| Abstract: | During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO2 per liter air, was 1.45 milligrams CO2 evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO2 per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO2 enriched compared with CO2 normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO2 enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO2 enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO2 adapted plants, but not in the control plants. Mitochondria, isolated simultaneously from the leaves of CO2 enriched and control plants, showed no difference in NADH or malate-glutamate dependent O2 uptake, and there were no observed differences in the specific activities of NAD+ linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O2 uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO2 enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status. |
