A/Ci curve
In order to evaluate the contribution of photorespiration, carbon assimilation rates were measured as a function of the intercellular CO2 (A/C i curve) under 21% and 2% oxygen (Fig. 7a). In 21% O2, the transgenic plants had higher photosynthetic capacity (the maximum rate of photosynthesis under CO2 saturation at saturating light) than the VC plants. At saturating C i(~1000 ppm), CO2 assimilation rate increased by ~19% in CAx3 and ~22% in CAx5 transgenic plants than in the VC (Fig. 7a ). The carboxylation efficiency (CE ), as calculated from the initial slope of A/C i curve (i.e. between 50 and 200 µmol mol-1 CO2), in VC was 0.038 mol m–2 s–1. It increased (by ~17%) to 0.044 mol m–2s–1 in the transgenics under 21% O2. Further, there was no significant difference in the CO2compensation point of the VC and the transgenic plants. Similarly, theV cmax (Bernacchi et al., 2001) was 34 µmol m-2 s-1 for VC and the same increased to 40 µmol m-2 s-1 i.e., by ~18% in transgenics. The electron transport rateJ max, derived from theA/C i curve, was 67 µmol m-2s-1 in VC and it increased to 84 µmol m-2 s-1 i.e., by ~18% in the overexpressors.
In 2% oxygen, the maximum rate of CO2 assimilation at saturating CO2 was similar to that under 21% O2 in all plants. However, the net CO2 assimilation was 22% –25% higher in the transgenics than in the VC. The CE significantly increased with the decrease in the O2 level, from 21% to 2%. TheCE in VC was 0.045 mol m–2s–1 and it increased to 0.054-0.056 mol m–2 s–1 (19%-23%) in the transgenics under 2% O2.