The movement of tritiated water in plants

Abstract

Photosynthate translocation in the soybean was studied using tritiated water as the only isotopic compound. Data obtained for the distribution of THO and T-photosynthate of the intact soybean plants under the semi-closed system were similar to that of the closed system (petiole and stem sections were greased by lanolin). Though the soybean petiole was found to be capable of water exchange with the surrounding moisture via the stomata, there was little or no THO loss since the THO was never present at any significant distance from the leaflet. In the absence of light, trans­ piration was observed at a very slow rate as judged by the THO administration of soybean petiole. THO transport in ex­ cised petiole sections was studied by means of an agardiffusion technique. The pattern of this transport can be best described by the model of THO self-diffusion in a path 2 mm in diameter. An attempt was made to distinguish between diffusion and mass flow of water during photosynthate translocation by means of the following mathematical models. Model I: Selfdiffusion of THO, using Pick's Law. The water and photosyn­ thate are considered as moving independently. The former is considered as an equilibrated single pool, in an environment of constant specific activity, generating a radioactive pro­