by Ann Barnes, EMGV
Plants “keep their cool” by the evaporation of water that is released from leaves through stomata – small openings that allow carbon dioxide to enter the plant during photosynthesis. This process is called transpiration. As water evaporates, a negative pressure or tension is created in the leaf surfaces. Because of this tension, more water is pulled from the soil into roots, stems, and leaves of the plant. Along with water, nutrients from the soil are taken up and moved throughout the entire plant. In similar fashion, sucking liquid through a straw moves a refreshing drink from a parched gardener’s glass to mouth – negative pressures and the physical properties of liquids are at work in both places.
About 95% of water taken up by plants will be used in the process of transpiration. The remaining 5% is used in chemical reactions that take place in cells and is also held within cells. When plants have sufficient moisture, the water inside cells pushes on the plant’s cell walls like inflated inner tubes inside bicycle tires. The resulting pressure of water on cell walls, called turgor pressure, gives stems and leaves the structure they need to remain upright. When plants are unable to obtain enough water to replace what was lost to transpiration, turgor pressure becomes too low for the plants to be rigid, resulting in wilting – imagine a bicycle with a flat tire. During periods of extremely hot weather, a plant can lose water through transpiration faster than its roots can take water from the soil, which is why we see wilting on hot days even when we’ve had ample rainfall. If water is available in soil, plants will recover in the evenings when temperatures are cooler, but can wilt again during the heat of the day. If soil moisture remains low, plants can suffer lasting damage.