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-etiolation process – 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 (P r  P fr ) 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 Ca 2+ 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