Improving water-use efficiency of young lemon trees by shading with aluminised-plastic nets

The experiment was carried out in a research field near Murcia, Spain, over a 3-week period between August 29 and September 17, 2001. Measurements obtained by the continuous monitoring of sap flow and parameters derived from trunk diameter fluctuations were compared with discrete measurements of conventional plant–water status indicators in potted young lemon trees (Citrus limon (l.) Burm. Fil, cv. Verna) grafted on sour orange (Citrus aurantium L.) rootstock. Eight trees were used in the experiment, four of which were placed under a rectangular shading net, while the other four were maintained in the open air. The decrease of sap flow in shaded trees with respect to the exposed trees was evident every day. The net also affected trunk diameter changes, with the shaded trees showing lower values of maximum daily shrinkage. High radiation reduced the leaf water potential. In the early morning, the shaded plants opened their stomata later than the plants growing in the open air, and so the transpiration in the latter was higher than in the former during the first hours of the day. However, the net photosynthesis rate was not increased in the exposed plants during these hours of the morning. In the central hours of the day, the leaf stomatal conductance values of exposed plants were lower, as were their instantaneous photosynthesis rates, than the corresponding values in the shaded trees. As a result of the photosynthesis/transpiration balance, it was observed that the best integrated daily water-use efficiency corresponded to the shaded treatment.