This summer is shaping up to be one of the most stressful to Missouri crops in recent years. The combination of low precipitation and high atmospheric demand for water makes 2012 feel much like 1988. One of the driving forces for increased atmospheric demand is air temperatures much higher than normal. These high temperatures often have direct effects plants on plants, as well as aggravating water stress.
It is important to remember that plants react to temperature differently than humans. Humans must evaporate water to dissipate heat. High humidity reduces evaporation and greatly affects the way a particular temperature feels. Thus, weather stations report heat indices that are an attempt to estimate how air temperature “feels” to humans. High humidity translates into heat indices that are often five or more degrees above air temperature. Heat indices have little relationship to the direct effects of temperature on plants.
Sometimes leaf temperature is more important to plants then air temperature. Leaves function as solar collectors, that is, they are designed to absorb light energy. They do this in order to build sugars and produce other products necessary for life (and yield). However, very little of the light energy is actually used to do this work (photosynthesis). Light energy not used for photosynthesis causes leaf temperature to rise.
Plants dissipate heat through water evaporation from cell surfaces, convection, and conduction. Changing liquid water to water vapor requires substantial energy and this energy loss causes the cooling effect. Conduction means that the warm leaf surface gives energy to the air touching the leaf if the air temperature is less than the leaf temperature. Convection means that cooler air is moved closer to the leaf surface and displaces warmer air. These three methods of heat dissipation are very much interrelated, and without them the leaf temperature would quickly rise to the point where plants could not survive.
During the day it is not uncommon for leaf temperature to be higher than air temperature, especially on bright sunny days with little wind. With good moisture supply, evaporation will be fast enough to keep leaf temperatures fairly close to air temperature. With limited moisture, however, water evaporation may not be fast enough to cool the leaf.
Our crop plants have several mechanisms to reduce the amount of sunlight energy hitting their surfaces. Leaves of grass plants, such as corn, roll into a cylinder. This reduces leaf effective surface area and tilts leaves upward. Broad-leafed plants, such as soybean, move their leaves so that they are parallel with the incoming sunlight. Sometimes they will flip leaves so that their lighter colored bottom surfaces face upward. Although we see these responses during water stress, it is actually an attempt to reduce sunlight absorption, and thus, leaf temperature. Of course, the reason to reduce leaf temperature is to reduce water evaporation so the two factors are interrelated.