In plants, the non-photochemical quenching of chlorophyll fluorescence (NPQ) induced by high light reveals the occurrence of a multiplicity of regulatory processes of photosynthesis, primarily devoted to photoprotection of photosystem I and II (PSI and PSII). The study of NPQ relaxation in darkness allows the separation of three kinetically distinct phases: the fast relaxing high-energy quenching qE, the intermediate relaxing phase and the nearly non-relaxatable photoinhibitory quenching. Several processes can underlie the intermediate phase. In the ancient vascular plant Selaginella martensii (Lycopodiophyta) this component, here termed qX, was previously proposed to reflect mainly a photoprotective energy-spillover from PSII to PSI. It is hypothesized that qX is induced by an over-reduced photosynthetic electron transport chain from PSII to final acceptors. To test this hypothesis the leaves were treated with the reductant dithiothreitol (DTT) and the chlorophyll fluorescence changes were analysed during the induction with high irradiance and the subsequent relaxation in darkness. DTT treatment caused the well-known decrease in NPQ induction and expectedly resulted in a disturbed photosynthetic electron flow. The relaxation curves of Y(NPQ), formally representing the quantum yield of the regulatory thermal dissipation, revealed a DTT dose-dependent decrease in amplitude not only of qE, but also of qX, up to the complete disappearance of the latter. Modelling of the relaxation curves under alternative scenarios led to the conclusion that DTT is permissive with respect to qX induction but suppresses its dark relaxation. The strong dependence of qX on the chloroplast redox state is discussed with respect to its proposed energy-spillover photoprotective significance in a lycophyte.

在植物中，强光诱导的叶绿素荧光（NPQ）的非光化学猝灭揭示了光合作用调控过程的发生，其主要致力于光系统I和II（PSI和PSII）的光保护。在黑暗中研究NPQ弛豫可以​​分离三个动力学上不同的相：快速弛豫高能猝灭qE，中间弛豫相和几乎不可松弛的光抑制猝灭。中间阶段可能有几个过程。在古代维管植物卷柏中（Lycopodiophyta）此组分，以前称为qX，先前被提议主要反映从PSII到PSI的光保护能量溢出。假设qX是由从PSII到最终受体的过度还原的光合电子传输链诱导的。为了检验该假设，用还原剂二硫苏糖醇（DTT）处理叶片，并在诱导过程中以高辐照度分析了叶绿素荧光变化，随后在黑暗中放松。DTT处理导致众所周知的NPQ感应降低，并有望导致光合电子流受到干扰。Y（NPQ）的弛豫曲线正式代表了调节性散热的量子产率，它揭示了DTT剂量依赖性的幅度降低，不仅qE，而且qX都减小，直到后者完全消失。在替代方案下对弛豫曲线进行建模得出的结论是，DTT相对于qX诱导是允许的，但抑制了其暗弛豫。关于qX对叶绿体氧化还原状态的强烈依赖性，就其在拟生植物中的能量外溢光防护意义进行了讨论。