Plant Hormones and External Factors

Roles of Auxin

Cell elongation, apical dominance, lateral root formation, tropisms

Roles of Cytokinin

Regulate cell division, modify apical dominance (inhibits auxin), delay leaf aging, used in tissue culture, cell differentiation

Roles of Ethylene

Promote leaf/fruit abscission, cause fruit ripening

Roles of Abscisic Acid

prevents premature seed germination, induces stomatal closing

Roles of Gibberellins

stimulates cell elongation+division, hastens seed germination

Apical Dominance

phenomenon in which the closer a bud is to the stem's tip, the more its growth is inhibited; controlled by auxin

Roles of Brassinosteroids

steroid hormones in plants that have a variety of effects, including cell elongation, retarding leaf abscission, and promoting xylem differentiation.

Tropism

A growth response of a plant toward or away from a stimulus

Gravitropism (roots)

Growth towards (positive) gravity; amyloplasts sense gravity at bottom of cell, auxin inhibits elongation on lower side of cell

Gravitropism (shoots)

Growth away (negative) from gravity, auxin promotes elongation on lower side

Hydrotropism

a plants growth response to water; root grows towards the water

Thigmotropism

A growth response to touch

Nastic movement

Response of plant to stimulus; its direction of growth is independent of stimulus; can be due to turgor pressure

Sleep mvmts = leaves droop at night (turgor pressure)

Heliotropism

tropism + nastic movement; tracking of sun via turgor pressure

Photoperiodism

Flowering response due to changes in day length

Phytochrome

responsible for photoperiodism

Pr= inactive, absorbs red light, converted to Pfr

Pfr= active, absorbs far red light, converted to Pr

Pr=Pfr during day, flashes of far red light at night will reverse flowering behavior

circadian rhythm

24hr clock reset by phytochrome system, helps plant tell what time of day