sticks or branches and grow toward the object seeking support. This is called thigmotropism.

Hormones

Auxins – Auxins are essential for cell growth, affecting both cell division and celluar expansion in shoots, roots and fruit. Auxins directly stimulate or inhibit the expression of specific genes (a signalling molecule).

Auxins are responsible for the growth of tips toward light. The auxin moves away from the direction of light, causing the cells on the ‘dark’ side to elongate under turgor pressure. It also promotes initiation of adventitious roots

Auxins act with or against other hormones: .

Hormones

Ratio of auxin to cytokinin determines initiation of root versus shoot growth

In large amounts will stimulate production of ethylene

If gibberellin is present then cell elongation is stronger.

If cytokinins are present then auxin will stimulate cell division

Auxins play a minor role in flower inititation.

Hormones

Gibberellins initiate cell growth and are therefore responsible for the regulation of growth of the plant, sex determination, dormancy, enzyme induction, germination of seeds, flowering and fruit enlargement.

Produced in fruit, seeds, growth buds, elongating stems

Hormones

Cytokinins – antagonise auxins on leaf buds, promoting cell division and differentiation and the growth of lateral branches. Involved in shoot and root growth, chloroplast maturation, cell enlargement, auxillary bud release and senescence.

Produced in roots and developing fruits.

Hormones

Abscisic acid – inhibits growth. It is responsible for seed dormancy, vernalisation and drought tolerance in plants. ABA produced in terminal buds in response to the onset of winter slows plant growth. ABA produced in roots in response to low soil water potential causes the closure of stomata.

Produced in Chloroplasts.

Hormones

Ethylene – is a gas produced by plants that causes fruit ripening. Interestingly it is produced by ripening fruit and accelerates the ripening of adjacent fruit making seed dispersal easier.