Under natural field conditions, plants usually experience fluctuating light (FL) under moderate heat stress in summer. However, responses of photosystems I and II (PSI and PSII) to such combined stresses are not well known. Furthermore, the role of water-water cycle (WWC) in photoprotection in FL under moderate heat stress is poorly understood. In this study, we examined chlorophyll fluorescence and P700 redox state in FL at 42 °C in two orchids, Dendrobium officinale (with high WWC activity) and Bletilla striata (with low WWC activity). After FL treatment at 42 °C, PSI activity maintained stable while PSII activity decreased significantly in these two orchids. In D. officinale, the WWC could rapidly consume the excess excitation energy in PSI and thus avoided an over-reduction of PSI upon any increase in illumination. Therefore, in D. officinale, WWC likely protected PSI in FL at 42 °C. In B. striata, heat-induced PSII photoinhibition down-regulated electron flow from PSII and thus prevented an over-reduction of PSI after transition from low to high light. Consequently, in B. striata moderate PSII photoinhibition could protected PSI in FL at 42 °C. We conclude that, in addition to cyclic electron flow, WWC and PSII photoinhibition-repair cycle are two important strategies for preventing PSI photoinhibition in FL under moderate heat stress.

中文翻译：

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两种兰花铁皮石斛和白芨的光系统 I 在中等热应激下耐受光波动

在自然田间条件下，植物在夏季中等热胁迫下通常会经历波动的光 (FL)。然而，光系统 I 和 II（PSI 和 PSII）对这种组合应力的响应并不为人所知。此外，人们对水-水循环 (WWC) 在中等热应激下 FL 光保护中的作用知之甚少。在这项研究中，我们检查了 42 °C 下两种兰花，铁皮石斛（具有高 WWC 活性）和白芨（具有低 WWC 活性）的 FL 中的叶绿素荧光和 P700 氧化还原状态。在42°C FL处理后，这两种兰花的PSI活性保持稳定，而PSII活性显着降低。在 D. officinale 中，WWC 可以快速消耗 PSI 中多余的激发能量，从而避免在任何光照增加时过度减少 PSI。因此，在 D. officinale 中，WWC 可能在 42 °C 时保护了佛罗里达州的 PSI。在 B. striata 中，热诱导的 PSII 光抑制下调了来自 PSII 的电子流，从而防止了从低光到高光过渡后 PSI 的过度减少。因此，在 B. striata 中，适度的 PSII 光抑制可以在 42°C 保护 FL 中的 PSI。我们得出的结论是，除了循环电子流之外，WWC 和 PSII 光抑制-修复循环是在中等热应力下防止 FL 中 PSI 光抑制的两种重要策略。