Comparison of deficit and saline irrigation strategies to confront water restriction in lemon trees grown in semi-arid regions

The physiological and agronomic responses to two irrigation strategies - regulated saline irrigation (RSI) and regulated deficit irrigation (RDI), intended to confront water restriction - were compared in 15-year-old 'Fino 49' lemon trees (Citrus limon (L.) Burm. fil.) grafted on Citrus macrophylla Wester. Three independent treatments were applied: Control (100% ETc, non-saline water); RDI (25% ETc, non-saline water) and RSI (145% ETc, saline water-40 mM NaCl). The RDI and RSI treatments were maintained along the crop season except during the high evapotranspiration (ET0) period (corresponding to phase II of fruit growth-cell elongation), when the irrigation dose applied was 100% ETc, with non-saline water. The application of these irrigation strategies produced fresh water savings of 31.5% and 39% for the RDI and RSI treatments, respectively. The use of saline water during the stress periods in RSI trees did not affect the plant water status but decreased leaf photosynthesis, due to high leaf Cl− accumulation, and altered the leaf mineral nutrition; whereas, in RDI trees, the soil water deficit affected negatively the plant water status and, in consequence, the gas exchange parameters. In RSI trees, the cumulative salt stress decreased yield much more than vegetative growth, while in RDI trees the yield and vegetative growth reductions were related to the irrigation water savings. The total production was affected similarly by both treatments, but the yield reduction was greater in RSI than in RDI trees in the second year. Fruit quality was not affected significantly by RSI but the effects of RDI delayed fruit maturation, based on the smaller fruit diameter, lower juice content and higher titratable acidity and total soluble solids relative to the fruits of control trees. Therefore, based on these results, RDI would be the best irrigation strategy for a long water restriction period, while RSI could be successful for a period of not more than one year.