Why We Water: Maintaining Plant Turgidity and Nutritional Sufficiency
Understanding the factors that determine water uptake and movement within plants can serve as a guide for determining when and how much water is needed to ensure sufficient amounts are available at all times in your garden. Dr. J. Benton Jones, Jr. shares a few tips on what to keep in mind when it comes to watering.
Maintaining water turgidity in plants is essential for maintaining their nutritional status and growth potential. The water content of herbaceous plants is between 90 and 95%, and it is this water content that maintains the plant’s physical shape.
As water content declines from full turgidity, the plant will begin to change its physical shape, commonly known as wilting. With wilting can also come a nutrient element deficiency because it is the transpiration stream that carries essential plant nutrient element ions drawn from the soil water solution into the plant’s roots, and from there they move up through all of the other parts of the plant.
One of the best illustrations of this function is the occurrence of blossom-end rot in tomatoes and peppers. The elemental deficiency is calcium, but the triggering parameter can be moisture stress, which reduces the amount of calcium carried into the developing fruit. Other major elemental deficiencies, such as nitrogen and magnesium, are not uncommon occurrences when plants are under periodic moisture stress, particularly when this stress occurs during periods of rapid plant growth.
High humidity in the greenhouse, growroom or growth chamber can also result in nutrient element inefficiencies as water movement into and up the plant is impaired. If water turgidity is not maintained, even when visual wilting symptoms do not appear, plant growth can be adversely affected.
Plants do not grow well in stagnant air, partially due to impaired element ion movement as water loss from the surface of leaves slows the transpiration stream. Wind or generated air movement over plant leaf surfaces strips away trapped air on the leaf surface, stimulating transpiration as well as bringing carbon dioxide into contact with plant leaf surfaces. However, hot, dry air moving across a leaf surface can result in leaf wilting.
Water moves up the plant in the xylem vessels, drawn up as water transpires from the leaf surfaces, mainly through open stomata. The characteristics of the leaf surface will determine how easily water is transpired. The extent of water loss will be determined by the atmospheric conditions surrounding the plant, such as air temperature, relative humidity and wind movement over the leaf surfaces. The vapor pressure deficient is a calculable parameter that can be used to determine what will be the potential water loss based on the measurement of various atmospheric factors.
The ability and quantity of water absorbed by plant roots depends on a number of factors, both external and internal. The external factors are aeration, temperature and ion content of the surrounding water, the inhibiting factor being high salt content, levels of which can be determined by an electrical conductivity (EC) measurement.
The effect of temperature on water availability can result in plant wilting during high temperatures, atmospheric demand following a rain shower or after applied irrigation water. Even though rain showers cool the atmosphere, reducing the atmospheric demand, the cool water entering the soil reduces the ability of the roots to absorb that cool water, and the plant wilts—the extent of which depends on both the plant and the atmospheric conditions.
In addition, a heavy rainfall or an irrigation application can create a temporary anaerobic condition that impairs root function, and therefore water absorption that is followed by plant wilting.