Osmotic Adjustment: Effect of Water Stress on Carbohydrates in Leaves, Stems and Roots of Apple

Abstract

Potted apple (Malus domestica Borkh. cv. Jonathan) trees were subjected to water stress in a greenhouse. Midday leaf water potential (Ψ_W), osmotic potential (Ψ_S), soluble carbohydrates, and starch content of expanding and mature leaves, stems, and roots were measured to determine whether active osmotic adjustment occurred and if water stress affected carbohydrate metabolism. Mature leaves had the highest total soluble carbohydrate level (357 mM) and lowest Ψ (-1.85 MPa), followed 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 all organs ranging from 53% of total soluble carbohydrate in young leaves to 73% in mature leaves. When Ψ_W decreased from -1.0 to -3.2 MPa, active osmotic adjustments of 0.3-0.4 MPa were observed in mature leaves, stems, and roots while 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 Ψ_W decreased from -1.0 to -2.5 MPa, and then remained relatively stable or decreased slightly as Ψ_W decreased from -2.5 to -3.2 MPa. However, the percentage of soluble carbohydrate as sorbitol in roots decreased in response to water stress. Sucrose concentration decreased in mature leaves and stems, but increased in young leaves and roots as Ψ_W decreased. Starch concentrations in stems and roots also decreased as water stress developed. The sorbitol to sucrose ratios increased in mature leaves, but decreased in roots in response to water stress.