Plant Tropism and Defenses

1. Plant Tropism and Defenses • Lights important affect on plants • Photosynthesis… conversion of CO2 & H2O to energy containing carbon compounds and O2 • Phototropism… movement of plants toward light • Photomorphogenesis… the induction of important key development and growth stages due to the presence, intensity, and duration of light

2. Light Frequencies • Action spectrum… the wavelengths useful to the plant • Red and blue light for most plants • blue-light photoreceptors • Active in phototropism & slowing hypocotyl elongation when seedlings break ground • Phytochromes • Activate de-etiolationprocess • Signals germination in many seed types depending on type of red light present (near vs far) • Allows plants to alter metabolism at different times of the day (shade avoidance)

3. Circadian Clock • Circadian clocks (rhythms) or biological cycles that are independent of extraneous environmental signals • Are internal to all Eukaryotic organsims • Can be reset by outside stimulus… light (PrDPfr) • Duration of night in some flowering plants • May be due to transcription factors that inhibit further production through allosteric inhibition. • Causes an increase in the morning and a fall in the evening • Clock will operate in the absence of day night although timing may be off

4. Response to External Stimuli • Gravitropism… causes stem to travel up and root to travel down • Orchestrated through statoliths (heavy starch grains) that settle in the bottom of cells • Causes a concentration of Ca2+ and auxin increasing growth on the upper portion resulting in bending of root • Thigmomorphogenisis (Mechanical stimuli)… • causes tree trunks to thicken and growth to be stunted • Plant leaves to curl when touched… action potentials resembling nerve impulses • Vines to grasp… thigmotropism

5. Environmental Stresses • Drought… closes stomata and inhibits shallow root growth (deep root growth continues) • Flooding… fills pore spaces and drowns roots • Oxygen in the air spaces of soil is needed to support cellular respiration by root systems • Salt stress… reduces water potential of soil (becomes hypertonic to roots) dehydrates plant and reduces the plants ability to bring water to the stems and leaves • Some plants can produce solutes to counteract increases in salinity • Heat stress… denatures needed enzymes • Plants common to hot regions produce heat-shock proteins (chaperonins) to reduce the effect of the heat • Cold stress… affects solute transport as membranes become rigid • Production of unsaturated lipids increases (like cholesterol) to reduce crystallization and keep the membrane fluid

6. Plant Defenses • Herbivory • Thorns • Toxins • kill of disable insect or animal • Attract predators that feed on possible eaters • Released to start production of toxins in neighboring plants