Ceusters Nathalie, Valcke Roland, Frans Mario, Claes Johan E, Van den Ende Wim, Ceusters Johan
Department of Biosystems, Division of Crop Biotechnics, Research Group for Sustainable Crop Production & Protection, KU Leuven, Geel, Belgium.
Molecular and Physical Plant Physiology, UHasselt, Diepenbeek, Belgium.
Front Plant Sci. 2019 Aug 6;10:1012. doi: 10.3389/fpls.2019.01012. eCollection 2019.
Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis characterized by improved water use efficiency mediated by major nocturnal CO fixation. Due to its inherent metabolic plasticity CAM represents a successful physiological strategy for plant adaptation to abiotic stress. The present study reports on the impact of drought stress and different light intensities (PPFD 50 and 200 μmol m s) on the photosynthetic performance of the obligate CAM orchid "Edessa" by integrating diel gas exchange patterns with assessments of the light reactions by analyzing fast chlorophyll fluorescence induction. Parameters such as PI (performance index), different energy fluxes per active reaction centre (RC) reflecting the electron flow from photosystem II to photosystem I and the energetic communication between PSII complexes defined as connectivity were considered for the first time in a CAM plant. A higher PS II connectivity for plants grown under low light ( ∼ 0.51) compared to plants grown under high light ( ∼ 0.31) brought about similar specific energy fluxes of light absorbance, dissipation and processing through the electron transport chain, irrespective of the light treatment. With a 25% higher maximum quantum yield and comparable biomass formation, low light grown plants indeed proved to process light energy more efficiently compared to high light grown plants. The performance index was identified as a very reliable and sensitive parameter to indicate the onset and progress of drought stress. Under restricted CO availability (due to closed stomata) leaves showed higher energy dissipation and partial inactivation of PSII reaction centres to reduce the energy input to the electron transport chain and as such aid in avoiding overexcitation and photodamage. Especially during CAM idling there is a discrepancy between continuous input of light energy but severely reduced availability of both water and CO, which represents the ultimate electron acceptor. Taken together, our results show a unique flexibility of CAM plants to optimize the light reactions under different environmental conditions in a dual way by either attenuating or increasing energy flux.
景天酸代谢（CAM）是一种特殊的光合作用模式，其特点是通过主要在夜间固定二氧化碳来提高水分利用效率。由于其固有的代谢可塑性，CAM代表了植物适应非生物胁迫的一种成功生理策略。本研究通过将昼夜气体交换模式与通过分析快速叶绿素荧光诱导对光反应的评估相结合，报道了干旱胁迫和不同光强（光合光子通量密度为50和200 μmol m⁻² s⁻¹）对专性CAM兰花“埃代萨”光合性能的影响。首次在CAM植物中考虑了诸如性能指数（PI）、反映从光系统II到光系统I的电子流的每个活性反应中心（RC）的不同能量通量以及定义为连接性的光系统II复合物之间的能量通信等参数。与在高光下生长的植物（约0.31）相比，在低光下生长的植物具有更高的光系统II连接性（约0.51），这使得无论光照处理如何，通过电子传递链的光吸收、耗散和处理的比能量通量相似。低光下生长的植物最大量子产率高25%且生物量形成相当，确实证明与高光下生长的植物相比能更有效地处理光能。性能指数被确定为指示干旱胁迫开始和进展的非常可靠和敏感的参数。在二氧化碳供应受限（由于气孔关闭）的情况下，叶片表现出更高的能量耗散和光系统II反应中心的部分失活，以减少输入到电子传递链的能量，从而有助于避免过度激发和光损伤。特别是在CAM空转期间，光能持续输入，但水和二氧化碳（代表最终电子受体）的可用性都严重降低，两者之间存在差异。综上所述，我们的结果表明CAM植物具有独特的灵活性，能够通过减弱或增加能量通量以两种方式在不同环境条件下优化光反应。
