Carboxylation Capacity Can Limit C₃ Photosynthesis at Elevated CO₂ throughout Diurnal Cycles

The response of carbon fixation in C₃ plants to elevated CO₂ is relatively larger when photosynthesis is limited by carboxylation capacity (V_C) than when limited by electron transport (J). Recent experiments under controlled, steady-state conditions have shown that photosynthesis at elevated CO₂ may be limited by V_C even at limiting PPFD. These experiments were designed to test whether this also occurs in dynamic field environments. Leaf gas exchange was recorded every 5 min using two identical instruments both attached to the same leaf. The CO₂ concentration in one instrument was controlled at 400 μmol mol^-1 and one at 600 μmol mol^-1. Leaves were exposed to ambient sunlight outdoors, and cuvette air temperatures tracked ambient outside air temperature. The water content of air in the leaf cuvettes was kept close to that of the ambient air. These measurements were conducted on multiple, mostly clear days for each of three species, Glycine max, Lablab purpureus, and Hemerocallis fulva. The results indicated that in all species, photosynthesis was limited by V_C rather than J at both ambient and elevated CO₂ both at high midday PPFDs and also at limiting PPFDs in the early morning and late afternoon. During brief reductions in PPFD due to midday clouds, photosynthesis became limited by J. The net result of the apparent deactivation of Rubisco at low PPFD was that the relative stimulation of diurnal carbon fixation at elevated CO₂ was larger than would be predicted when assuming limitation of photosynthesis by J at low PPFD.