Growth in elevated CO sub(2) can both increase and decrease photochemistry and photoinhibition of photosynthesis in a predictable manner. Dactylis glomerata grown in two levels of nitrogen nutrition

Biochemically based models of C sub(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO sub(2) partial pressure (pCO sub(2)) will increase light-saturated linear electron flow through photosystem II (J sub(t)). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J sub(c)) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J sub(o)). Where elevated pCO sub(2) increases J sub(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) pCO sub(2), and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J sub(t) was significantly higher in plants grown and measured at elevated pCO sub(2) than for plants grown and measured at ambient pCO sub(2). This was due to a significant increase in J sub(c) exceeding any suppression of J sub(o). This increase in photochemistry at elevated pCO sub(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J sub(t ) was significantly lower in plants grown and measured at elevated pCO sub(2) than for plants grown and measured at ambient pCO sub(2). Elevated pCO sub(2) again suppressed J sub(o); however growth in elevated pCO sub(2) resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J sub(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO sub(2).