Growth in Elevated CO(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(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO(2) partial pressure (pCO(2)) 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 pCO(2) 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) pCO(2), 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 pCO(2) than for plants grown and measured at ambient pCO(2). This was due to a significant increase in J(c) exceeding any suppression of J(o). This increase in photochemistry 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 ambient pCO(2). Elevated pCO(2) again suppressed J(o); however growth in elevated pCO(2) 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 pCO(2).

基于生物化学原理的C3光合作用模型可用于预测：当光合作用受核酮糖二磷酸羧化酶/加氧酶（Rubisco）含量限制时，升高大气CO₂分压（pCO₂）会提升光饱和状态下光系统II（Photosystem II）的线性电子流速率（J(t)）。这是因为电子流向光合碳还原循环（J(c)）的促进效应，大于其向光呼吸碳氧化循环（J(o)）所受到的竞争性抑制效应。若升高的pCO₂提升了J(t)，则光化学耗散的吸收光能与非光化学耗散的吸收光能的比值会升高。上述预测在完全可控环境中培养的鸭茅（Dactylis glomerata）上得到了验证，实验设置了两种CO₂分压条件：环境CO₂分压（35 Pa）与升高CO₂分压（65 Pa），同时设置了两个氮营养水平。正如预测所示，在高氮条件下培养的鸭茅，在升高CO₂分压环境中培养并测定的植株，其J(t)显著高于在环境CO₂分压环境中培养并测定的植株。这一现象源于J(c)的显著提升幅度超过了J(o)所受到的所有抑制作用。升高CO₂分压带来的光化学活性提升，可抵御高光强环境下的光抑制现象。对于低氮条件下培养的鸭茅，在升高CO₂分压环境中培养并测定的植株，其J(t)则显著低于在环境CO₂分压环境中培养并测定的植株。升高CO₂分压同样对J(o)产生了抑制作用；然而，在升高CO₂分压环境中培养会引发叶片Rubisco含量的驯化性下降，从而抵消了J(c)原本可获得的所有促进效应。与光化学活性下降的结果一致，低氮培养的鸭茅叶片在经过3小时光抑制处理后，在升高CO₂分压环境中的恢复速率更为缓慢。