Differential photosynthetic response of two mediterranean species (Arbutus unedo and Viburnum tinus) to sulphur dioxide

The responses of young plants of Arbutus unedo and Viburnum tinus to long-term (80120 days) exposure of sulphur dioxide were investigated. Constant levels of the pollutant of 25 and 60 ppb were supplied to the plants, in comparison to charcoal-filtered air. At regular intervals, the photosynthetic responses were analyzed on fully expanded leaves developed during the fumigation. No visible symptoms were detected on the leaves of the treated plants. The photosynthetic activity and growth of A. unedo was markedly reduced in connection to a partial stomatal closure. In V. tinus, CO2 assimilation did not change although stomatal conductance significantly decreased at the end of treatment. A mechanism of repair was supposed. Results are discussed on the basis of natural drought stress resistance of plants typical of mediterrannean environments.