Timing irrigation to suit citrus phenology: a means of reducing water use without compromising fruit yield and quality?

This paper addresses the question of whether a citrus crop has the same need for water at all stages of development or whether it is possible to withhold water at times when the crop is less sensitive to water stress, thus, reducing total water use and improving water use efficiency while still maintaining yield. To answer this question water applied by irrigation was reduced by up to 33% relative to standard full irrigation by extending the intervals between applications from 3 to 17 days during fruit growth stages II and III in the annual growth cycle. As expected, the longer intervals resulted in greater depletion in soil moisture and significant water stress developed as soil water deficits approached the lower limits of plant available water. Stressed trees exhibited mean pre-dawn water potential (psi(1)) values of - 0.93 MPa and midday Psi(1) values decreased to between - 2.0 and - 2.5 MPa. Periodic soil water deficits in late summer and autumn reduced shoot growth, but fruit yield was unaffected, and there was no evidence of reduced canopy size. Water use efficiency ( mass of fruit produced per unit water applied) improved, but fruit growth was extremely sensitive to moisture stress and extended irrigation intervals in summer and autumn reduced fruit size. Fruit juice quality was also affected, as there was an increase in both total soluble solids and juice acidity, but the practical consequences of these were limited because there were only small changes to the sugar : acid ratios. This work has demonstrated that deficient irrigation during summer can be used to manipulate growth and reduce water use, but at the risk of a marginal reduction in fruit size.