Effects of irrigation deprivation during the harvest period on leaf persistence and function in mature almond trees

In nut tree orchards in California, irrigation is typically withheld during the harvest period to reduce the likelihood of bark damage during mechanic al shaking of the trees. The ensuing water stress, however, may result in p remature defoliation and subsequent yield declines. Our objective was to es tablish and quantify the water stress resulting from irrigation deprivation and determine its impact on leaf function and persistence in mature almond trees (Prunus dulcis (NEU.) D.A. Webb cv. Nonpareil) during a 3-year field experiment. The severity of the water stress was characterized by measurem ents of predawn leaf (psi (pd)) and midday stem (psi (ms)) water potentials , stomatal conductance (g(s)), net CO2 assimilation rate (A) and leaf absci ssion. During 1995, psi (ms) of fully irrigated (FI) trees was maintained a bove -1.0 MPa. In trees in the moderate(MS) and severe-stress (SS) treatmen ts, psi (ms) was reduced to -1.4 to -2.0 MPa and -2.0 to -2.6 MPa, respecti vely. After 18 days of irrigation deprivation,A was reduced by 32 and 58% a t midday and early afternoon, respectively, compared with morning values. A significant decrease in morning values of A only occurred after 30 days of irrigation deprivation. Water-use efficiency and A declined as evaporative demand increased from morning to afternoon. Assimilation also declined sea sonally as leaves aged. Midday stem water potential was highly correlated w ith A, but less so with g(s),. The coefficient of determination between psi (ms) and g(s) improved considerably when vapor pressure deficit and wind w ere multiply regressed with Tm,. Although A recovered rapidly when MS trees were irrigated, recovery in SS trees was slower and incomplete, Integratin g the MS and SS effects for an extended period during 1995 resulted in 14 a nd 30% declines in A, and 6 and 20% declines in g(s), respectively. The app arent psi (ms) threshold for leaf abscission was -1.8 MPa. Daily canopy lig ht interception declined with decreasing psi (ms) as a result of premature defoliation (and perhaps altered leaf angles) from 67.9% in FI trees to 61. 4 and 60.7% in MS and SS trees, respectively.