Responses of Young Almond Trees to Two Drought Periods in the Field

Abstract

SummaryWater potential, conductance to water vapour diffusion and 14CO2 uptake rate were measured in the field on leaves of young, trickle-irrigated almond trees subjected to drought periods. Treatments imposed were: T. two one-month drying cycles with an interim period of recovery; T2, a single one-month drying cycle; T3, a single 2.5-month drying cycle; and T4, daily irrigation. Midday leaf water potential in T, and T2 trees decreased 0.8 to 1.0 MPa below that of the control, and that of T3 trees was further decreased by about 0.7 MPa. Turgor pressure was partially maintained in the stressed treatments through osmotic adjustment. Osmotic potential at full turgor in leaves of unirrigated trees was about 0.3 MPa lower than in the controls and was the same at the end of both drought cycles. Decreases in osmotic potential were accompanied by increases in dry weight per unit leaf area. Despite partial maintenance of turgor, water stress decreased leaf conductance and 14CO2 uptake. Leaf conductance and 14CO2 uptake correlated well over a wide range of water-stress conditions. Stomatal closure during both drying periods occurred gradually over a wide range of decreasing leaf water potential. Water stress resulted in leaf rolling and shedding. Both effects caused the solar radiation intercepted by the trees to decrease by the end of the second drought period from 80% in the control trees to less than 50% for trees that had leaf water potential below —3.0 Mpa.