Diurnal variations in water relations of deficit irrigated lemon trees during fruit growth period

Field-grown lemon trees ( Citrus limon (L.) Burm. fil. cv. Fino) were subjected to different drip irrigation treatments: a control treatment, irrigated daily above crop water requirements in order to obtain non-limiting soil water conditions and two deficit irrigation treatments, reducing the water applied according to the maximum daily trunk shrinkage (MDS) signal intensity (actual MDS/control treatment MDS) threshold values of 1.25 (T1 treatment) and 1.35 (T2 treatment), which induced two different drought stress levels. Daily variations in leaf (Y leaf ) and stem (Y stem ) water potentials, leaf conductance, net photosynthesis, sap flow (SF) and trunk diameter fluctuations were studied on four occasions during the lemon fruit growth period. Y stem and Y leaf revealed a diurnal pattern in response to changes in evaporative demand of the atmosphere. Both water potentials decreased in response to water deficits, which were more pronounced in the T2 treatment. Y stem was seen to be a better plant water status indicator than Y leaf . The difference between the two values of Y (Y stem - Y leaf  = DY) was closely correlated with sap flow, making it a suitable measure of leaf transpiration. Using the slope of this relationship, the canopy hydraulic conductance (K C ) was estimated. When other continuously recorded plant-based indicators are not accessible, the concurrent measurement of leaf and stem water potentials at midday, which are relatively inexpensive to measure and user-friendly, act as sufficiently good indicators of the plant water status in field grown Fino lemon trees.