Daily and seasonal patterns of leaf water relations and gas exchange of regulated deficit-irrigated almond trees under semiarid conditions

The diurnal courses of leaf water relations and gas exchange of long-term regulated deficit-irrigated field-grown almond trees (Prunus dulcis (Mill.) D.A. Webb) were investigated at three different times of the growing season (Spring, Summer and Autumn) under different irrigation conditions (surface and subsurface drip irrigation) during the year 2000 in a semiarid region of SE Spain. Leaf water relations and stomatal behaviour were altered significantly more by irrigation treatment than by irrigation system (surface or subsurface). Leaf water status and gas exchange parameters responded diurnally and seasonally to variations in soil water status and evaporative demand. Leaf, xylem water potential (Ψl, Ψx) and leaf relative water content (RWC) were closely correlated with air vapour pressure deficit (VPD) in well-irrigated periods (Spring and Autumn), but not under severe water stress (Summer). Leaf photosynthesis (A), stomatal conductance (gs) and transpiration (E) were also positively correlated with air VPD, with a threshold level for stomatal closure around 2 kPa. Mesophyll water status and gas exchange activity were more affected by factors related to soil water availability than by atmospheric factors under severe water stress. A low correlation between daily changes of leaf water status parameters (Ψx, Ψl and RWC) and gs and A in the three seasons was also observed. Soil water deficit reduced significantly Ψl, Ψx, RWC and gas exchange levels during the day and the maximum daily peak of gas exchange was moved to earlier in the morning. This increased significantly gas exchange efficiency in the deficit treatments compared to the control in the early and mid-morning. Daily stomatal behaviour was modified in the treatment watered the least, as a consequence of long-term cumulative water stress effect. Preconditioning to water stress may also play a main role in diurnal gas exchange responses of field-grown almond trees under long-term regulated deficit irrigation (RDI). After four years of RDI, this treatment maintained, at post-harvest, similar mean daily levels of photosynthesis and gas exchange efficiency than the control, with a 50% irrigation reduction, optimising water use. Intrinsic gas exchange efficiency (A/gs) was less variable during the day and less influenced by VPD compared to A/E, being better indicator of the regulation of plant water use at the leaf level. The daily and seasonal variation of A/gs during the growing season would be used as a physiological tool for optimising RDI strategies in semiarid conditions.