Diurnal and seasonal changes in the impact of CO(2) enrichment on assimilation, stomatal conductance and growth in a long-term study of Mangifera indica in the wet-dry tropics of Australia

We studied assimilation, stomatal conductance and growth of Mangifera indica L. saplings during long-term exposure to a CO(2)-enriched atmosphere in the seasonally wet-dry tropics of northern Australia. Grafted saplings of M. indica were planted in the ground in four air-conditioned, sunlit, plastic-covered chambers and exposed to CO(2) at the ambient or an elevated (700 micro mol mol(-1)) concentration for 28 months. Light-saturating assimilation (A(max)), stomatal conductance (g(s)), apparent quantum yield (phi), biomass and leaf area were measured periodically. After 28 months, the CO(2) treatments were changed in all four chambers from ambient to the elevated concentration or vice versa, and A(max) and g(s) were remeasured during a two-week exposure to the new regime. Throughout the 28-month period of exposure, A(max) and apparent quantum yield of leaves in the elevated CO(2) treatment were enhanced, whereas stomatal conductance and stomatal density of leaves were reduced. The relative impacts of atmospheric CO(2) enrichment on assimilation and stomatal conductance were significantly larger in the dry season than in the wet season. Total tree biomass was substantially increased in response to atmospheric CO(2) enrichment throughout the experimental period, but total canopy area did not differ between CO(2) treatments at either the first or the last harvest. During the two-week period following the change in CO(2) concentration, A(max) of plants grown in ambient air but measured in CO(2)-enriched air was significantly larger than that of trees grown and measured in CO(2)-enriched air. There was no difference in A(max) between trees grown and measured in ambient air compared to trees grown in CO(2)-enriched air but measured in ambient air. No evidence of down-regulation of assimilation in response to atmospheric CO(2) enrichment was observed when rates of assimilation were compared at a common intercellular CO(2) concentration. Reduced stomatal conductance in response to atmospheric CO(2) enrichment was attributed to a decline in both stomatal aperture and stomatal density.