THE RELATION BETWEEN RATE OF TRANSPIRATION AND RATE OF ABSORPTION OF WATER IN PLANTS

Abstract

THE MOST FIUNDAMENTAL phase of plant water relations in respect to effect on growth is the internal water balance or degree of saturation of the tissues. This is determined by the relative rates of absorption and transpiration. The ratio of absorption to transpiration is therefore more important than the absolute rate of either process. Rapid transpiration may not be harmful in itself if accompanied by sufficiently rapid absorption so that no long-continued saturation deficit or loss of turgidity results. On the other hand, very moderate transpiration may be injurious if the rate of absorption is very slow, as in frozen soil, and a serious saturation deficit is developed in the plant. In spite of recognition of the importance of the ratio between transpiration and absorption, very few simultaneous studies of the two processes have been made on plants rooted in soil. Vesque (1878), who seems to have carried out the first careful investigation of this problem, observed the behavior of bean and Ner-iwm plants by placing their roots in water in a potometer. He found that absorption was not always proportional to transpiration, but that usually near mid-day transpiration much exceeded absorption. If transpiration was decreased or increased by varying environmental conditions, corresponding changes in absorption soon occurred. Vesque concluded that his results supported the view that transpiration is instrumental in bringing about the absorption of water. By the use of potometers MVlontfort (1922) and Lachenmeier (1932) investigated the relative rates of absorption and transpiration of plants growing in liquid media, but they obtained rather contradictory results. Montfort found that transpiration tended to exceed absorption, while Lachenmeier found that over a period of time, both in light and in darkness, absorption exceeded transpiration. Werner (1936) developed a technique for growing plants in such a manner that the shoots could be weighed independently of the roots. He reported variations in weight of the shoot in the course of a day, which were ascribed to changes in water content resulting from inequality in rates of transpiration and absorption. The relation between absorption and transpiration of cut branches has been studied by several investigators. Lloyd (1912) reported that the transpiratio.n of cut branches of Fouqueria splendens exceeded absorption during the day, but absorption exceeded transpiration at night. Stefanoff and Stoickoff (1932) compared absorption and water loss of cut branches of a number of tree species and found that absorption lagged considerably behind transpiration in some species, but very little in others. They ascribed the variation to differences in efficiency of the conducting systems. Measurements made on cut