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Canopy Position and Leaf Age Affect Stomatal Response and Water Use of Citrus

Summary We investigated the responses of citrus stomata to leaf age, leaf position, canopy size and local microclimate. Large differences existed in the stomatal response due to leaf age within a 15-year-old canopy. Much smaller differences due to leaf age were evident in 7-year-old and 2-year-old trees. Likewise, position in the canopy played a significant role in determining stomatal response within a 15-year-old tree, but had little influence in the smaller trees. Stomatal conductance data were in good agreement with values calculated using a previously published stomatal-response function based on incident photosynthetically active radiation and the vapor pressure deficit of air. The model of stomatal conductance was used to calculate total daily plant water use of 2-year-old trees based on the Penman-Monteith model. Calculated values of plant water use were close to actual values for these 2-year-old trees as measured using weighing lysimeters. These results confirm the robustness of the Penman-Monteith model for the evaluation of citrus water use. We then scaled our calculations up to the 15-year-old field grown trees. Our study highlights the differences in stomatal response between various populations of leaves within a citrus canopy and indicate the important role stomatal conductance has in determining water use of citrus trees.