Apple sap flow in different light environments

Three photoselective, 50% shading, nets (red, white, blue) and a black, 20% shading, commercial hail net (CTRL) were placed on 6 apple trees (Malus domestica 'Rosy Glow') each, during summer 2016. Total and specific sap flows were monitored, using the thermal balance approach, on 4 branches per treatment at different phenological stages. In addition, daily leaf and stem water potentials as well as leaf gas exchange were determined three times during the season. During the warmest periods, trees under the red net showed the highest branch sap flow, followed by the CTRL, the blue, and the white. The higher sap flow under the red net might have been due to a steeper temperature rise during the morning hours. In fact, spectral light transmission under this net was higher in the infrared region compared to the blue and the white. Water potential data support this hypothesis, as lower values were recorded under the red net. However, leaf transpiration and stomatal conductance were not different among the four treatments. This discrepancy between leaf gas exchange and sap flow may be explained by the fact that the former was measured on full light leaves, whereas the latter integrated the whole branch transpiration, thus also transpiration from shade leaves. Under the red net these leaves might have had higher transpiration due to the higher infrared absorption, thus explaining the higher sap flow recorded under this net. On the other hand, the white net showed lower water use, probably due to the benefit of shading. This work shows how photoselective nets can significantly alter the canopy water use and suggests the possibility to use them as tools to improve orchard water use efficiency.