Lilley R M, Walker D A
The University of Sheffield, Department of Botany, Sheffield S10 2TN, United Kingdom.
Plant Physiol. 1975 Jun;55(6):1087-92. doi: 10.1104/pp.55.6.1087.
The relationship between rate of photosynthesis and CO(2) concentration has been reinvestigated using isolated spinach (Spinacia oleracea) chloroplasts. The apparently low CO(2) concentration required for half-maximal photosynthesis is shown to result partly from a ceiling imposed by electron transport. In double reciprocal plots of rate against CO(2) concentration, this ceiling results in departures from linearity at high CO(2) concentrations. If these rate limitations are disregarded in extrapolation the "true" CO(2) concentration required for half maximal carboxylation by intact chloroplasts is approximately 46 mum (CO(2)).When assayed under comparable conditions, ribulose bisphosphate carboxylase from these chloroplasts also shows an apparent Km (CO(2)) of approximately 46 mum, suggesting that its characteristics are not modified by extraction. An improved assay for ribulose bisphosphate carboxylase yielded rates of carboxylation considerably higher than those previously reported, the highest maximal velocities recorded approaching 1000 mumoles CO(2) fixed mg(-1) chlorophyll hr(-1) at 20 C. With such Km and V(max), values the carboxylase would be able to achieve, at concentrations of CO(2) less than atmospheric, rates of CO(2) fixation equal to those displayed by the parent tissue or by the average plant under favorable conditions in its natural environment.
利用分离的菠菜(Spinacia oleracea)叶绿体,对光合作用速率与二氧化碳浓度之间的关系进行了重新研究。结果表明,光合作用达到半最大值所需的明显较低的二氧化碳浓度,部分是由电子传递所施加的上限导致的。在速率对二氧化碳浓度的双倒数图中,这种上限导致在高二氧化碳浓度下偏离线性关系。如果在推断时忽略这些速率限制,完整叶绿体进行半最大羧化反应所需的“真实”二氧化碳浓度约为46微摩尔(二氧化碳)。在可比条件下进行测定时,这些叶绿体中的核酮糖二磷酸羧化酶的表观米氏常数(二氧化碳)也约为46微摩尔,这表明其特性在提取过程中未被改变。一种改进的核酮糖二磷酸羧化酶测定方法得到的羧化速率明显高于先前报道的速率,在20℃时记录到的最高最大速度接近1000微摩尔二氧化碳固定毫克(-1)叶绿素小时(-1)。有了这样的米氏常数和最大速度值,在二氧化碳浓度低于大气浓度时,羧化酶能够实现与母组织或自然环境中处于有利条件下的普通植物所显示的二氧化碳固定速率相当的速率。