The influence of enriched atmospheric CO₂ concentration on gas exchange, water relations and growth of the tropical tree species Mangifera indica, cv Irwin

Abstract

The concentration of atmospheric CO₂ is increasing because of anthropogenic activities. Elevated atmospheric CO₂ is a major contributor to the enhanced greenhouse effect and global climate change. Carbon dioxide is central to the physiology of plants. Few field studies have been undertaken over several growing seasons in the tropics. This study investigates the long-term impact (28 months) of CO₂ enrichment (700 µmol mol-1) upon assimilation, stomatal conductance (gs), water relations and growth of grafted Mangifera indica L. (cv Irwin) trees growing in the ground in temperature controlled chambers in the wet-dry tropics of Australia. Investigation of source-sink relations through manipulation of fruit set, shoot pruning and reciprocal transfer experiments was undertaken to assess the importance of source-sink relations in determining the assimilation response of M. indica.

There was no evidence of downward acclimation of assimilation over the 28 month period of study. Light saturated assimilation (A_max) was increased by approximately 20%, although the percentage stimulation of A_max was largest in the dry season. Similarly, apparent quantum yield (ф) was increased by approximately 33% and the percentage stimulation of ф was largest in the dry season compared to the wet season. Stomatal conductance was reduced in response to C0₂ enrichment. However, the relative impact of C0₂ enrichment on g_s was larger in the wet season compared to the dry season. A_max and ф were reduced in the afternoon compared to the morning irrespective of treatment, but the reduction was largest for trees grown under CO₂-enriched conditions. The diurnal response of assimilation was found to be largely independent of gs and plant water status. 
Maintenance of enhanced A_max throughout the study was reflected in continued enhancement of trees growth with CO₂ enrichment. Canopy area and canopy dry weight initially increased with CO₂ enrichment but were not significantly different between treatments by the end of the 28 month study. 
Tree water status was enhanced by CO₂ enrichment in the dry season. In addition whole plant hydraulic conductance was decreased by CO₂ enrichment. However, tree water status was essentially close to zero at all times, and so was unlikely to have had a significant impact on the response of assimilation to CO₂ enrichment. The maintenance of an active sink (either through seasonal alternation of dry matter allocation to the roots or shoots, developing fruit, or shoot regrowth following pruning) was central to the continued enhancement of assimilation in response to CO₂ enrichment.

Date of Award	Dec 1997
Original language	English
Supervisor	Derek Eamus (Supervisor)