Resistance to Water Uptake in a Mature Citrus Tree

Abstract

Resistance to water uptake in an irrigated and a water-stressed 22-year-old citrus tree was examined during 7 weeks. Frequent irrigation maintained the soil water potential of one citrus tree between 0.0 and −0.016 MPa in the upper 0.6 m, while the other tree received no water for 44 d. Sap flow in the trunks was measured once a week from dawn to dusk at hourly intervals with a calibrated heat pulse technique. At the same time leaf water potential was measured on sunlit and shaded leaves. Sap flow in the stressed tree decreased with time at a higher rate than the water potential difference between soil and leaf, which implies an increase in the resistance of the transport system. Diurnal sap flow and diurnal leaf water potential of the irrigated and stressed trees were linearly related throughout the experimental period. The slope of the linear relationship between flow and potential for both trees remained nearly constant, indicating that the resistance within the tree remained constant. The effect of drying appeared to be a progressive decline of the leaf water potential at zero flow (computed by linear regression). This was ascribed to a reduction of the transport of water in the soil toward the roots.