Impact of fruiting on gas exchange, water fluxes and frond development in irrigated date palms

The cultivation of date palms in Israel's Arava Valley, which is characterized by a high evaporative demand, is widespread and exclusively depends on high-frequency irrigation. Fruit sets are usually thinned early in their development to produce large, high-quality dates. However, no effort has yet been made to comprehensively examine fruit load effects on date palm water use, CO2 fluxes and growth. The objective of this study was to investigate the effects of fruit load on these factors. Twelve date palms, six with fruit removed (“without fruit”) and six untreated (“with fruit”), were irrigated with equal amounts of water at sufficient levels for maintaining optimal soil water conditions. Sap flow and frond elongation were continuously measured. Gas exchange parameters (i.e., stomatal conductance and CO2 assimilation rate) and fruit growth (i.e., fruit size, mass, and sugar content) were monitored periodically. No significant differences were found in gas exchange, water consumption and frond elongation between the two treatments several weeks after the onset of the fruit load differentiation. However, a pronounced increase in stomatal conductance and CO2 assimilation rates in palms with fruit compared to those without fruit was identified during the sugar accumulation and post-harvest periods. Continuously increasing water consumption in palms with fruit was also observed during these periods, probably as a result of the progressive recovery of depleted carbohydrate and water storage in the tree. In addition, the frond elongation rate of palms with fruit was remarkably lower than those without fruit until the end of the harvest. It is concluded that crop load has a pronounced effect on physiological behaviors and water use of the cultivated date palm. Therefore, irrigation management must consider fruit load to achieve optimal yield.