Movement and distribution of tritiated water in rooted poplar stems

Abstract

Water movement in plants has a long history of investigation. However, the mechanism by which this quantity of water is moved within plants, in particular to reach heights greater than that which can be supported by atmospheric pressure, remains to be clarified. Presently held theories of the mechanism of water movement may be divided into two groups. In one group the living cell is considered to be actively involved in the process and consequently the driving force for water movement may come from either the upper or lower part of the plant. In the second group it is considered that water movement occurs in response to a purely physical "pull from above" caused by transpiration. Dixon and Joly (1894) formed the basis of this latter explanation in what they called the Cohesion Theory. Greenidge (1957, p 237) considers this theory is "the most widely accepted of current generalisations", and this is also indicated by the fact that since its proposition, most work done on water movement has been done in terms of this theory. This paper uses tritium as a tracer in the study of water movement in plants, specifically Lombardy poplar (Populus nigra var italica Du Roi). The slow movement of water in these small plants does suggest that the mechanism involved is not as simple as the physical system described by the Cohesion Theory. However, that transpiration is an important influencing factor is indicated.