Response of orchard 'Washington Navel' orange, Citrus sinensis (L.) Osbeck, to saline irrigation water. I. Canopy characteristics and seasonal patterns in leaf osmotic potential, carbohydrates and ion concentrations

Abstract

Effects of irrigation water salinity on tree canopy volume, leaf area, rates of leaf abscission and production, as well as seasonal patterns in leaf osmotic potential (=), starch, soluble sugars and sodium and chloride concentrations were determined for 24-year-old 'Washington Navel' orange trees (Citrus sinensis [L.] Osbeck) on sweet orange (C. sinensis) rootstock. Trees had been irrigated with water containing either 5 or 20 mol NaCl m-3 for 5 years prior to measurements.Trees irrigated with 20 mol NaCl m-3 had a greater number of vegetative flushes in spring. This occurred at the expense of flowering, as numbers of reproductive and mixed flushes were reduced by salinity. Despite a high number of vegetative buds on trees irrigated with 20 mol NaCl m-3, leaf area was still less than low salinity trees.Extensive abscission of spring flush leaves occurred from mid-summer onwards for trees irrigated with 20 mol NaCl m-3. This was not a consequence of leaf water deficit, as more negative leaf osmotic potentials resulting from increased foliar sodium and chloride concentrations resulted in maintenance of leaf turgor. Excessive concentrations of sodium and/or chloride may have been responsible for abscission observed. Some acclimation of foliage to salinity was evident.Irrespective of salinity treatment, leaf osmotic potential became more negative as the season progressed. This was partly due to increased concentrations of soluble sugars in foliage during autumn and winter. Levels of soluble sugars and starch were consistently lower in leaves on trees irrigated with high salinity water, indicating that production rather than utilization of carbohydrate may limit citrus productivity under saline conditions.