In forests, understory plants are usually exposed to sunflecks on timescales of seconds or minutes. However, it is unclear how understory plants acclimate to fluctuating light. In this study, we compared chlorophyll fluorescence, PSI redox state and the electrochromic shift signal under fluctuating light between an understory plant Paris polyphylla (Liliaceae) and a light-demanding plant Bletilla striata (Orchidaceae). Within the first seconds after transition from low to high light, PSI was highly oxidized in P. polyphylla but was highly reduced in B. striata, although both species could not generate a sufficient trans-thylakoid proton gradient (ΔpH). Furthermore, the outflow of electrons from PSI to O2 was not significant in P. polyphylla, as indicated by the P700 redox kinetics upon dark-to-light transition. Therefore, the different responses of PSI to fluctuating light between P. polyphylla and B. striata could not be explained by ΔpH formation or alternative electron transport. In contrast, upon a sudden transition from low to high light, electron flow from PSII was much lower in P. polyphylla than in B. striata, suggesting that the rapid oxidation of PSI in P. polyphylla was largely attributed to the lower PSII activity. We propose, for the first time, that down-regulation of PSII activity is an important strategy used by some understory angiosperms to cope with sunflecks.

中文翻译：

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光系统II活性的降低有利于下层植物巴黎七叶植物对光的波动的适应。

在森林中，林下植物通常会在数秒或数分钟的时间内暴露在日光下。然而，目前还不清楚林下植物如何适应光的波动。在这项研究中，我们比较了林下植物巴黎多叶植物（百合科）和对光需求高的植物白B（兰科）之间的波动光下的叶绿素荧光，PSI氧化还原状态和电致变色转换信号。在从弱光过渡到强光后的最初几秒钟内，PSI在多叶红假单胞菌中被高度氧化，而在条纹双歧杆菌中被高度还原，尽管这两种物种都无法产生足够的跨类囊体质子梯度（ΔpH）。此外，电子从PSI到O2在多叶红假单胞菌中的流出并不显着，如从暗到亮过渡时的P700氧化还原动力学所表明的那样。所以，ΔpH的形成或替代性电子传输无法解释PSI对多叶青假单胞菌和条纹双歧杆菌之间脉动光的不同响应。相反，在从低光到高光的突然转变时，多叶红假单胞菌中PSII的电子流远低于条纹双歧杆菌，这表明多叶红假单胞菌中PSI的快速氧化很大程度上归因于较低的PSII活性。我们首次提出，PSII活性的下调是一些下层被子植物用来应对日晒的重要策略。这表明，P。polyphylla中PSI的快速氧化很大程度上归因于较低的PSII活性。我们首次提出，PSII活性的下调是一些下层被子植物用来应对日晒的重要策略。这表明，P。polyphylla中PSI的快速氧化很大程度上归因于较低的PSII活性。我们首次提出，PSII活性的下调是一些下层被子植物用来应对日晒的重要策略。