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

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Tree Physiology, 17:291–299

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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

John Goodfellow, D. Eamus and G. Duff

School of Biological Sciences, Northern Territory University, Darwin, NT 0909, Australia / Received December 14, 1995

Summary

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 (A_max), stomatal conductance (g_s), 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 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 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, A_max 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 A_max 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.

Keywords: acclimation, apparent quantum yield, down-regulation, elevated CO₂, gas exchange.