Partial Root Zone Drying with and without Salinity Affects Water Use Efficiency of Citrus

A partial root zone drying (PRD) experiment was conducted in a greenhouse with and without saline water on split-root 'Swingle' citrumelo [Citrus paradisi Macfad. x Poncirus trifoliata (L.) Raf.] seedlings potted in an autoclaved fine sandy soil to evaluate leaf water use efficiency (WUE) and photosynthetic responses. Roots of all plants were split in half and the following five treatments were applied: 1) both root halves irrigated with 50% ETc each [100% ETc control or well-watered (WW)], 2) one-half irrigated with 100% ETc while the other received no water (PRD), 3) one-half irrigated with 50% ETc water plus 50 mM NaCl while the other received no water (PRS), 4) both sides irrigated with 50% ETc plus 50 mM NaCl (SS), or 5) one-half irrigated with 50% ETc and one-half irrigated with 50% ETc plus 50 mM NaCl (WS). Citrus seedlings in the PRD and PRS treatments used 33% to 42% less water than the WW controls. PRD and PRS treatments restricted shoot growth while root growth was less affected, leading to an increase in root: shoot ratio. Dry weight (DW) of PRD and PRS plants were 23% to 33% less compared to plant DW of WW controls. PRD and PRS dry sides induced higher levels of abscisic acid (ABA) in roots but not in leaves. Leaves above the PRD dry side had higher net CO2 assimilation rate (A(CO2)) compared to the WW controls and the PRD wet side. Leaves in both halves of PRS plants were lower ACO(2) than in WW plants. PRD resulted in water savings without loss of leaf function, but the mechanism that regulates stomatal closure and transpiration appeared to be independent of ABA concentrations in roots.