Effects of shading on leaf temperature, photosynthesis and water relations of two navel orange [Citrus sinensis (L.) Osbeck] cultivars

Between January and February 2007, diel variability in carbon dioxide (CO2) and water vapour (H2O) exchange with leaf temperature on four potted 5-year-old navel orange trees [Citrus sinensis (L.) Osbeck] on an open-air laboratory on the roof of the Physics Department (University of Zimbabwe) was measured. Two of the four trees were of the Baianinha variety while the other two were Navelina. An automated dynamic chamber (cuvette) was installed sequentially on each of the four trees with flux measurements on a selected branch of each tree typically taking ten days. Out of the ten days on each tree, three days consisted of measurements under full solar radiation, three days under a single layer plastic mesh shade net whose transmittivity was found to be 24 % of full solar radiation, and the remainder of the ten day period under a double layer net (7 % transmittivity). Leaf temperature was measured as well as branch and stem sap flow rates. Results showed that photosynthesis was highest when leaf temperatures were between 25 and 30 °C while transpiration was also increased at such temperatures. There was less than ten percent difference between the two varieties of navel orange trees in terms of net exchange of fluxes. Although the shade nets were able to effectively lower leaf temperature, they also significantly lowered the levels of photosynthetic flux density to levels much less than the level at which photosynthesis saturates in citrus trees (600-700 µmol m-2 s-1). Photosynthetic water productivity was found to be highest under the unshaded conditions. A model was run in April 2007 on two trees to predict stem sap flow rates under different shading conditions using leaf water potential, branch sap flow and soil water potential as inputs. Data for the model was collected over two days for each tree: first under full solar radiation conditions then with a single layer shade net. The results showed that the model was, to a large extent, able to predict the stem sap flow. Optimization of the model yielded the hydraulic parameters for each tree under the different radiation regimes. Overall, photosynthetic photon flux density was found to be a more important factor than leaf temperature in optimizing photosynthesis and water productivity