Department of Applied Biological Science, Graduate School for Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe, 657-8501, Japan.
Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 7 Gobancho, Tokyo, 102-0076, Japan.
J Plant Res. 2022 Jul;135(4):565-577. doi: 10.1007/s10265-021-01357-6. Epub 2021 Nov 15.
It is still a controversial issue how the electron transport reaction is carried out around photosystem I (PSI) in the photosynthetic electron transport chain. The measurable component in PSI is the oxidized P700, the reaction center chlorophyll in PSI, as the absorbance changes at 820-830 nm. Previously, the quantum yield at PSI [Y(I)] has been estimated as the existence probability of the photo-oxidizable P700 by applying the saturated-pulse illumination (SP; 10,000-20,000 µmol photons m s). The electron transport rate (ETR) at PSI has been estimated from the Y(I) value, which was larger than the reaction rate at PSII, evaluated as the quantum yield of PSII, especially under stress-conditions such as CO-limited and high light intensity conditions. Therefore, it has been considered that the extra electron flow at PSI was enhanced at the stress condition and played an important role in dealing with the excessive light energy. However, some pieces of evidence were reported that the excessive electron flow at PSI would be ignorable from other aspects. In the present research, we confirmed that the Y(I) value estimated by the SP method could be easily misestimated by the limitation of the electron donation to PSI. Moreover, we estimated the quantitative turnover rate of P700 by the light-to-dark transition. However, the turnover rate of P700 was much slower than the ETR at PSII. It is still hard to quantitatively estimate the ETR at PSI by the current techniques.
在光合作用电子传递链中，围绕光系统 I（PSI）进行电子传递反应的方式仍存在争议。PSI 中可测量的组件是氧化的 P700，PSI 中的反应中心叶绿素，其在 820-830nm 处的吸光度发生变化。以前，通过应用饱和脉冲照明（SP；10,000-20,000µmol 光子 m s），PSI 中的量子产率 [Y(I)] 被估计为可光氧化 P700 的存在概率。PSI 中的电子传递速率（ETR）是从 Y(I) 值估计的，它大于 PSII 的反应速率，被评估为 PSII 的量子产率，特别是在 CO 限制和高光强等胁迫条件下。因此，人们认为 PSI 中的额外电子流在胁迫条件下得到增强，并在处理过量光能方面发挥了重要作用。然而，一些证据表明，PSI 中的过量电子流在其他方面可能可以忽略不计。在本研究中，我们证实了 SP 方法估计的 Y(I)值很容易受到 PSI 电子供体的限制而产生错误估计。此外，我们通过光暗转换来估计 P700 的定量周转率。然而，P700 的周转率比 PSII 的 ETR 要慢得多。目前的技术仍然难以定量估计 PSI 的 ETR。
