The impact of Ca<sup>2+</sup> on the protective mechanisms of the photosystem under drought stress
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External calcium ions (Ca<sup>2+</sup>) play a crucial role in mitigating drought stress in plants, as revealed through physiological and transcriptomic analyses using tobacco (Nicotiana tabacum L) as a model system. Under drought stress, leaves wilted, chlorophyll content decreased. The maximum photochemical efficiency of PSII (<i>F</i><sub>v</sub>/<i>F</i><sub>m</sub>) and the photosynthetic performance index decreased, fluorescence intensities at the K and L points increased, <i>Y</i><sub>(NPQ)</sub> increased, net photosynthetic rate (<i>P</i><sub>n</sub>) decreased, and PSI activity (ΔI/Io) was reduced. Upon application of external calcium under drought conditions, leaf wilting was reduced, chlorophyll content increased, <i>F</i><sub>v</sub>/<i>F</i><sub>m</sub>, PIabs, and PItotal improved, fluorescence intensities at the K and L points as well as <i>Y</i><sub>(NPQ)</sub> decreased, cyclic electron transport and <i>P</i><sub>n</sub> were enhanced, stomatal aperture increased, and Δ<i>I</i>/<i>I</i>o improved. Transcriptomic analysis revealed a significant reduction in the expression of genes related to the PSI and PSII reaction centers, ATP synthesis, electron transport, and antenna proteins under drought stress. The application of calcium significantly alleviated the drought-induced downregulation of these genes. In summary, Ca<sup>2+</sup> reduces water loss in plants, increases chlorophyll content in leaves, restores PSII reaction center activity, ensures efficient photosynthetic electron transport, maintains thylakoid membrane stability, enhances cyclic electron transport, and reduces damage to the photosystem.
以烟草（Nicotiana tabacum L.）为模式植物开展生理与转录组分析后发现，外源钙离子（Ca²+）在缓解植物干旱胁迫过程中发挥关键作用。干旱胁迫下，植物叶片出现萎蔫，叶绿素含量下降；光系统II（PSII）最大光化学效率（Fv/Fm）与光合性能指数均降低，K点与L点荧光强度升高，非光化学淬灭量子产额（Y(NPQ)）上升，净光合速率（Pn）下降，光系统I（PSI）活性（ΔI/Io）降低。干旱胁迫下施加外源钙离子后，叶片萎蔫程度减轻，叶绿素含量回升，Fv/Fm、光合性能指数（PIabs、PItotal）均得到改善，K点、L点荧光强度及Y(NPQ)均下降，循环电子传递与Pn得以增强，气孔开度升高，ΔI/Io得到恢复。转录组分析结果显示，干旱胁迫下，与光系统I、光系统II反应中心、ATP合成、电子传递及天线蛋白相关的基因表达量显著下调；施加钙离子可显著缓解上述基因受干旱诱导的下调表达。综上，Ca²+可减少植物水分散失，提升叶片叶绿素含量，恢复光系统II反应中心活性，保障光合电子传递高效进行，维持类囊体膜稳定性，增强循环电子传递，降低光合系统受损程度。
