Transpiration as related to Internal Water Content

Abstract

THE valuable concept of water movement through plants as a catenary process¹ explicitly assumes no change of plant moisture content. While this assumption is adequate for many purposes, the quantities of water within larger plants are considerable, and variation over a period of time may be important. From Ovington's² results for plantations in southern England, the moisture content of the aerial parts of the trees (expressed as the equivalent depth of water) averaged 3.3 in., with a maximum value of about 7 in. (47-year-old Picea abies). The needles of young Pinus sylvestris in pots well supplied with water had a variation over a 24-h period in summer amounting to 5 per cent of the saturated moisture content³. If similar fluctuations occur in larger trees in the field, these would amount to 0.05 in. of water or 1 l./tree at 2,000 trees/ha. in a total water content of the canopy of 40,000 kg/ha. When the soil moisture tension in the experiment with potted pines exceeded an atmosphere, internal drying of the leaves occurred and the lost water was not replaced during the night. Should the moisture tension of soil of the rooting zone in a plantation reach the wilting point, perhaps as much as another 0.1 in. of the canopy moisture might be lost if the leaf water deficits were similar to those for the potted trees.