Long-term effects of saline irrigation water on growth, yield, and fruit quality of Valencia orange trees

Citrus is regarded as a salt-sensitive crop, but its yield response to salinity is affected by variety, rootstock, duration of salt exposure, irrigation management, soil type, and climate. This study quantified the yield response of mature Valencia [Citrus sinensis (L. Osbeck)] orange trees on sweet orange (C. sinensis) rootstock to increased levels of sodium chloride in irrigation water in the Sunraysia area of the Murray Valley in south-eastern Australia. The orchard was planted on a loamy sand and trees were irrigated and fertilised with a well-managed under-tree microsprinkler system. Four levels of salt, ranging from the river-water control (0.44 dS/m) to 2.50 dS/m, were applied over a 9-year period. Overall yield effects were smaller than expected, and did not conform well to the often used bent-stick model. Relative to the control, yield was initially higher (by up to 9%) in the intermediate salt treatments, and 3% lower in the highest treatment. However, relative yields of salinised trees decreased with time, and in the final year of the experiment, yield of the highest salt treatment was 9% lower than the control. Yield increases in the intermediate treatments resulted from increases in fruit number. All 3 salt treatments decreased average fruit weight by 4% and decreased juice content but increased juice sugar and acid content. Salt treatment strongly reduced trunk growth, and the effect increased with time. Our results show that with appropriate irrigation management, soils, and rootstocks, citrus trees can maintain productivity at salinity levels of 2.0 dS/m or more, but fresh fruit profitability is likely to be lower because of a reduction in average fruit size.