Plant responses to Internal and External Stimuli

Response  Stimulus Results in survival of species respond by changing their growth pattern Tropism – growth toward or away from a unidirectional stimulus, toward +, away - Phototropism - light Gravitropism - gravity Thigmotropism – touch Reception  transduction  response

Reception – Transduction - Response

Transduction – a multistep pathway that amplifies the signal Reception – cell signals are detected by receptors that undergo changes in shape in response to specific stimulus. Transduction – a multistep pathway that amplifies the signal Response – accomplished by 2 mechanisms: Turning genes on or off Activation existing enzyme molecules

Plant Hormones Chemical signals produced in very low concentrations for communication

Auxin affect many aspects of plant growth and development Indoleacetic acid(IAA) most common occurring Apical dominance – prevents lateral buds Promotes root growth if applied Promotes growth of fruit Induce development of fruit without pollination Weed control (Agent Orange) Gravitropism and phototropism moves to shady side, 2nd messengers lead to production of growth factors, elongation of stem on shady side, bends toward light

Ethylene Involved in abscission, stimulates enzymes which cause leaves, fruit or flowers drop Used to speed up ripening of fruit Applied to citrus to keep color Release gas at wound, speeds up ripening of fruit around fruit

Nastic movements Movements that do not involve growth, not dependent on direction of stimulus Seismonastic movements – touching, shaking, light or thermal stimulation Mimosa plant, prayer plant Changes in turgor pressure, K+ Circadian rhythm – biological rhythm with 24 hours cycle Biological clock – mechanism by which the biological rhythm is maintained

Photoperiodism Physiological response prompted by changes of length of day or night Germination – breaking of bud dormancy Can influence flowering 3 groups of flowering plants Short day – day length 14 hours or shorter Long day – 14 hours or longer Day neutral - not dependent on day length If dark period is interrupted, flowering will not result, not the same result if reversed

Photoperiodic control of flowering

Phytochrome blue/green leaf pigment, absorb red has 2 forms that absorbs red light at different wavelengths Phytochrome red – 660 nm Phytochrome far-red – 730 nm Allows a plant to detect photoperiod changes Other functions of phytochrome Promotes seed germination Inhibits stem elongation

Plant defenses Thorns Chemicals such as bitter or poisonous compounds Airborne attractants to recruit other animals to kill herbivores Epidermal layer – protects against viruses by sealing off the pathogen and then killing the cells in the area – brown spots…