Application of partial rootzone drying to improve irrigation water use efficiency in grapefruit trees

Partial rootzone drying (PRD) has been reported to potentially improve crop water use efficiency (WUEcrop) compared to full irrigation in different fruit trees; however, field studies on the effect of PRD in citrus are scarce. In this field study, three irrigation strategies were tested in an orchard of mature grapefruit trees during two consecutive seasons (2013/2014 and 2014/2015): drip PRD (two drip lines, alternating irrigation between lines every month), microsprinkler irrigation, and double-line drip irrigation (control). Irrigation was applied during the fruit enlargement stage (April-August). The aims of this field study were: 1) to provide a quantitative comparison of irrigation water productivity among irrigation treatments; and 2) to study their effects on fruit quality, yield, tree growth, and flowering. Drip PRD saved 43 to 47% of irrigation water compared to control irrigation, and microsprinkler irrigation saved 12 to 18% of water compared to control irrigation PRD-irrigated trees maintained or increased yield compared to microsprinkler-irrigated and control trees, depending on the experimental season. Therefore, WUEcrop in PRD-irrigated trees was significantly higher than in control and microsprinkler-irrigated trees at the end of both seasons. Fruit and juice quality parameters were statistically similar among all treatments. PRD irrigation did not reduce the flowering potential of the trees although it delayed the onset of flowering in trees relative to the other treatments in 2013/2014. There was a similar timing of flowering among treatments in 2014/2015 and there were no differences in vegetative growth among the irrigation treatments by the end of March (86 Julian data) in both experimental seasons. Our results suggest that PRD can be economically beneficial for citrus growers who use double-line drip irrigation systems, and a strategy to sustain tree growth, tree health and yield during seasons of extreme drought or when high water restrictions are placed in citrus-producing areas.