RESUMEN
Methanol emissions from several deciduous tree species with predominantly mature leaves were measured under laboratory and field conditions. The emissions were modulated by temperature and light. Under constant light conditions in the laboratory, methanol emissions increased with leaf temperature, by up to 12% per degree. At constant temperatures, emissions doubled when light intensity (PAR) increased from darkness to 800 micromol x m(-2) x s(-1). A phenomenological description of light and temperature dependencies was derived from the laboratory measurements. This description was successfully applied to reproduce the diel cycle of methanol emissions from an English oak measured in the field. Labelling experiments with (13)CO(2) provided evidence that less than 10% of the emitted methanol was produced de novo by photosynthesis directly prior to emission. Hence, the light dependence of the emissions cannot be explained by instantaneous production from CO(2) fixation. Additional experiments with selective cooling of plant roots indicated that a substantial fraction of the emitted methanol may be produced in the roots or stem and transported to stomata by the transpiration stream. However, the transpiration stream cannot be considered as the main factor that determines methanol emissions by the investigated plants.
