Effects of regulated deficit irrigation under subsurface drip irrigation conditions on water relations of mature almond trees

The influence of several regulated deficit irrigation (RDI) strategies under subsurface drip irrigation (SDI) conditions on water relations and gas exchange activity was analysed during a four year period for mature trees of a local variety of almond (Prunes dulcis (Mill.) D.A. Webb, cv. Cartagenera), in a commercial plantation in the district of Aljorra, Murcia (,Southern Spain). Five treatments were applied: T1 (surface drip), which was irrigated at 100% ETc (crop evapotranspiration) for the full season, applying 603 mm per year; T2 (surface drip), irrigated at 100% ETc, except during, the kernel-filling stage (from early June to early August), when 20% ETc was applied; T3 (SDI), equal to T2, but in subsurface drip irrigation; T4 (SDI), irrigated at 100% ETc, except during kernel-filling (20% ETc) and post-harvest (75% ETc); T5 (SDI), equal to T4, but a greater reduction of post-harvest irrigation (50% ETc). The severity of water stress was characterised by measurements of soil water content (theta(v)), predawn leaf water potential (Psi(pd)), osmotic potential (Psi(pi)), relative water content (RWC), and gas exchange rates. Under stress (during the kernel-filling stage) and non-stress conditions (during active growth and post-harvest periods) in treatments T2, T3;and T4 there were not significant differences in the soil water content, nor in the plant water status and gas exchange parameters measured. Only T5 (SDI) showed a significant reduction in gas exchange activity at the end of the kernel-filling stage (point of maximum stress). This response was closely correlated with the severity of the stress reached. Minimum values of Psi(pd) reached in this period were -2.37 MPa in T2, -2.26 MPa in T3, -1.95 MPa in T4 and -2.52 MPa in T5. Maximum reductions in photosynthesis rate (A), with regard to the control, were 64%, 61%, 58% and 75%, respectively. Reductions in A, E and stomatal conductance (g(s)) in response to severe water stress were reversible. Although soil and plant water status recovered rapidly when trees were irrigated post-harvest, gas exchange activity (g,,, E and A) recovered more slowly in all treatments. T5 showed a recovery of soil water status that was slower and incomplete compared to the other treatments, post-harvest (early August-early September), although gas exchange activity was not affected in this period. These results indicate that these RDI strategies, with a severe irrigation deprivation during kernel-filling (20% ETc), and a recovery post-harvest at 75% ETc or up to 50% ETc under SDI, can be adequate in this orchard under semiarid conditions, due to a higher water application efficiency of this irrigation system, saving between 220-273 mm year(-1) irrigation water.