STOMATAL RESPONSES OF VARIEGATED LEAVES TO CO2 ENRICHMENT

The responses of stomatal density and stomatal index of five species of ornamental plants with variegated leaves grown at two mole fractions of atmospheric CO2 (350 and 700 mu mol mol(-1)) were measured. The use of variegated leaves allowed any potential effects of mesophyll photosynthetic capacity to be uncoupled from the responses of stomatal density to changes in atmospheric CO2 concentration. There was a decrease in stomatal density and stomatal index with CO2 enrichment on both white (unpigmented) and green (pigmented) leaf areas. A similar response of stomatal density and index was also observed on areas of leaves with pigmentation other than green indicating that any differences in metabolic processes associated with coloured leaves are not influencing the responses of stomatal density to CO2 concentrations. Therefore the carboxylation capacity of mesophyll tissue has no direct influence on stomatal density and index responses as suggested previously (Friend and Woodward 1990 Advances in Ecological Research 20: 59-124), instead the responses were related to leaf structure. The stomatal characteristics (density and index) of homobaric variegated leaves showed a greater sensitivity to CO2 on green portions, whereas heterobaric leaves showed a greater sensitivity on white areas. These results provide evidence that leaf structure may play an important role in determining the magnitude of stomatal density and index responses to CO2 concentrations.