Drought induces stomata closure and lowers the CO2 concentration in the mesophyll, limiting CO2 assimilation and favouring photorespiration. The photosynthetic apparatus is protected under drought conditions by a number of downregulation mechanisms like photosynthetic control and activation of cyclic electron transport leading to the generation of a high proton gradient across the thylakoid membrane. Here, we studied photosynthetic electron transport by chlorophyll fluorescence, thermoluminescence and P700 absorption measurements in spinach exposed to moderate drought stress. Chlorophyll fluorescence induction and decay kinetics were slowed down. Under drought conditions an increase of the thermoluminescence AG-band and a downshift of the maximum temperatures of both, the B-band and the AG-band was observed when leaves were illuminated under conditions that maintained the proton gradient. When leaves were frozen prior to the thermoluminescence measurements, the maximum temperature of the B-band was upshifted in drought-stressed leaves. This shows a stabilization of the QB /QB •- redox couple in accordance with the slower fluorescence decay kinetics. We propose that, during drought stress, photorespiration exerts a feedback control on photosystem II via the binding of a photorespiratory metabolite at the non-heme iron at the acceptor side of photosystem II. According to our hypothesis, an exchange of bicarbonate at the non-heme iron by a photorespiratory metabolite such as glycolate would not only affect the midpoint potential of the QA /QA •- couple, as shown previously, but also that of the QB /QB •- couple. This article is protected by copyright. All rights reserved.

中文翻译：

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中等干旱胁迫稳定光系统 II 中的初级醌受体 QA 和次级醌受体 QB

干旱会导致气孔关闭并降低叶肉中的 CO2 浓度，限制 CO2 同化并有利于光呼吸。在干旱条件下，光合作用装置受到许多下调机制的保护，例如光合作用控制和循环电子传递的激活，导致跨类囊体膜产生高质子梯度。在这里，我们通过叶绿素荧光、热释光和 P700 吸收测量研究了暴露于中等干旱胁迫的菠菜的光合电子传递。叶绿素荧光诱导和衰减动力学减慢。在干旱条件下，热释光 AG 带的增加和两者的最高温度的下降，在保持质子梯度的条件下照射叶子时，观察到 B 带和 AG 带。当叶子在热释光测量之前被冷冻时，B 波段的最高温度在干旱胁迫的叶子中上移。这显示了根据较慢的荧光衰减动力学，QB /QB •-氧化还原对的稳定性。我们提出，在干旱胁迫期间，光呼吸通过光呼吸代谢物与光系统 II 受体侧的非血红素铁的结合对光系统 II 施加反馈控制。根据我们的假设，光呼吸代谢物（例如乙醇酸盐）在非血红素铁处交换碳酸氢盐不仅会影响 QA /QA •- 对的中点电位，如前所示，还会影响 QB /QB 的中点电位•- 夫妻。本文受版权保护。版权所有。