INVOLVEMENT OF MALATE AND MANNITOL IN THE DIURNAL REGULATION OF THE WATER STATUS IN MEMBERS OF OLEACEAE

This study examines water status regulation in plants of the Oleaceae family and in some other co-occurring species that are exposed to high solar radiation, in the same habitat. Fraxinus excelsior L., one of : he most studied Oleaceae in this field exhibited, during the growing s eason, a close relationship between diurnal variations in leaf water p otential and changes in malate, mannitol and K+ levels, depending on t he weather conditions. On sunny days, similar variations can be observ ed in leaves of the other Oleaceae, with a concomitant decrease in the osmotic potential between predawn and solar noon. Malate, mannitol an d the well-known osmoticum K+, contribute greatly to the osmotic poten tial decrease. This mechanism, which can be related to the osmotic adj ustment described for both drought and salt-affected plants, appears a s a general response in plants of the Oleaceae family. Among the other co-occurring species investigated, only Quercus robur L. displayed a similar mechanism under the same environmental conditions, but two oth er organic compounds, quinic and shikimic acids, are presumably involv ed. Alnus glutinosa (L.) Gaertn. and Robinia pseudacacia L. responded to a vapor deficit by partial stomatal closure, as transpiration progr essed through the morning.