Partitioning of [¹⁴C]Glucose Into Sorbitol and Other Carbohydrates in Apple Under Water Stress

Abstract

Sorbitol plays an important role in osmotic adjustment in mature apple leaves under water stress. This study was conducted to determine whether water stress increases the conversion of glucose to sorbitol in mature apple leaves. A solution of [¹⁴C]glucose or [¹⁴C]sorbitol was introduced into the cut end of detached apple (Malus domestica Borkh. 'Red Jonathan') shoots which had previously experienced either water stress or no stress. The cut shoots were then placed in sterile deionised water to maintain well-watered conditions or in no water to continue water-stressed conditions. When shoots were labelled with [¹⁴C]glucose, 38% of [¹⁴C]glucose was recovered as glucose in the leaves at a leaf water potential (Ψ_w) of -1.0 MPa following a 30-min labelling. The remaining [¹⁴C]glucose was converted to sucrose (24%), fructose (21%), and sorbitol (17%). Water stress altered the partitioning of [¹⁴C]glucose between sorbitol and sucrose, increasing the ratio from 0.8 at Ψ_w = -1.0 to 1.7 at Ψ_w = -3.0 MPa. When shoots were supplied with [¹⁴C]sorbitol, <10% of [¹⁴C]sorbitol was converted to other soluble carbohydrates. Water stress inhibited the conversion of both [¹⁴C]glucose and [¹⁴C]sorbitol into starch. The results suggest that sorbitol accumulation may result from the preferential conversion of glucose to sorbitol rather than to sucrose and starch.