Effects of regulated deficit irrigation under subsurface drip irrigation conditions on vegetative development and yield of mature almond trees

The influence of several regulated deficit irrigation (RDI) strategies, applied under subsurface drip irrigation (SDI), on vegetative development and yield parameters in mature almond (Prunus dulcis (Mill.) D.A. Webb, cv. Cartagenera) trees was analysed during a 4-year field experiment. Five treatments were applied: T1 (100% crop evapotranspiration (ETc), full season); T2 (irrigated at 100% ETc except in the kernel-filling stage (20% ETc)); T3 (equal to T2 but in SDI); T4 (SDI, 100% ETc, except in the kernel-filling stage (20% ETc) and post-harvest (75% ETc)); T5 (SDI, 100% ETc except in the kernel-filling stage (20% ETc) and post-harvest (50% ETc). A close correlation between applied water, plant water status (Psi(pd)) and tree growth parameters was observed. After four years, the vegetative development in T5 was reduced significantly due to a larger annual cumulative effect of water stress on growth processes, resulting in a smaller tree size (trunk and branch growth, canopy volume and pruning weight) compared to other treatments. Moreover, water stress during kernel-filling produced a significant reduction in the leaf expansion rate and a stimulation of premature leaf abscission, resulting in a smaller tree leaf area in this treatment. SDI produced a greater horizontal distribution of fine roots in the soil profile than surface drip system. The RDI practices applied under subsurface drip irrigation stimulated a deeper root development (40-80 cm) than surface treatments (0-40 cm), producing also a higher root density in the subsurface treatments watered the least (T4 and T5). Water stress during the pre- and post-harvest periods had no important effect on bud development, bloom, fruit growth or fruit abscission. Moreover, there were no significant reductions in kernel dry weight or kernel percentage. Reductions in kernel yield were significant compared to T1, being of 11% in T2, 15% in T3, 20% in T4 and 22% in T5. Water use efficiency (kg m(-3)) was increased significantly in the SDI treatments T4 and T5. A significant correlation between kernel yield and tree growth parameters was observed. We conclude that the application of overall reductions of water use of up to 50% during high water stress sensitivity periods (such as post-harvest) under SDI system, is a promising alternative for water management in semiarid regions in order to improve water use efficiency. Nevertheless prolonged water stress during kernel-filling and post-harvest can reduce excessively the vegetative development of almond, negatively affecting the long-term yield response.