A. Andersen
One of the problems faced by all organisms is to maintain a balance between the sources and sinks of heat around them. The study of energy or heat transfer has become an important discipline, and there seems to be growing enthusiasm for the subject. Physical laws have been applied and predictions have been made relating to convection, transpiration, and radiation - the three principal means whereby energy is received and disposed of by plants. The thermal radiation or reradiation depends on the plant surface temperature and we have very little influence on the rate of reradiation. The energy exchange by convection is only a very limited part of the total heat transfer and depends mostly on windspeed and leaf size. Transpiration is the most important factor, because to a certain extent we can control it.

Transpiration is very important to a plant for keeping the temperature of the fully sunlit leaves below the lethal value. The importance of transpiration as a factor for regulating the temperature has been a topic of much discussion and disagreement among plant physiologists and other scientists. Many older research workers considered transpiration as an evil which could not be avoided. Barnes wrote in 1902: "As plants developed on land they had to contend with this necessary evil". The points of view have changed since then.

Transpiration cools the leaf and reduces the effective radiation load by some amount. For each 1.7 x 10-4g cm-2min-1 of transpiration at 20°C, the effective radiation load on the leaf is reduced by 0.10 cal cm-2 min-1. This amount of transpiration together with a wind velocity of 10–20 cm sec-1 is sufficient to drop the leaf temperature about 5°C below the temperature which would occur in the absence of transpiration.

Transpiration is a physical process which within the boundaries of a normal climate follows the physical laws. Only under extreme conditions it deviates from this and becomes influenced by physiological conditions (Mellor et al, 1964). The actual rate of transpiration will therefore, at potential water supply, be determined by the rate of solar radiation and the vapour pressure gradient between leaf and surrounding air. In the intercellular space of a plant, when in equilibrium with the surrounding cells, there will be a relative humidity close to 100%. The vapour pressure will therefore solely be determined by the leaf temperature which often differs considerably from the temperature of the surrounding air. The vapour pressure in the surrounding air varies extremely and depends on the total content of vapour and the temperature. The vapour pressure gradient or vapour pressure difference can be expressed by:

Andersen, A. (1969). TRANSPIRATION AND WATERING PROBLEMS. Acta Hortic. 15, 27-33
DOI: 10.17660/ActaHortic.1969.15.6