Plant Repsonses & HormonesNancy G. MorrisVolunteer State Community CollegeCampbell, 5th edition, Chapter 39

Plant HormonesHormone = Gr. to excite1) active in small amounts2) produced in one part of plant & transported to another for action3) action is specific for that site

Each has a Multiplicity of EffectsDepending on site of actionDevelopmental stage of plantConcentration of hormone

Auxinsstimulate growth but too much inhibits growthfunctions: 1) root initiation, stem elongation2) retard abscission (loss) of leaves & fruits (Figure 39.10)3) stimulates cell differentiation4) apical dominance (Figure 39.6)

Apical dominance

Gibberellinsdiscovered due to a fungus Giberella which causes Japanese foolish rice seedling diseaseFigure 39.7induces floweringstimulates 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

Cytokinins1) induces cell division (cytokinesis)2) affects root growth & differentiation3) stimulates germination4) delays senescence (aging); the progression of irreversible change that eventually leads to death

Abscisic Acid1) stimulates abscission2)converts vegetative buds (active) to dormant buds3) inhibits growth

39.16 Abscission

Ethylene1) promotes fruit ripening2) stimulates production of cellulase

PhotochromesPlant pigmentImportant in processes where light is a critical factor, such as, flowering in long day short night plantsFigure 39.16

Plant Movements due to growthPhototrophism movement toward (positive) or movement away from (negative) lightGeotrophism movement toward (positive) or away from (negative) center of the earthThigmotrophism directional growth responding to contact (tendrils)

Photoperiodic control of Flowering

Plant Movements not due to GrowthThigmonastic response touch stimulus, e.g. mimosa leavesPhotonastic response shamrockSleep response caused by change in osmotic pressure

Sleep movements of a bean plant:

Movement of materialsPath from roots:1) epidermis of root & root hair cells2) roots absorb water, minerals, gases3) stems conductXylem & phloem are vertical conductorsParenchyma of cortex, pith, rays are lateral conductors

Casparian Strip (Figure 36.6)

MovementRoot pressure osmotic pressure & pressure created by active absorption & secretion by root cells; drives materials up xylemTranspiration evaporation of water through leaf stomata; cohesion of water molecules exerts a pull on columns of water in xylemGuttation release of water droplets at leaf tips; occurs when too much water is absorbed by plant & when humidity is highTranslocation movement of material from one location to another

Guttation

Fighting Back.Response to herbivory (animals eating plants):1) physical barriers thorns & spines2) chemical defenses distasteful or toxic compounds3) 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)