Consequence of salinity and excess boron on growth, evapotranspiration and ion uptake in date palm (Phoenix dactylifera L., cv. Medjool)

Yield and transpiration of juvenile date palms (Phoenix dactylifera L., cv. Medjool) were studied under conditions of increasing salinity and boron in lysimeters. Twenty seedlings were planted and grown in 20 lysimeters and irrigated with combinations of four salinity and five B irrigation concentrations. A linear decrease was found for both yield and for transpiration in response to increased soil saturated paste salinity for the treatments having lower B concentrations (0.0278, 0.185 and 0.4625 mmol 1^-1). Yield and transpiration also decreased with increased B concentration. While increases in soil saturated paste B from 0.3 to 1.5 mmol l^-1 caused substantial declines in yield and transpiration, subsequent increased B to 3 mmol l^-1 caused only minor reductions. Response to salinity and to excess B was witnessed from the lowest tested levels when each of the variables was isolated. Growth response to combined conditions of salinity and B behaved according to the dominant of the two stress causing factors and did not show additive effects. Dynamics of plant water uptake and tree growth observed for salinity and boron occurring independently and together were summarized by decreased water uptake but not ion accumulation for NaCl and CaCl salts and by boron that was accumulated in leaves and subsequently was associated with reduced tree size. It is suggested that while mechanisms for plant response to salinity are dominated by lowered soil water potential (osmotic stress), boron becomes toxic as it accumulates to a threshold level in plant tissue.