Irrigation management and rootstock effects on navel orange [Citrus sinensis (L.) Osbeck] fruit quality

Bellamy navel orange trees grafted on five rootstocks within an orchard in south-western NSW, Australia, were irrigated with low-level sprinklers using three irrigation strategies during the 1999/2000 and 2000/2001 seasons to determine the effects of deficit irrigation (DI) and partial rootzone drying (PRD) on fruit development and quality. Treatments were 10 ML ha−1 season−1 of water applied on both sides of the tree at each irrigation event (control—C), 5.9 ML ha−1 season−1 also applied across the orchard floor (DI) and 5.5 ML ha−1 season−1 applied by alternately wetting each side of the tree during alternate irrigation events (PRD). Mean crop load based on the weight and number of fruit per canopy m2 decreased in the second season (from 7.5 kg and 43 fruit to 4.4 kg and 34 fruit m−2), across all irrigation treatments. Across seasons, trees on citrange and trifoliate orange rootstocks were most affected. Fruit size and fresh weight were significantly affected by water volume, but not irrigation strategy at Stage I/Stage II transition and colour break, with C fruit being 4% larger and weighing 10% more than the DI and PRD fruit. At maturity, fruit from the PRD treatment were the smallest and weighed the least, while fruit from the C were the largest and weighed the most. Total soluble solids and titratable acidity in juice at maturity were significantly higher in the DI and PRD fruit (average 14.4 °Brix, 1.27 g L−1 titratable acidity) compared with C fruit (13.1 °Brix, 1.12 g L−1 titratable acidity). Fruit rind thickness varied depending on rootstock and sampling time. Reduced water volumes were associated with thinner rinds at the end of the season and lower incidences of moderate and severe symptoms of albedo breakdown.