Effects of water stress during the harvest period on the physiology and behavior of yield components of almond (Prumus dulcis (Mill.) Webb) trees

The effect of water stress on the physiology and yield of mature almond trees was studied during a 3-year field experiment in which different irrigation treatments (full irrigation and moderate and severe water stress) were imposed during the harvest period (August–September). The experimental approach was to: (1) relate irrigation treatments to actual, measured, stress in the trees. (2) relate degree of stress to carbohydrate reserves in dormant trees, and to reproductive behavior and yield components in the subsequent three years.; Water-stress-related parameters were characterized. Midday stem water potential (Ψ_ms) was found to be a practical and reliable stress index for almonds since it integrated the tree response to soil water content and environmental parameters. Ψ_ms was closely related to leaf CO₂ exchange and a threshold for defoliation was detected at about Ψ _ms = 1.8 MPa.; As a result of reduced photosynthesis and premature defoliation due to water stress, total whole tree non-structural carbohydrates (TNC) content for severely-stressed trees was calculated to be 26.1% less than that of fully-irrigated trees, with larger differences for roots (34.9%) than for the aerial parts (21.1%). Root TNC concentration had an apparent linear association with the lowest (more negative) values of Ψ_ms experienced by trees. The more severe the stress, the lower the root TNC concentration.; Water stress had no apparent effect on return bloom, fruit set, or spur mortality. Fewer fruiting positions per tree were, however, associated with stress. Only 10% of the original spur population tagged in summer of 1995 bore fruits three years later. By this time, 66.1% of the spurs died, indicating the importance of spur renewal for maintaining crop productivity. A strong biennial behavior of both spurs and branches was consistently observed throughout the evaluation years. A consistent negative linear relationship was found between the seasonal minimum Ψ_ms and individual tree yield, with yield reductions being evident after two years of stress. The yield reduction due to water stress appears to be a compound, multi-year effect, with the renewal of fruiting positions playing a major role.