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 nutrition

Biochemically based models of C-3 photosynthesis can be used to predict tha t when photosynthesis is limited by the amount of Rubisco, increasing atmos pheric CO2 partial pressure (PCO2) will increase light-saturated linear ele ctron flow through photosystem II (J(t)). This is because the stimulation o f electron flow to the photosynthetic carbon reduction cycle (J(c)) will be greater than the competitive suppression of electron flow to the photoresp iratory carbon oxidation cycle (J(o)). Where elevated pCO(2) increases J(t) , then the ratio of absorbed energy dissipated photochemically to that diss ipated non-photochemically will rise. These predictions were tested on Dact ylis glomerata grown in fully controlled environments, at either ambient (3 5 Pa) or elevated (65 Pa) pCO(2) and at two levels of nitrogen nutrition, A s was predicted, for D. glomerata grown in high nitrogen, J(t) was signific antly higher in plants grown and measured at elevated pCO(2) than for plant s grown and measured at ambient pCO(2) This was due to a significant increa se in J(c) exceeding any suppression of J(o). This increase in photochemist ry at elevated pCO(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J(t) was significantly lower in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambie nt pCO(2), Elevated PCO2 again suppressed J(o); however growth in elevated pCO(2) resulted in an acclimatory decrease in leaf Rubisco content that rem oved any stimulation of J(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory trea tment was slower at elevated pCO(2).