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 atmosph ere in the seasonally wet-dry tropics of northern Australia. Grafted s aplings of M. indica were planted in the ground in four air-conditione d, sunlit, plastic-covered chambers and exposed to CO2 at the ambient or an elevated (700 mu mol mol(-1)) concentration for 28 months. Light -saturating assimilation (A(max)), stomatal conductance (g(s)), appare nt quantum yield (phi), biomass and leaf area were measured periodical ly. After 28 months, the CO2 treatments were changed in all four chamb ers from ambient to the elevated concentration or vice versa, and A(ma x) and g(s) were remeasured during a two-week exposure to the new regi me. Throughout the 28-month period of exposure, A(max) and apparent qu antum yield of leaves in the elevated CO2 treatment were enhanced, whe reas stomatal conductance and stomatal density of leaves were reduced. The relative impacts of atmospheric CO2 enrichment on assimilation an d stomatal conductance were significantly larger in the dry season tha n in the wet season. Total tree biomass was substantially increased in response to atmospheric CO2 enrichment throughout the experimental pe riod, but total canopy area did not differ between CO2 treatments at e ither the first or the last harvest. During the two-week period follow ing the change in CO2 concentration, A(max) of plants grown in ambient air but measured in CO2-enriched air was significantly larger than th at of trees grown and measured in CO2-enriched air. There was no diffe rence in A(max) between trees grown and measured in ambient air compar ed 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 conducta nce in response to atmospheric CO2 enrichment was attributed to a decl ine in both stomatal aperture and stomatal density.