Water Relations and Related Leaf Characteristics of Healthy and Blight Affected Citrus Trees1

Abstract Citrus blight or young tree decline, is a wilt-like disease of unknown etiology which is characterized by restricted water movement and an upset in normal zinc distribution patterns. Diurnal leaf and fruit water potentials and leaf stomatal conductances of sweet orange Citrus sinensis (L.) Osbeck leaves on trees in various stages of decline were characterized to determine the progression of this disorder. All blight affected trees, regardless of severity of tree condition, had similar diurnal water relations. Blight affected trees have fewer and smaller leaves, less leaf area per tree, lower stomatal conductances, and lower diurnal transpiration rates than healthy trees. These differences did not result in any apparent changes in specific leaf weight, leaf osmotic potentials or in the critical leaf water deficits at which leaf turgor was lost. At equivalent transpirational fluxes, leaf water potential was much lower in blight affected trees than in healthy trees. Therefore, the water stress symptoms associated with blight are related to increased resistances in the water transport system and are not a result of lost stomatal function or changes in water relations characteristics of leaves that remain on blight affected trees.