High boron in irrigation water mitigates drought stress effects on 'Carrizo' citrange seedlings

Cultivating citrus in semi-arid areas exposes them to drought periods and sometimes irrigation with non-conventional water sources (e.g. reclaimed water and desalinated seawater) containing high boron (B) concentration. This study aims to determine how high B influences plant water relations and water transport in well-watered (WW) and drought-stressed (DS) ‘Carrizo’ citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata L.] seedlings during a stress period and after rewatering. Seedlings were well-watered with control B (0.11 mg·L⁻¹) or high B concentration (5 mg·L⁻¹) for 29 days. Then irrigation was withheld from half the plants for 7 days (drought-stressed, DS), followed by a 10-day re-irrigation period using control B. High B irrigation increased endogenous B accumulation, without reaching toxic levels as chlorophyll fluorescence parameters remained unaltered and plant water status was unaffected. However, partial stomatal closure mediated by a 1.8-fold increase in foliar ABA accumulation decreased transpiration and photosynthesis. Under drought stress, plants irrigated with high B concentration better maintained their leaf water status, with leaf water potential decreasing by only 0.9 MPa compared to 1.8 MPa decrease in Control B-DS plants relative to their respective well-watered plants. Thus, high B enhanced drought tolerance through ABA-mediated stomatal regulation, allowing more conservative water use during drought. Plant water status of high B plants recovered faster and more completely than control B plants, which experienced greater drought-induced damage.