Fan Da-Yong, Hope Alexander B, Smith Paul J, Jia Husen, Pace Ronald J, Anderson Jan M, Chow Wah Soon
Photobioenergetics Group, Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia.
Biochim Biophys Acta. 2007 Aug;1767(8):1064-72. doi: 10.1016/j.bbabio.2007.06.001. Epub 2007 Jun 8.
The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.
通过两种方法测定了菠菜叶片片段中光系统II（PSII）与光系统I（PSI）反应中心的化学计量比，这两种方法均可用于监测给定样品中两种光系统的存在情况。一种方法基于快速电致变色（EC）信号，在毫秒时间尺度上，它代表了由于两个光系统中的电荷分离而导致的类囊体膜上离域电势差的变化。该方法应用于叶片片段，从而避免了与类囊体膜分离相关的任何潜在假象。此方法的两个变体，即通过预先光灭活抑制PSII活性（在菠菜和杨树叶片片段中）或通过背景远红光对P700（PSI中的叶绿素二聚体）进行光氧化来抑制PSI（在菠菜、杨树和黄瓜叶片片段中），分别给出了每个光系统对快速EC信号的单独贡献；通过该方法获得的PSII/PSI化学计量比在这三种植物中为1.5 - 1.9，菠菜尤其为1.5 - 1.8。第二种基于电子顺磁共振（EPR）的方法，得出菠菜的该比值在1.7 - 2.1的可比范围内。第三种方法是分别通过单次周转闪光的氧气产量测定叶片片段中功能性PSII的含量，并通过从相应叶片分离的类囊体中P700的光氧化来测定PSI的含量，得出的PSII/PSI化学计量比（1.5 - 1.7）与上述值一致。结论是，与瑞典报道的在远红光不足的光照条件下生长在培养箱中的菠菜（由EPR测定其PSII/PSI比值低至惊人的0.88）相比，典型高等植物中PSII与PSI反应中心的比率远高于1。我们认为，瑞典菠菜在远红光不足且PSII含量较低的光照条件下生长，其低PSII/PSI比值并非如作者所暗示的那样是由于EPR测量方法的更高准确性，而是由于生长条件所致。
