BIOL 2060 Lecture Notes - Lecture 7: Stoma, Cactus, Water Potential
1 Feb 2017
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High vpd = high evaporation in warm and/or dry environments, low humidity leads to a high vpd low vpd = low evaporation in cool and/or moist environments, high humidity leads to low vpd. But organisms can cope with dry environments by changing the permeability of the skin or epidermis. So the evaporative water loss is affected by vpd and the permeability of membranes i. e. evaporative water losses in tiger beetles. Water moves from high to low water potential. Water potential usually decreases from soil to plant to air. Water evaporates from higher (leaf) to lower (air) water potential. Because of hydrogen bonding, negative pressure pulls water reduction in leaf water potential creates negative pressure through xylem. Negative pressure at the root pulls water from the soil. Water balance: ws = wd + wf + wa - we - ws. Example: stomatal closure by desert cacti tradeoff? limited gas exchange. Example: thickened cacti stems with lower surface area/volume (sa:v) ratio.
