Diurnal and seasonal changes in the impact of CO2 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 CO2-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 CO2 at the ambient or an elevated (700 μmol mol−1) concentration for 28 months. Light-saturating assimilation (Amax), stomatal conductance (gs), apparent quantum yield (Φ), biomass and leaf area were measured periodically. After 28 months, the CO2 treatments were changed in all four chambers from ambient to the elevated concentration or vice versa, and Amax and gs were remeasured during a two-week exposure to the new regime.Throughout the 28-month period of exposure, Amax and apparent quantum yield of leaves in the elevated CO2 treatment were enhanced, whereas stomatal conductance and stomatal density of leaves were reduced. The relative impacts of atmospheric CO2 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 CO2 enrichment throughout the experimental period, but total canopy area did not differ between CO2 treatments at either the first or the last harvest.During the two-week period following the change in CO2 concentration, Amax of plants grown in ambient air but measured in CO2-enriched air was significantly larger than that of trees grown and measured in CO2-enriched air. There was no difference in Amax between trees grown and measured in ambient air compared to trees grown in CO2-enriched air but measured in ambient air. No evidence of down-regulation of assimilation in response to atmospheric CO2 enrichment was observed when rates of assimilation were compared at a common intercellular CO2 concentration. Reduced stomatal conductance in response to atmospheric CO2 enrichment was attributed to a decline in both stomatal aperture and stomatal density