Responses of transpiration and transpiration efficiency of almond trees to moderate water deficits

The majority of world almond acreage is grown under rainfed conditions but most of the production is obtained under irrigation. Increased water scarcity is reducing water availability for irrigation thus the need to characterize the responses of almond to water deficits. Several works have defined well the stomatal closure in almond leaves under water deficits, but the behavior at the canopy level is not well understood. A field experiment was conducted in an almond (cv. Guara) orchard in Cordoba (Spain) under four different levels of irrigation supply to investigate the responses of almond tree transpiration (T) and transpiration efficiency (TE) to water stress. Stem water potential (Psi(x)), whole tree transpiration (T) and leaf gas-exchange were periodically measured throughout the 2013 growing season. Tree T decreased linearly as midday T decreased below a threshold value of about -1.1 MPa, and declined to about 50% of the Control value when midday Psi(x) reached -1.6 MPa. The quick decline in T in response to the lowering of mix suggests a high sensitivity of almond T to water deficits. The instantaneous transpiration efficiency (TE) of almond leaves was unaffected by water and varied essentially with vapor pressure deficits. On a daily scale, the leaf TE of stressed trees followed the same pattern as in the non-stressed trees. Then, contrary to the behavior observed in olive and citrus, there were no instantaneous or daily TE increments in almond trees in response to water deficits.