Kopecky J, Azarkovich M, Pfündel E E, Shuvalov V A, Heber U
Institute of Microbiology, Academy of Sciences, Department of Autotrophic Microorganisms, Opatovicky mlyn, 379 81 Trebon, Czech Republic.
Plant Biol (Stuttg). 2005 Mar;7(2):156-67. doi: 10.1055/s-2005-837471.
Modulated chlorophyll fluorescence was used to compare dissipation of light energy as heat in photosystem II of homoiohydric and poikilohydric photosynthetic organisms which were either hydrated or dehydrated. In hydrated chlorolichens with an alga as the photobiont, fluorescence quenching revealed a dominant mechanism of energy dissipation which was based on a protonation reaction when zeaxanthin was present. CO2 was effective as a weak protonating agent and actinic light was not necessary. In a hydrated cyanobacterial lichen, protonation by CO2 was ineffective to initiate energy dissipation. This was also true for leaves of higher plants. Thus, regulation of zeaxanthin-dependent energy dissipation by protonation was different in leaves and in chlorolichens. A mechanism of energy dissipation different from that based on zeaxanthin became apparent on dehydration of both lichens and leaves. Quenching of maximum or Fm fluorescence increased strongly during dehydration. In lichens, this was also true for so-called basal or Fo fluorescence. In contrast to zeaxanthin-dependent quenching, dehydration-induced quenching could not be inhibited by dithiothreitol. Both zeaxanthin-dependent and dehydration-induced quenching cooperated in chlorolichens to increase thermal dissipation of light energy if desiccation occurred in the light. In cyanolichens, which do not possess a zeaxanthin cycle, only desiccation-induced thermal energy dissipation was active in the dry state. Fluorescence emission spectra of chlorolichens revealed stronger desiccation-induced suppression of 685-nm fluorescence than of 720-nm fluorescence. In agreement with earlier reports of , fluorescence excitation data showed that desiccation reduced flow of excitation energy from chlorophyll b of the light harvesting complex II to emitting centres more than flow from chlorophyll a of core pigments. The data are discussed in relation to regulation and localization of thermal energy dissipation mechanisms. It is concluded that desiccation-induced fluorescence quenching of lichens results from the reversible conversion of energy-conserving to energy-dissipating photosystem II core complexes.
利用调制叶绿素荧光来比较同水合和异水合光合生物在水合或脱水状态下，光系统II中光能作为热量的耗散情况。在以藻类为光合共生体的水合绿藻地衣中，荧光猝灭揭示了一种主要的能量耗散机制，该机制基于叶黄素存在时的质子化反应。二氧化碳作为一种弱质子化剂有效，且不需要光化光。在水合蓝细菌地衣中，二氧化碳质子化对启动能量耗散无效。高等植物的叶子也是如此。因此，质子化对叶黄素依赖的能量耗散的调节在叶子和绿藻地衣中有所不同。在地衣和叶子脱水时，一种不同于基于叶黄素的能量耗散机制变得明显。脱水过程中，最大或Fm荧光的猝灭强烈增加。在地衣中，所谓的基础或Fo荧光也是如此。与叶黄素依赖的猝灭不同，脱水诱导的猝灭不能被二硫苏糖醇抑制。如果在光照下发生干燥，叶黄素依赖的猝灭和脱水诱导的猝灭在地衣中协同作用，以增加光能的热耗散。在不具有叶黄素循环的蓝藻地衣中，只有干燥诱导的热能耗散在干燥状态下是活跃的。绿藻地衣的荧光发射光谱显示，干燥诱导的对685纳米荧光的抑制比对720纳米荧光的抑制更强。与早期报告一致，荧光激发数据表明，干燥减少了从光捕获复合物II的叶绿素b到发射中心的激发能流动，比从核心色素的叶绿素a的流动减少得更多。这些数据结合热能耗散机制的调节和定位进行了讨论。得出的结论是，地衣的干燥诱导荧光猝灭是由能量守恒的光系统II核心复合物向能量耗散的核心复合物的可逆转变引起的。
