Control of photosynthesis in leaves as revealed by rapid gas exchange and measurements of the assimilatory force FA
Control of photosynthesis in leaves as revealed by rapid gas exchange and measurements of the assimilatory force FA
复制标题
通过快速气体交换和同化力 FA 的测量揭示了叶片光合作用的控制
作者：
K. Siebke;A. Laisk;V. Oja;O. Kiirats;Klaus Raschke;Ulrich Heber
测定了向日葵（Helianthus annuusL.）叶片CO2气体交换的快速瞬变过程。在平行实验中的同化力FA，这是磷酸化电位和NADPH/NADP的氧化还原比的产品，已计算出从测量的比率磷酸二羟丙酮在叶绿体基质和叶片磷酸甘油。结果表明：（1）在2000 μl · l ~（-1）CO_2的空气中，在稳态光合作用条件下，光依赖的基质碱化度随光合作用量子通量密度（辐照度）的增加而增加。这与在暗-光瞬变期间在暗适应的叶中测量的光依赖性基质碱化形成对比（Laisk等，1989，Planta 177，350-358），其在远低于饱和光合作用所需的量子通量密度时达到最大值。这一最大值比光和CO2饱和光合作用下的最大基质碱化值高约3倍。(ii)同化力FA的准确计算需要考虑基质的pH值。然而，在许多条件下，由于光合通量的变化而引起的基质pH值的变化可以忽略不计，因为它们很小。(iii)基质中磷酸二羟丙酮与磷酸甘油的比例一般低于叶提取物中测得的比例。由基质代谢物计算的FA值比由细胞代谢物计算的FA值低约30%。尽管如此，从叶提取物中的代谢物测量值计算FA似乎足以满足许多目的。(iv)在光中，光合器的催化能力被调节到辐照度的水平。碳同化对辐照度大幅增加的反应是缓慢的，因为它需要酶的激活。辐照度降低引起的卡尔文循环失活比活化慢。(v)催化能力和底物（如CO2）的可用性或水平的变化会改变卡尔文循环的通量阻力。焊剂电阻的降低解释了为什么当碳通量增加时，FA通常不会增加很多，实际上可能会降低。调节卡尔文循环中的通量阻力和电子传递链中的光系统-II活性允许在低水平的ATP和NADPH下改变光合作用速率。由于NADP仍然可用，导致电子传递光失活的过度还原的危险被最小化。
The rapid transients of CO2gas exchange have been measured in leaves ofHelianthus annuusL. In parallel experiments the assimilatory force FA, which is the product of the phosphorylation potential and the redox ratio NADPH/NADP, has been calculated from measured ratios of dihydroxyacetone phosphate to phosphoglycerate in the chloroplast stroma and in leaves. The following results were obtained: (i) When the light-dependent stroma alkalization was measured under steady-state conditions for photosynthesis in air containing 2000 μl · l-1CO2, alkalization increased with photosynthesis as the quantum flux density (irradiance) was increased. This contrasts to the light-dependent stroma alkalisation measured in dark-adapted leaves during the dark-light transient (Laisk et al. 1989, Planta177, 350–358) which reached a maximum at a quantum flux density far below that necessary to saturate photosynthesis. This maximum was about three times higher than the maximum stroma alkalization at light- and CO2-saturated photosynthesis. (ii) Accurate calculations of the assimilatory force FArequire a consideration of the stromal pH. However, under many conditions, changes in the stromal pH resulting from changes in photosynthetic flux can be neglected because they are small. (iii) Stromal ratios of dihydroxyacetone phosphate to phosphoglycerate are generally lower than ratios measured in leaf extracts. The value of FAcalculated from stromal metabolites was about 30% lower than FAcalculated from cellular metabolites. Still, it appears sufficient for many purposes to calculate FAfrom metabolite measurements in leaf extracts. (iv) In the light, the catalytic capacity of the photosynthetic apparatus is adjusted to the level of irradiance. The response of carbon assimilation to large increases in irradiance is slow because it requires enzyme activation. Deactivation of the Calvin cycle induced by decreases in irradiance is slower than activation. (v) Changes in catalytic capacity and in the availability or level of substrates such as CO2alter the flux resistance of the Calvin cycle. A decrease in flux resistance explains why FAoften does not increase by much and may actually decrease when carbon flux is increased. Adjustments of flux resistances in the Calvin cycle and of photosystem-II activity in the electron-transport chain permit varying rates of photosynthesis at low levels of ATP and NADPH. As NADP remains available, the danger of over-reduction which leads to photoinactivation of electron transport is minimized.