Sap flow, gas exchange, and hydraulic conductance of young apricot trees growing under a shading net and different water supplies

Summary

The experiment was carried out in a research field near Murcia, Spain, over a 3-week period between September 26 and October 16, 2000. Sixteen trees were used in the experiment, eight of which were placed under a rectangular shading net, while the other eight were maintained in the open air. Trees were irrigated once per day and, after October 5th, water was witheld from eight trees (four shaded and four unprotected for 5 days). The leaf stomatal conductance and the photosynthesis rates were higher in the shaded trees than in the exposed plants, probably because the leaf water potential was lower in the unshaded plants. This higher leaf conductance partially compensated for the effect of low radiation on transpiration, and the reduction of daily sap flow registered in shaded trees was only around 10–20%. The net also affected trunk diameter changes, with the shaded trees showing lower values of maximum daily shrinkage. Soil water deficit and high radiation had a similar effect on plant water parameters, lowering leaf water potential, leaf stomatal conductance, and the photosynthesis rate. The effects of both conditions were accumulative and so the exposed water-stressed plants showed the lowest values of total hydraulic resistance and water use efficiency, while the shaded well-irrigated trees registered the highest values for both parameters. For this reason, we think that net shading could be extended to apricot culture in many areas in which irrigation water is scarce and insolation is high.

Introduction

Apricot trees are characterized by their large canopies, which represent a large evaporative surface, while their stem and root hydraulic conductivities are low (Alarcón et al. 2000; Domingo et al., 2002). These factors, especially in adverse conditions, may result in a tree's transpirational losses exceeding its absorptive capacity, thereby giving rise to a plant water deficit (Kriedemann and Barrs, 1981).

The adverse conditions that may result in the decoupling of transpiration and root absorption include strong insolation and high temperatures, both of which are typical of Mediterranean climates. In summer and autumn, with average midday temperatures of about 30 °C, crop transpiration may well exceed root water uptake even in well-irrigated soils, leading to temporary but excessive water deficits. In fact, previous studies have demonstrated that crop productivity in different species may be reduced in environments with high radiation, high temperature, and high water vapour pressure deficits (Syvertsen and Lloyd, 1994; Bustan, 1996). To avoid this situation in horticultural crops, shading nets have been used to reduce the radiation load (Cohen et al., 1997). It has been shown that such nets distribute the radiation to the plants growing underneath with greater efficiency (Allen and Lemon, 1974) and inhibit turbulence (Tanny and Cohen, 2003), creating a humidity blanket which contributes to a lower environmental evaporative demand (Allen, 1975). Therefore, if the net radiation of a tree can be reduced, the direct effect should be a proportional reduction in transpiration (following Penman, 1948), perhaps avoiding a water deficit. However, since the light underneath shading nets is altered, the effects of such an alteration on the physiological processes related to plant growth must be investigated. Although the reduction in radiation would probably reduce transpiration, it was thought that it might also affect photosynthesis and, therefore, plant growth.

The aim of this study was to evaluate the use of reflective aluminized polypropylene shading nets on the water use efficiency of young apricot trees. The variations in leaf conductance, photosynthesis, whole tree transpiration, plant hydraulic resistance, and trunk diameter under different radiation levels were measured and related in both well-irrigated and non-irrigated plants.