Effects of water stress on sorbitol production and anatomical changes in the nuclei of leaf and root cells of young loquat trees

The effects of water stress on the production of sorbitol in the leaves and roots of one-year-old grafted loquat trees were investigated. Concurrently anatomical changes in the nuclei of the leaves and the root apical meristem were followed by staining with 4'-6-diamidino-2-phenylindole (DAPI) and observing the results by fluorescence microscopy. The degree of water stress was graded into three levels: a) unstressed plots (control), b) moderately stressed plots (m-plots, no irrigation period for 20 days), and c) severely stressed plots (s-plots, no irrigation period for 25 days). The degree of water stress was estimated by the water saturated deficiency (WSD) of the leaves. With an increase in WSD, the sorbitol content in the leaves and roots increased, the accumulation occurring in the roots. Although WSD for the s-plots was higher than that for the m-plots, there was no significant difference in sorbitol content in leaves and roots between the two treatments. After recovery from water stress, the sorbitol content decreased gradually in roots and leaves. Leaves of m-plots abscised little after irrigation was resumed, whereas leaves of s-plots abscised severely; only a few trees died. New roots developed in control plants, but not in stressed-plants. Water stress resulted in an acceleration of nuclear degradation and a decrease in starch accumulation in the mesophyll parenchyma and in the apical meristem from the root tip to 2-3 mm.