OSMOTIC ADJUSTMENT IN FRAXINUS-EXCELSIOR L - MALATE AND MANNITOL ACCUMULATION IN LEAVES UNDER DROUGHT CONDITIONS

In leaves of Fraxinus excelsior L., malate and mannitol were character ized by C-13 NMR spectroscopy and enzymatic specific assays as the maj or constituents of a soluble carbon fraction involved in an osmotic ad justment. During a summer drought where predawn leaf water potential o f adult trees growing in a mesoxerophilic stand fell to - 4 MPa in Aug ust, malate and mannitol leaf contents increased by a factor of 1.8 an d 2.2 respectively, compared to control trees growing on a flood plain . This drought stress led to concentrations as high as 280 mM and 600 mM for mannitol and malate, respectively. The effects of gradually dev eloping water deficit were also studied in a semi-controlled environme nt in 3-year-old seedlings. When predawn leaf water potential reached -6 MPa, leaves displayed a low turgor pressure but stomatal conductanc e was still measurable. Malate and mannitol were also the main osmotic um involved. After rewatering, gas exchange capacities were largely re stored. Altogether, these results show that the strong water-stress to lerance of Fraxinus excelsior is in part related to an accumulation of malate and mannitol.