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 (psi(w)), leaf osmotic potential (psi(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 (psi(p)) could not be maintained through the osmotic adjustment when psi(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 osmotics for adjustment. Sorbitol was a primary carbohydrate in the cell sap and accounted for >50% of total osmotic adjustment. The partitioning of newly fixed C-14-labeled photosynthates in mature leaves was not affected by water stress immediately after the 30-min (CO2)-C-14 treatment. All the C-14-labeled carbohydrates decreased in the labeled leaves very rapidly after (CO2)-C-14 labeling. The decrease in C-14-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 C-14-sorbitol increased while the percentages of sucrose, starch, glucose, and fructose decreased significantly with increasing levels of stress. The ratio of C-14-sorbitol in leaves with psi(w) = -3.5 MPa to leaves with psi(w) = -0.5 MPa was significantly higher than that of C-14-sucrose, C-14-glucose, C-14-fructose, or C-14-starch.