The distribution of leaf area, radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. Part II. Photosynthesis, transpiration, and the effect of row shape and direction

Abstract

The influence of the distribution of radiation in an orange canopy on transpiration and photosynthesis was examined by developing a model of these processes. The leaf energy balance, microclimate relationships and climatic data are combined with radiation, leaf conductance, and leaf carbon uptake models to simulate orchard photosynthesis and transpiration over 2 days. Calculated hourly values of transpiration showed good agreement with measured values of sap flow in the orange orchard. Calculated carbon uptake during the six summer months was 22 kg CO 2 per tree; however, experimental estimates of annual dry matter production yield 55 kg CO 2 per tree. The calculated figure is therefore considerably in error and indicates that present information used in carbon balance modeling of Citrus is inadequate. Even so, it is shown that radiation levels deep in the canopy, where a significant amount of leaf area and transpiration is located, are too low for significant carbon uptake to occur. As an example of the usefulness of the model, the distributions of photosynthesis, transpiration and photosynthetic radiation were simulated in hedgerow canopies of three different shapes following current pruning practices in Israel. The distribution of foliage inside the given hedgerow cross-section was calculated based on the relationship of average measured foliage density to calculated diffuse photosynthetic irradiance in the canopy. The simulation was run for rows oriented north-south and east-west and for climatic conditions of midsummer. The results of the simulation indicated that: (a) The highest photosynthesis in citrus orchards is obtained by covering the largest ground areas possible with a thick canopy, i.e., maximum leaf area index (LAI). Under such conditions most photosynthesis occurs in the upper 1 m of the canopy. (b) Although rows with slanted walls do not have the highest photosynthesis, they allow more light penetration into the canopy and have productive regions on the periphery of the canopy at all heights within the orchard. (c) Whereas row orientation has little influence on total photosynthesis of the orchard, a N-S orientation allows more light penetration into rows with slanted walls and/or wide inter-row alleys, thus reducing spatial variation in the computed photosynthesis. (d) Water use of vertically pruned citrus orchards can be decreased significantly without seriously affecting photosynthesis by reducing canopy height to as low as 3 m.