P lant Repsonses & H ormones Nancy G. Morris Volunteer State Community College Campbell, 5 th edition, Chapter 39

P lant H ormones Hormone Hormone = Gr. “to excite” 1) active in small amounts 2) produced in one part of plant & transported to another for action 3) action is specific for that site

E ach has a M ultiplicity of E ffects Depending on site of action Developmental stage of plant Concentration of hormone

A uxins stimulate growth but too much inhibits growth functions: 1) root initiation, stem elongation 2) retard abscission (loss) of leaves & fruits (Figure 39.10) 3) stimulates cell differentiation 4) apical dominance (Figure 39.6)

Apical dominance

G ibberellins discovered due to a fungus Giberella which causes Japanese “foolish” rice seedling disease Figure 39.7 induces flowering stimulates growth by increasing cell size & numbers (Figure 39.8)

“Foolish seedling disease” in rice

39.11 Effect of Gibberellin

Dwarf pea plant treated with gibberellin

C ytokinins 1) induces cell division (cytokinesis) 2) affects root growth & differentiation 3) stimulates germination 4) delays senescence (aging); the progression of irreversible change that eventually leads to death

A bscisic A cid 1) stimulates abscission 2)converts vegetative buds (active) to dormant buds 3) inhibits growth

39.16 Abscission

E thylene 1) promotes fruit ripening cellulase 2) stimulates production of cellulase

Photochromes Plant pigment Important in processes where light is a critical factor, such as, flowering in long day short night plants Figure 39.16

Plant Movements due to growth Phototrophism – movement toward (positive) or movement away from (negative) light Geotrophism – movement toward (positive) or away from (negative) center of the earth Thigmotrophism Thigmotrophism – directional growth responding to contact (tendrils)

Photoperiodic control of Flowering

Plant Movements not due to Growth Thigmonastic response – touch stimulus, e.g. mimosa leaves Photonastic response – shamrock Sleep response – caused by change in osmotic pressure

Sleep movements of a bean plant:

Movement of materials Path from roots: 1) epidermis of root & root hair cells 2) roots absorb water, minerals, gases 3) stems conduct Xylem & phloem are vertical conductors Parenchyma of cortex, pith, rays are lateral conductors

Movement Root pressure –osmotic pressure & pressure created by active absorption & secretion by root cells; drives materials up xylem Transpiration – evaporation of water through leaf stomata; cohesion of water molecules exerts a pull on columns of water in xylem Guttation – release of water droplets at leaf tips; occurs when too much water is absorbed by plant & when humidity is high Translocation – movement of material from one location to another

Guttation

Fighting Back…. Response to herbivory (animals eating plants): 1) physical barriers – thorns & spines 2) chemical defenses – distasteful or toxic compounds 3) chemical signaling – salivary enzyme from herbivore triggers a pathway that produces a volatile attractant that recruits a parasitoid wasp. The wasp lays eggs within the caterpillar, and the larvae upon hatching destroy the caterpillar by consuming its tissues. (Figure 39.22)