Environmental and stomatal control of transpiration, canopy conductance and decoupling coefficient in young lemon trees under shading net

Sap flow, as a measure of transpiration, was monitored in 2-year-old lemon trees growing in pots. 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. Daily averages of canopy conductance and photosynthesis were not affected by shading; however, the daily transpiration was reduced in shaded plants, which displayed an increase in water use efficiency compared with exposed trees. The decoupling coefficient was higher in the shaded trees, indicating that the transpiration of lemon trees was efficiently controlled by stomata in exposed plants, while the transpiration rate was mainly influenced by radiation in the plants growing under the net. This influence was more pronounced in the afternoon, when the whole tree transpiration was largely dominated by equilibrium transpiration in the plants under netting, and the relationship between transpiration and radiation showed a steeper slope in shaded trees. The reduction in transpiration and the maintenance of photosynthesis in shaded plants with respect to exposed trees indicated that screen structures in semi-arid and arid environments could be considered as an intermediate solution for reducing plant water stress and increasing water use efficiency.