EFFECTS OF ATMOSPHERIC CO2 ENRICHMENT ON PHOTOSYNTHESIS, RESPIRATION, AND GROWTH OF SOUR ORANGE TREES

Numerous net photosynthetic and dark respiratory measurements were made over a period of 4 years on leaves of 24 sour orange (Citrus aurantium) trees; 8 of them growing in ambient air at a mean CO2 concentration of 400 microliters per liter, and 16 growing in air enriched with CO2 to concentrations approaching 1000 microliters per liter. Over this CO2 concentration range, net photosynthesis increased linearly with CO2 by more than 200%, whereas dark respiration decreased linearly to only 20% of its initial value. These results, together with those of a comprehensive fine-root biomass determination and two independent above-ground trunk and branch volume inventories, suggest that a doubling of the air's current mean CO2 concentration of 360 microliters per liter would enhance the growth of the trees by a factor of 3.8.