Water Balance inCucumisPlants, Measured by Nuclear Magnetic Resonance, I

Abstract

In this paper we demonstrate the study of plant water balance by the non-invasive measurement of tissue water content and water flow using proton nuclear magnetic resonance (NMR). Sap velocity and flux were measured independently in the presence of an excess of stationary tissue water. The instrumentation described allows automated and unattended measurement of flow- and water content-variables in a well-defined region of the plant over periods of several days, with a time resolution between successive measurements of c. 5 s. Using this apparatus the effect of changes in light intensity (day/night rhythm) and relative humidity on stem tissue water content as well as on the velocity and flux of xylem sap in the stem were investigated in a cucumber plant. The results are in agreement with predictions from a simple model for plant water balance, which is based on water potential, flow rate and resistance to flow. As long as only transpiration is varied, flow rate and water content (or potential) are affected in opposite ways as demonstrated in this paper. In contrast, the model predicts that changes in uptake (resulting from changes in, for example, root resistance) will induce changes in water content and flow in the same direction. An experimental verification of this prediction is given in a subsequent paper, where, in addition, the NMR results are compared to those obtained with a dendrometer.