Abiotic stresses such as drought and salinity are major factors affecting plant growth and development, leading to the reductions in crop productivity. It is generally believed that plant water stress is exacerbated by saline soils. However, sodium ions absorbed by plant cells are partmentalized into vacuoles, and function as an effective osmoregulator, increasing the ability of plants to absorb and retain water. Thus, salt uptake in plants may potentially alleviate the adverse effects of drought. In this study, pot grown cotton plants were watered with NaCl solutions of different concentrations at a plant height of ca. 20 cm, followed by a sustained drought treatment. Parameters such as chlorophyll fluorescence, photosynthesis and gas exchange, plant water status, tissue Na+content, etc. were measured during drought stress were taken to assess whether Na+could alleviate drought induced photosystem dysfunction in leaves and identify the mechanisms involved. The leaf photosynthetic rates and the gas exchange of the plants were investigated using a TPS-1 portable photosynthesis system ( PP System, USA), and chlorophyll fluorescence parameters were measured with a handy PEA (Hansatech, UK). The net photosynthetic rate declined as drought stress progressed in plants not treated with NaCl. By contrast, the decline in photosynthetic rate induced by drought was significantly lower in plants treated with NaCl. In addition, in plants not treated with NaCl, CO2 concentrations in leaf intercellular spaces increased rapidly, while the stomatal limitation value decreased rapidly 20 days after the drought treatment commenced;the chlorophyll fluorescence parameter Fo ( the initial fluorescence) increased rapidly after 10 days of drought treatment, but decreased rapidly after 20 days;the other chlorophyll fluorescence parameters, such as Fv ( the variable fluorescence ) , Fv/Fm ( maximum quantum efficiency of photosystemⅡ) , Fv/Fo ( maximum primary yield of photochemistry of photosystemⅡ) , RC/CSo ( the density of the active photosynthetic reaction centers) and PI ( the performance index) all showed sharp declines as drought stress became obvious, while the ABS/RC ( the effective antenna size) increased consistently 10 days after drought treatment began. These results indicate that the photosynthetic apparatus suffered serious damage in the late stages of drought stress in plants not watered with salt solution. By contrast, the salt treated plants subjected to the same drought stress showed smaller or insignificant changes in these parameters, indicating that the photosynthetic apparatus of the salt treated plants suffered less or no damage. Salt absorption and plant water status have always been extremely important in the study of salt stress. Our results show that Na+accumulation in salt treated plants increased significantly, which lowered leaf osmotic potential, facilitating the maintenance of turgor pressure. Compared to salt treated plants, cells in plants not treated with NaCl lost their turgor with the advancement of drought stress and the plants wilted seriously. It is therefore concluded that the cotton plants treated with NaCl absorbed and accumulated Na+, lowering tissue osmotic potential and maintaining some turgor pressure during sustained drought, thus mitigating damage to the photosynthetic apparatus caused by drought stress.%以盆栽棉花为材料,在植株高约20 cm时用不同浓度NaCl溶液浇透后进行持续干旱处理。在干旱处理期间测定叶片叶绿素荧光参数、光合气体交换参数的变化以及植株水分状况和Na+含量,以分析土壤NaCl施入引起的棉花Na+吸收和积累量的增加对干旱胁迫导致的叶片光合系统损伤的缓解作用及可能原因。结果表明,未用NaCl处理的棉花植株,其叶片净光合速率随着干旱的延续而持续下降、光合机构在干旱处理后期出现了严重损伤；而NaCl处理的棉花植株,其叶片净光合速率下降幅度明显小于未用NaCl处理的,光合机构受损伤程度也较轻或无明显损伤。对各处理棉花植株Na+的吸收和水分状况的测定分析表明,NaCl处理的植株,其叶片Na+积累显著增加、渗透势降低,细胞膨压显著高于未用NaCl处理的植株。由此可见,在土壤浇灌NaCl溶液后的持续干旱条件下,棉花植株吸收和积累Na+增加,降低了组织渗透势、维持了一定的细胞膨压,从而有效缓解了干旱胁迫对叶片光合机构的损伤。
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