Download
1 / 26
260 likes | 426 Views
This guide explores the function and effects of plant hormones, detailing their active roles in various processes. Major hormones discussed include auxins, gibberellins, cytokinins, abscisic acid, and ethylene. The document explains how these hormones influence growth, flowering, and stress responses in plants, showcasing mechanisms like apical dominance and phototropism. Additionally, it highlights plant movements, nutrient transport, and defensive strategies against herbivory. Understanding these complexities is crucial for the study of plant biology and horticulture.
E N D
Plant Repsonses & Hormones Nancy G. Morris Volunteer State Community College Campbell, 5th edition, Chapter 39
Plant Hormones • 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
Each has a Multiplicity of Effects • Depending on site of action • Developmental stage of plant • Concentration of hormone
Auxins • 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)
Gibberellins • 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)
Cytokinins • 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
Abscisic Acid • 1) stimulates abscission • 2) converts vegetative buds (active) to dormant buds • 3) inhibits growth
Ethylene • 1) promotes fruit ripening • 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 – directional growth responding to contact (tendrils)
Plant Movements not due to Growth • Thigmonastic response – touch stimulus, e.g. mimosa leaves • Photonastic response – shamrock • Sleepresponse – caused by change in osmotic pressure
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
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)
