Plant Physiol, November 2001, Vol. 127, pp. 1204-1211

Growth in Elevated CO₂ Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition¹

Department of Biological Sciences, John Tabor Laboratories, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, United Kingdom

Biochemically based models of C₃ photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO₂ partial pressure (pCO₂) will increase light-saturated linear electron flow through photosystem II (J_t). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J_c) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J_o). Where elevated pCO₂ increases J_t, then the ratio of absorbed energy dissipated photochemically to that dissipated non-photochemically will rise. These predictions were tested on Dactylis glomerata grown in fully controlled environments, at either ambient (35 Pa) or elevated (65 Pa) pCO₂, and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J_t was significantly higher in plants grown and measured at elevated pCO₂ than for plants grown and measured at ambient pCO₂. This was due to a significant increase in J_c exceeding any suppression of J_o. This increase in photochemistry at elevated pCO₂ protected against photoinhibition at high light. For plants grown at low nitrogen, J_t was significantly lower in plants grown and measured at elevated pCO₂ than for plants grown and measured at ambient pCO₂. Elevated pCO₂ again suppressed J_o; however growth in elevated pCO₂ resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J_c. Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO₂.