Lecture 9 Notes
Plant Water Relations
I. Theories of water movement in plants
A. How does water get up tall plants? Here are several potential theories. Only one is correct. 1. Pushed - the root pressure hypothesis 2. Capillary Action - it wicks up the plant 3. Vapor Movement - it vaporizes up the xylem 4. Cohesion-Tension Theory of Sap Ascent - plants "pull" water up B. Root Pressure - what is it and can it push water to the tops of tall plants? 1. Remember the structure of a root? See Lecture 8. The endodermis has that suberized layer (Casparian Strip). Water and the dissolved nutrients contained therein must pass through the living cells in the endodermis to get to the stele. At night, when there is no transpiration, the membranes on these cells are still transporting nutrients, such as K+, and NO3- into the stele. These act as osmotica, and can cause osmosis to begin. This results in water entering the stele, even at night. Since the plant is not transpiring, the pressure builds up in the root. The pressure that builds up is known as root pressure. If you decapitate a plant, then the root pressure will force water out of the cut stem. Stephen Hales did this experiment with grape back in the 1700's, and showed that there was enough pressure to push water to a height of 27'. It would have gone even higher had the "tube not leaked". It takes a pressure of about 1.5 psi to push water up about 3'. So this means that the plant generated a root pressure of about 13.5 psi, or just below one atmosphere of pressure. Some plants can be 100 m tall or more (say 300' or more). That would require a pressure of 150 psi, or 10 or more bars (1 bar is 14.7 psi). But no has ever measured much more than 5-6 bars (75-90 psi) root pressure. In addition, root pressure is too slow to provide trees with the amount of water that we know they transpire. SO - root pressure can't be the way that water gets to the tops of tall trees. 2. Capillary Action - ever dip a corner of a napkin in water and then watched how the water wicks its way up the napkin? This is capillary action at work. Water is strongly attracted to other objects because of charges on the water. This is called adhesion. Since the walls of xylem are cellulose, it is plausible to suggest that perhaps water wicks its way up trees. However, some physics and math quickly show that (1) it's too slow, and (2) gravity would stop the upward movement after only about 1 meter (3') of rise. SO - capillary action can not explain how trees get water to their tops. 3. Vapor Diffusion - what if the xylem in trees didn't contain liquid water, but instead, was composed of vapor? Then, maybe the vapor could diffuse to the tops of tall trees. But anatomical studies show that the xylem is indeed filled with liquid water, and besides, diffusion is WAY TOO SLOW. SO - vapor diffusion is not the answer. 4. Cohesion-Tension Theory - Well, that leaves just this theory. How does this theory work? We know that in an ordinary pipe, like that attached to a well pump, you can only "pull" water to a height of 33' (10 m) with a vacuum. After that, gravity pulls on the water column and it breaks under it's own weight. So if this theory is to work, we have to explain how a tree can violate this property. First off, trees don't have large diameter tubes inside. Instead, they have tracheids and vessel elements. The diameters of these cells range from 20 um to nearly 500 um, depending on species. Studies with water in capillary tubes show that water in small diameter tubes can withstand tensions of up to 300 bars (or 4500 psi tension!!). Tension is simply negative pressure. They found this out by putting water in small diameter tubes, bent at the ends, and then spun in a centrifuge. By knowing how fast the centrifuge spun, and lengths of the tubes, scientists could calculate the tension on the water inside. So, water in small diameter tubes does not break. When it does, that is called a cavitation event, and the result is an embolism. When water transpires from the leaf, water molecules leave the cell walls of the cells below the stomata. They are replaced by other nearby water molecules through the process known as cohesion. Cohesion is the tendency of like molecules to attract each other. Because of charges, water is attracted to other water molecules. When a water molecule replaces one that is lost via transpiration, another takes its place. Then, another water replaces that one, and so on and son on down the line all the way to the root. This causes the entire water column in the plant from root to leaf to move up. All by cohesion. Since water can evaporate faster than it can be taken up, tension begins to build up in the xylem. Eventually, the plant is under quite severe tension. Studies show that these tensions "pull" water from the soil into the root, up the stem, and then to the leaf, where it evaporates. Henry Dixon and J. Joly first offered up this theory at the turn of the century, and it is still the most widely accepted theory for the ascent of sap in plants. What is needed for this theory to be viable? 1. negative pressures in the xylem - this has indeed been found. Trees shrink during the day when transpiring due to the tensions in the xylem. Further, if you cut into a tree, no water comes squirting out. In fact, if you put a drop of water over a cut, it gets sucked in! 2. water must be able to tolerate great tensions in the xylem - verified as noted above, and also recently by spinning not glass tubes in a centrifuge, but stems themselves, and finding no evidence that the water columns broke. 3. Tensions that develop must be of sufficient magnitude to move water fast enough to account for the transpiration rates that are measured. This has indeed been found. So, in conclusion - the cohesion-tension theory of sap ascent is the most widely accepted theory. 4. Some caveats - it is a passive process - that is, the plant need not expend any energy to bring water up the stem (makes sense since the xylem cells are dead at maturity). 5. There are trade-offs. Let's discuss these now. C. Cavitation and Embolisms - what happens when tensions run too high! 1. Remember, cavitation is when the water column breaks due to too high tension. 2. The air bubble left in the xylem after cavitation is called an embolism. a. when a xylem conduit cavitates, the tension is relieved, and water no longer moves up the plant in that tube. Thus cavitation is bad because it takes active xylem out of commission. 3. What causes cavitation? a. freezing i. when you freeze water, bubbles form (see your ice cubes). If this happens in a xylem cell, it can result in an embolism. 1. larger diameter cells are more prone to freezing-induced embolisms 2. easier for bubbles to form in large conduit xylem cells. This is one reason vines are a tropical phenomenon - they have large conduits and are very prone to freezing-induced embolisms. b. drought i. drought increases tension in the xylem. ii. when tension increases to a certain point, air can be sucked into the xylem from adjoining spaces, causing an embolism. iii. air sucked through pits. Pits with large pore diameters allow air to enter easier, so embolisms form more readily. Not related so much to cell diameter, like freezing-induced embolisms are. 4. Trade-Offs a. plants with large diameter xylem cells can move water very easily, but are prone to freezing-induced embolisms. May explain absence of vines from northern habitats, since vines have the largest xylem cell diameters. b. plants with small pit pore diameters more resistant to drought than those with big pores. But small pits inhibits the flow of water. So, plants with high resistance to drought may also have low capacity for water flow because of small pit pore diameters. c. remember, flow in a tube is proportional to the 4th power of the radius: Flow = r4
This means that cutting the diameter in half reduces flow by 16 times!
II. Stomatal
Physiology
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