Physiological Responses of Citrus to Partial Rootzone Drying Irrigation Strategies

The effect of partial rootzone drying (PRD) on growth and physiological responses of 2-year-old grapefruit trees was studied in two experiments under greenhouse conditions. First, we studied the effect of different irrigation volumes, including irrigation above and below tree evapotranspiration (ET_c) requirements, in order to determine how trees respond to excess of irrigation or to PRD + deficit irrigation (DI) strategies. Four treatments were applied for 12 weeks: 1) control (100% ET_c was applied, 50% on each pot); 2) PRD 100–0 (100% ET_c on one side, no water on the other side); 3) PRD 200–0 (200% ET_c on one side, no water on the other side); and  4) PRD+DI 50–0 (50% ET_c on one side, no water on the other side). All trees had their root system split in two halves and established in adjacent pots. Soil water content, tree water status, stomatal conductance, leaf ABA concentration, chlorophyll fluorescence and tree growth were measured. PRD 200-0 trees used 70% more water than control trees whereas PRD 100-0 and PRD+DI 50-0 trees decreased water use by 18% and 79% compared to control trees, respectively. PRD 100-0 or PRD 200-0 did not affect tree growth or any physiological parameter compared to control trees. Although tree growth was not affected by PRD+DI 50-0, leaf ABA concentrations and stomatal closure increased after 10 weeks of the experiment. In the second experiment, we studied the effect of alternate irrigation between each side of the tree root system. Three treatments were applied for 10 weeks: 1) control (100% ET_c, 50% on each pot); 2) alternate PRD1 100–0 (alternating irrigated/dry rootzones every month); and 3) alternate PRD2 100–0 (alternating irrigated/dry rootzones every two months). Tree establishment, growth conditions, and all parameters measured were the same as the first experiment. Trees under alternate PRD used 5% to 8% less water than control trees without decreasing growth or showing any changes in physiological parameters. Nevertheless, the timing of the alternation did not cause any differences in tree water use.