The Role of Carbohydrates in Active Osmotic Adjustment in Apple under Water Stress

Greenhouse grown 2-year-old potted `Jonathan' apple trees (Malus domestica Borkh.) were subjected to various levels of water stress in February. Midday leaf water potential (ψ W ), leaf osmotic potential (ψ S ), soluble sugars, and starch contents of mature leaves were measured throughout the development of water stress to determine whether active osmotic adjustment could be detected and whether carbohydrates were involved. Active adjustments of 0.6 MPa were observed 3 and 5 days, respectively, after water stress was initiated. Leaf turgor potential (ψ P ) could not be maintained through the osmotic adjustment when ψ W dropped below -1.6 MPa. Sorbitol, glucose, and fructose concentrations increased while sucrose and starch levels decreased significantly as water stress developed, strongly suggesting that sugar alcohol and monosaccharide are the most important osmotica for adjustment. Sorbitol was a primary carbohydrate in the cell sap and accounted for > 50% of total osmotic adjustment. The partitioning of newly fixed W-labeled photosynthates in mature leaves was not affected by water stress immediately after the 30-min 14 CO 2 treatment. All the W-labeled carbohydrates decreased in the labeled leaves very rapidly after 14 CO 2 labeling. The decrease in 14 C-sorbitol was greater than the decrease in other carbohydrates under both well-watered and stressed conditions. After 24 hours of water stress, however, the percentage of 14 C-sorbitol increased while the percentages of sucrose, starch, glucose, and fructose decreased significantly with increasing levels of stress. The ratio of 14 C-sorbitol in leaves with ψ W = -3.5 MPa to leaves with ψ W = -0.5 MPa was significantly higher than that of 14 C-sucrose, 14 C-glucose, W-fructose, or 14 C-starch.