Department of Biochemistry, University of Nebraska, 68583-0718, Lincoln, NE, USA.
Planta. 1988 Mar;173(3):411-8. doi: 10.1007/BF00401029.
Comparative (14)CO2 pulse-(12)CO2 chase studies performed at CO2 compensation (Γ)-versus air-concentrations of CO2 demonstrated a four-to eightfold increase in assimilation of (14)CO2 into the C4 acids malate and aspartate by leaves of the C3-C4 intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at Γ. Specifically, the distribution of (14)C in malate and aspartate following a 10-s pulse with (14)CO2 increases from 2% to 17% (P. milioides) and 4% to 16% (M. arvensis) when leaves are illuminated at the CO2 compensation concentration (20 μl CO2/l, 21% O2) versus air (340 μl CO2/l, 21% O2). Chasing recently incorporated (14)C for up to 5 min with (12)CO2 failed to show any substantial turnover of label in the C4 acids or in carbon-4 of malate. The C4-acid labeling patterns of leaves of the closely related C3 species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO2 from air to Γ. These data demonstrate that the C3-C4 intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO2 assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO2 compensation concentration than closely related C3 species. However, even at Γ, CO2 fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C3 cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C3-C4 intermediate species of Panicum and Moricandia.
在 CO2 补偿（Γ）与空气浓度下进行的（14）CO2 脉冲-（12）CO2 追踪研究表明，C3-C4 中间种 Panicum milioides Nees ex Trin.、P. decipiens Nees ex Trin.、Moricandia arvensis（L.）DC. 和 M. spinosa Pomel 的叶片对（14）CO2 的同化作用增加了 4 到 8 倍。具体来说，在 CO2 补偿浓度（20 μl CO2/l，21% O2）下，与空气（340 μl CO2/l，21% O2）相比，用（14）CO2 进行 10 秒脉冲后，苹果酸和天冬氨酸中（14）C 的分布从 2%增加到 17%（P. milioides）和 4%增加到 16%（M. arvensis）。用（12）CO2 追踪最近掺入的（14）C 长达 5 分钟，在 C4 酸或苹果酸的碳-4 中未发现任何明显的标记周转。与空气相比，在 CO2 补偿浓度下，与 CO2 补偿浓度相比，亲缘关系密切的 C3 种 Panicum laxum Sw. 和 M. moricandioides（Boiss.）Heywood 的叶片 C4-酸标记模式相对对 pCO2 从空气到 Γ 的变化不敏感。这些数据表明，与亲缘关系密切的 C3 种相比，Panicum 和 Moricandia 的 C3-C4 中间种在 CO2 补偿浓度下通过磷酸烯醇丙酮酸（PEP）羧化酶（EC 4.1.1.31）具有更大的 CO2 同化能力。然而，即使在 Γ 下，PEP 羧化酶固定的 CO2 也比核酮糖 1,5-二磷酸羧化酶（EC 4.1.1.39）和 C3 循环少，因此不太可能以主要方式促进 Panicum 和 Moricandia 的 C3-C4 中间种减少光呼吸的机制。
