Physiological responses of carambola trees to soil water depletion

Two-year-old 'Arkin' carambola (Averrhoa carambola L.) trees were grown in containers in a greenhouse and the field in a very gravelly loam soil. Trees in the field were subjected to four soil water depletion (SWD) levels which averaged, 10.5%, 26.5%, 41.0%, and 55.5% and trees in the greenhouse were maintained at field capacity or dried continuously to produce a range of SWD levels. The relationships between SWD and leaf (Ψ_L) and stem (Ψ_S) water potential, net CO₂ assimilation (A _CO2), stomatal conductance of water (g_s) and transpiration (E) were determined. Coefficients of determination values between physiological variables were higher for trees in the greenhouse than in the field, which may have been due to greater fluctuations in vapor pressure deficit (VPD) in the field. Soil water depletion levels above 50% caused a reduction in Ψ_S that subsequently decreased g_S. This reduction in Ψ_S was correlated with a linear reduction in E and a considerable decline in A_CO2 when g_S fell below about 50 mmol·m^-2·s^-1. Leaf gas exchange parameters were better correlated with Ψ_S than with SWD level. Therefore, Ψ_S may be a better predictor of carambola tree water status than SWD in a well-drained, very gravelly loam soil.