   Effects of this Summer's Drought - August 28, 2002 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Our current drought has affected many plants in both ornamental situations and natural stands. Early this spring, the scrub oak so abundant on the foothills, dropped their leaves. These plants have recovered better than other native shrubs. Manzanita and Wright's silktassel seem to have suffered some mortality. This can be seen in the reddish and yellowish hues these plants have taken on. Some of these drought-affected plants will likely resprout, but I fear that there has been some mortality. In urban landscapes, many plants that have received only intermittent irrigation have also died. Most evident are the pines and cypress trees. While the plant's actual death is often blamed on an insect or disease, drought is likely the precursor. According to NOAA's Climate Prediction Center, to forecast is grim with drought conditions continuing through November 2002. If you would like the bad news first-hand, the Climate Prediction Center's web site has maps and forecasts that can be viewed at www.cpc.noaa.gov. How does drought actual impact a plant at the physiological level? At the molecular level, all plant processes are driven by chemical reactions: compounds being changed and modified to create and store energy, transfer them from one point to another within the plant, and create structural components (cells) for growth. All chemical reactions that take place in a plant occur in a solution composed mainly of water. Photosynthesis depends upon water. Similarly, nutrient transport is reliant upon water. Soil nutrients are brought to the root in a solution of water; they enter the root dissolved in water and are carried through the plant in sap composed largely of water. Plant growth depends upon water. In fact, it is water pressure on the inside of cells which causes cell walls to stretch, and cells to grow. As each of the many thousands of cells forming a plant enlarges slightly, we see the effect as overall plant growth. A plant that does not have enough water pressing against its cell walls appears wilted. In this case, water acts in much the same way as air in a balloon. With abundant air, the walls of the balloon are stretched. When air is reduced, the walls become limp. Water therefore not only enables plants to grow, it allows them to stand upright. Water enters a plant through the roots. However, 99% of this water exits as water vapor through the leaves. This process is called transpiration. During transpiration, water vapor passes out of leaves through specialized opening called stomates. On a hot, dry, windy day, water loss to transpiration can be quite high, requiring relatively large amounts of soil moisture to prevent wilting. Between the roots and leaves of a plant, water is drawn through many thousands of cells called xylem vessels. Xylem cells can be thought of as soda straws stacked end to end to form long narrow pipelines running from the roots to the leaves. Because of water's strong tendency to hold together in drops, it takes less energy to pull the entire column of water upward than it would take to break the column. The chain of water molecules therefore remains intact and continues to pull slowly upward. This is how transpiration pulls water to the tops of plants. In order to get water to the leaves, the plant must lose water through evaporation. This evaporation also cools the plant like a swamp cooler. The remaining 1% that is retained by the plant is used for photosynthesis, and the myriad of other metabolic processes. If water is unavailable and/or environmental conditions become unfavorable, the column of water can be broken. This will effectively stop the upward movement of water within a plant. The term used for this disruption in the continuity of water molecules is called cavitation. I am aware of no scientific research that can prove that cavitation has occurred in our drought-killed plants, but it is a plausible explanation. The University of Arizona Cooperative Extension has publications and information on drought adapted trees and shrubs. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |