Moderate heat stress and fluctuating light are typical conditions in summer in tropical and subtropical regions. This type of stress can cause photodamage to photosystems I and II (PSI and PSII). However, photosynthetic responses to the combination of heat and fluctuating light in young leaves are little known. In this study, we investigated chlorophyll fluorescence and P700 redox state under fluctuating light at 25 °C and 42 °C in young leaves of tobacco. Our results indicated that fluctuating light caused selective photodamage to PSI in the young leaves at 25 °C and 42 °C. Furthermore, the moderate heat stress significantly accelerated photoinhibition of PSI under fluctuating light. Within the first 10 s after transition from low to high light, cyclic electron flow (CEF) around PSI was highly stimulated at 25 °C but was slightly activated at 42 °C. Such depression of CEF activation at moderate heat stress were unable to maintain energy balance under high light. As a result, electron flow from PSI to NADP+ was restricted, leading to the over-reduction of PSI electron carriers. These results indicated that moderate heat stress altered the CEF performance under fluctuating light and thus accelerated PSI photoinhibition in tobacco young leaves.

中文翻译：

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在波动的光线下，适度的热胁迫可加速烟草幼叶中光系统I的光抑制。

在热带和亚热带地区，夏季通常会出现中等程度的热应激和光线波动。这种压力会导致对光系统I和II（PSI和PSII）的光损坏。然而，鲜叶对热和波动光的组合的光合作用反应鲜为人知。在这项研究中，我们调查了在25°C和42°C波动的光下，烟草幼叶中叶绿素荧光和P700氧化还原状态。我们的结果表明，在25°C和42°C下，光线波动会导致幼叶对PSI的选择性光害。此外，适度的热应激在波动的光下显着加速了PSI的光抑制。从弱光转为强光后的前10秒钟内，PSI周围的循环电子流（CEF）在25°C时受到强烈刺激，但在42°C时被轻微激活。在中等的热应激下，这种CEF激活的抑制无法维持强光下的能量平衡。结果，限制了从PSI到NADP +的电子流，导致PSI电子载流子的过度还原。这些结果表明适度的热胁迫改变了在波动光下的CEF性能，从而加速了烟叶中PSI的光抑制作用。