Resistance to Water Uptake in a Mature Citrus Tree

Resistance to water uptake in an irrigated and a water-stressed22-year-old citrus tree was examined during 7 weeks. Frequentirrigation maintained the soil water potential of one citrustree between 0.0 and –0.016 MPa in the upper 0.6 m, whilethe other tree received no water for 44 d. Sap flow in the trunkswas measured once a week from dawn to dusk at hourly intervalswith a calibrated heat pulse technique. At the same time leafwater potential was measured on sunlit and shaded leaves. Sap flow in the stressed tree decreased with time at a higherrate than the water potential difference between soil and leaf,which implies an increase in the resistance of the transportsystem. Diurnal sap flow and diurnal leaf water potential ofthe irrigated and stressed trees were linearly related throughoutthe experimental period. The slope of the linear relationshipbetween 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 ofthe leaf water potential at zero flow (computed by linear regression).This was ascribed to a reduction of the transport of water inthe soil toward the roots. Key words: Transpiration, Leaf water potential, Plant water relations, Soil water stress