OSMOTIC ADJUSTMENT - EFFECT OF WATER-STRESS ON CARBOHYDRATES IN LEAVES, STEMS AND ROOTS OF APPLE

Potted apple (Malus domestica Borkh. cv. Jonathan) trees were subjecte d to water stress in a greenhouse. Midday leaf water potential (psi(W) ), osmotic potential (psi(S)), soluble carbohydrates, and starch conte nt of expanding and mature leaves, stems, and roots were measured to d etermine whether active osmotic adjustment occurred and if water stres s affected carbohydrate metabolism. Mature leaves had the highest tota l soluble carbohydrate level (357 mM) and lowest psi(S) (-1.85 MPa), f ollowed by young leaves (278 mM, -1.58 MPa), stems (115 mM, -1.02 MPa) , and roots (114 mM, -0.87 MPa). Sorbitol was the major component in a ll organs ranging from 53% of total soluble carbohydrate in young leav es to 73% in mature leaves. When psi(W) decreased from -1.0 to -3.2 MP a, active osmotic adjustments of 0.3-0.4 MPa were observed in mature l eaves, stems, and roots white a significantly higher adjustment of 1.0 MPa was detected in young leaves 5 days after the initiation of water stress. Sorbitol levels in leaves and stems gradually increased as ps i(W) decreased from -1.0 to -2.5 MPa, and then remained relatively sta ble or decreased slightly as psi(W) decreased from -2.5 to -3.2 MPa. H owever, the percentage of soluble carbohydrate as sorbitol in roots de creased in response to water stress. Sucrose concentration decreased i n mature leaves and stems, but increased in young leaves and roots as psi(W) decreased. Starch concentrations in stems and roots also decrea sed as water stress developed. The sorbitol to sucrose ratios increase d in mature leaves, but decreased in roots in response to water stress .