Oxygen and Carbon Dioxide Effects on the Pool Size of Some Photosynthetic and Photorespiratory Intermediates in Soybean (Glycine max [L.] Merr.)

The levels of ribulose 1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), glycolate, glycine, and serine were measured in soybean leaflets during photosynthesis in atmospheres ranging from 1 to 60% O(2) and from 0 to 500 microliters per liter CO(2.)The RuBP level remained constant as CO(2) concentration was decreased in atmospheres containing 20 or 60% O(2), but increased as CO(2) concentration was decreased in atmospheres containing 1% O(2.) PGA levels decreased at CO(2) concentrations near or below the CO(2) compensation point under all O(2) concentrations. The glycolate pool at 300 microliters per liter CO(2) increased slightly with increasing O(2) concentration, but remained nearly constant at very low CO(2). The serine pool showed no measurable change over the range of CO(2) or O(2) concentrations tested. The glycine pool did not change significantly with varying CO(2) concentration but increased linearly with increasing O(2) concentration.Measured RuBP levels indicate an RuBP concentration less than the estimated concentration of RuBP carboxylase/oxygenase active sites. The constant RuBP pool size in 20% O(2), however, indicates that RuBP level does not limit photosynthesis or photorespiration any more at 50 microliters per liter CO(2) than at 450 microliters per liter.