Response of citrus trees to modified radiation regime in semi-arid conditions

Citrus trees are characterized by a large canopy and low hydraulic conductivity. In Israel's semi-arid summer climate this could cause transpiration to exceed water uptake and cause temporary excessive water deficits. It was hypothesized that reduction of radiative load would reduce transpiration and thus reduce deficits. Net radiation of lemon trees in the hottest season was reduced by shading hedgerows with reflective nets for approximately one month in both 1994 and 1995. Stem sap flow and climate variables were measured continuously. Daily courses of leaf conductance and leaf water potentials were measured on selected days. Midday net radiation below the dense and sparse shade net treatments was 47% and 73% of that above the control trees. Midday 'sunlit' leaf temperatures below the nets were reduced by 2.7 and 1.6°C, respectively. The reduction in net radiation caused large changes in leaf conductance. Average midday sunlit leaf conductance measured in 1995 under the dense and sparse treatments and control were 4.1, 2.9 and 1.8 mm s -1 , respectively (significantly different at P <0.01). Similar differences in sunlit leaf conductance were found in 1994. Shade leaf conductance was not affected by the treatments. Daily total and midday sap flow under the dense net were reduced by 6-7% and 10-11%, respectively. Sap flow under the sparse net did not change significantly in 1994, but in 1995 daily and midday sap flows were reduced by 6% and 7%, respectively. Midday leaf water potentials increased by 0.2 and 0.1 MPa under dense shade in 1994 and 1995, respectively. Under sparse shade midday leaf water potentials increased by 0.1 MPa in 1994, but did not change significantly in 1995. A modified Penman-Monteith model evaluated transpiration if leaf conductance were constant in the different radiation environments. At leaf conductance levels found in the unshaded trees, dense shade was estimated to cause a 25% reduction in transpiration, while leaf conductance values found in trees under the dense shade would lead to an increase in transpiration of more than 35% in unshaded trees. The ability of the tree to maintain almost constant transpiration in different radiation environments and thus avoid water deficit by adjusting the conductance of sunlit leaves is discussed in terms of environmental influences and significance to the plant's water balance.