Plant Diversity and a little function Posted on web 3 18 07 at 5 30 pm Ecol 182 3 20 2007 Summary from last time We talked about Figure 29 10 The Evolution of Today s Plants Figure 35 9 Plant Cell Types Part 3 Why is a greater diameter a big deal for the evolution of plants Figure 35 10 Evolution of the Conducting Cells of Vascular Systems Figure 35 11 Sieve Tubes Angiosperms Flowering Plants Monocots a single embryonic cotyledon grasses cattails lilies orchids and palms Eudicots two cotyledons and include the majority of familiar seed plants Additional clades water lilies star anise and the magnoliid complex Big question in plant evolution what is the basal angiosperm Uptake and Movement of Water and Solutes Transport of Water and Minerals in the Xylem Transpiration and the Stomata Translocation of Substances in the Phloem General problem in plant function Need for H2O for photosynthesis Solute transport temperature control internal pressure for growth Plants obtain water and minerals from the soil via the roots in turn roots extract carbohydrates and other important materials from the leaves Water enters the plant through osmosis but the uptake of minerals requires transport proteins Uptake Movement of Water Solutes in Plants Water potential is the tendency of a solution to take up water from pure water Water potential of a system is the sum of the negative solute potential s and the usually positive pressure potential p s p Solute potential pressure potential and water potential are measured in megapascals Mpa Uptake Movement of Water Solutes in Plants Osmosis is the diffusion of water through a membrane primary means of water transport in plants Osmotic potential solute potential determines the direction of water movement across a membrane Potential refers to the potential energy potential to do work Dissolved solutes have the effect of lowering the concentration of water changing the potential energy Greater solute concentration results in a more negative solute potential and a greater the tendency of water to diffuse to the solution Figure 5 8 Osmosis Modifies the Shapes of Cells Figure 36 2 Water Potential Solute Potential and Pressure Potential Figure 36 4 Apoplast and Symplast Figure 36 5 Casparian Strips Transport of Water and Minerals in the Xylem The adhesion cohesion tension theory of water movement The concentration of water vapor is higher inside the leaf than outside so water diffuses out of the leaf through the stomata this is transpiration This creates a tension in the mesophyll that draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells The removal of water from the veins in turn establishes tension on the entire volume of water in the xylem so the column is drawn up from the roots Figure 36 8 The Transpiration Cohesion Tension Mechanism Transport of Water and Minerals in the Xylem Hydrogen bonding results in cohesion sticking of molecules to one another The narrower the tube the greater the tension the water column can stand Maintenance of the water column also occurs through adhesion of water molecules to the walls of the tube Transport of Water and Minerals in the Xylem The key elements in water transport in xylem Transpiration Tension Cohesion The transpiration cohesion tension mechanism does not require energy At each step water moves passively toward a region with a more negative water potential Transport of Water and Minerals in the Xylem Mineral ions in the xylem sap rise passively with the solution Transpiration also contributes to the plant s temperature regulation cooling plants in hot environments Figure 36 9 A Pressure Bomb Why is there a disconnect temporally between leaf root and soil Short and long term responses to water limitation When water is withheld the pressure potential of the cells declines hours to days and rates of cell expansion are reduced long term Rates of photosynthesis declines stomata close short New leaves are smaller with smaller cells long Profound change in patterns of allocation long Regulation of Transpiration by Stomata Leaf and stem epidermis has a waxy cuticle that is impermeable to water but also to CO2 Stomata or pores in the epidermis allow CO2 to enter by diffusion Guard cells control the opening and closing of the stomata Most plants open their stomata only when the light is intense enough to maintain photosynthesis Stomata also close if too much water is being lost Figure 36 11 Stomata Part 2 Stomatal aperture is regulated by controlling K concentrations in the guard cells Blue light activates a proton pump to actively pump protons out of the guard cells The proton gradient drives accumulation of K inside the cells Increasing K concentration makes the water potential of guard cells more negative and water enters by osmosis The guard cells respond by changing their shape and allowing a gap to form between them Abscisic acid a stress hormone can invoke stomatal closure in addition to changes in blue light Changes in guard cell photosynthesis can also invoke a stomatal response Leaf temperature PPFD CO2 demand VPD Transpiration Stomatal Conductance Hydraulic resistance Soil Conceptual understanding of stomatal function Optimization theory Cowan 1977 stomata work to optimize or maximize water exchanges for carbon dioxide Long distance transport hypothesis Tyree and Sperry stomata regulate water loss to maintain long distance water and nutrient transport Operate to avoid of catastrophic xylem dysfunction cavitation that occurs through the development of excessive tension Cavitation or Embolism Breakage of the xylem water column Entry of air into the conduit Primarily through the pit membrane Large tensions in the xylem stream Species and individuals differ in their vulnerability to cavitation trade offs produced relative to water flow rates This gymnosperm can continue to conduct water under extreme drought This angiosperm is slightly more drought tolerant than the streamside tree This angiosperm adapted to steamside conditions cannot conduct water in the presence of any drought Mechanisms of cavitation Desiccation induced vulnerability to cavitation is a function of air entry from pit membrane size and number of pits becomes the important traits Freeze thaw induced vulnerability occurs due to insoluble gases in sap that form bubbles under repeated low temperature conditions Differences in xylem diameter is important Ring Porous Trees Vessels confined to spring wood