Transpiration and canopy conductance in young apricot (Prunus armenica L.) trees subjected to different PAR levels and water stress

Water use and canopy conductance were monitored in 2-year-old apricot (Prunus armenica L.) trees growing in 70-L plastic pots. The experiment was carried out in a research field near Murcia, Spain, over a 3-week period between September 26 and October 16, 2000. 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. Trees were irrigated adequately until October 5 (day 279); later the water was withheld in all trees for five days. The estimation of transpiration from sap flow measurements was done under both radiation conditions along the experiment, recording a significant decrease of transpiration in shaded trees with respect to the exposed trees. The mild water stress affected the exposed and shaded plants in a similar manner. In both cases, the water deficit induced a significant reduction of transpiration after two days of water withholding. Total conductance was obtained from daily measurements of transpiration and vapor pressure deficit between September 28 and October 10, and was similar to canopy conductance. Maximum canopy conductance showed the lowest values at the end of mild water stress (October 10) (0.0017 and 0.0012 m s−1, shaded and sunny, respectively), and the highest values on September 28 when the trees were in well-irrigated condition (0.0024 and 0.0018 m s−1, shaded and sunny, respectively). Finally, the decoupling coefficient reached the highest values in the shaded trees and the lowest in sun-exposed trees. The results showed that water vapor exchange was strongly dominated by vapor pressure deficit and controlled by leaf conductance. It was also noticeable that trees under shade were more decoupled from the atmosphere than those fully exposed to the sun, meaning that exposed trees resulted in a better control of water loss. We think that net shading could be extended to apricot culture in many areas in which irrigation water is scarce and insolation is high, because the water use clearly decreased in shaded trees with respect to the exposed ones.