AIR-TEMPERATURE MODIFIES THE SIZE-ENHANCING EFFECTS OF ATMOSPHERIC CO2 ENRICHMENT ON SOUR ORANGE TREE LEAVES

Every other month for a period of 2 years, leaf area and dry weight measurements were made on the foliage of sour orange trees growing in ambient air and in air enriched with an extra 300 mul/l CO2. Leaf starch content measurements were made at approximate 2-month intervals for a period of 1 year. The data demonstrated that all three plant parameters were significantly increased by atmospheric CO2 enrichment, except in the coldest portion of the year. A plot of the ratio of CO2-enriched leaf dry weight to ambient-treatment leaf dry weight against the mean air temperature of the preceding month revealed this relationship with temperature to be linear. The relationship shows atmospheric CO2 enrichment to have a negligible effect on leaf dry weight at a mean air temperature of approximately 5-degrees-C. At a mean air temperature of 35-degrees-C, however, it shows individual CO2-enriched leaves of our experiment to weigh 40% more than their ambient-treatment counterparts. This phenomenon helps to explain the vastly different effects of atmospheric CO2 enrichment that have been reported for a number of diverse ecosystems.