Effects of salt stress on the photosynthetic physiology and mineral ion absorption and distribution in white willow (Salix alba L.)

Objective The purpose of this study was to explore the adaptive mechanism underlying the photosynthetic characteristics and the ion absorption and distribution of white willow (Salix alba L.) in a salt stress environment in cutting seedlings. The results lay a foundation for further understanding the distribution of sodium chloride and its effect on the photosynthetic system. Method A salt stress environment was simulated in a hydroponics system with different NaCl concentrations in one-year-old Salix alba L.branches as the test materials. Their growth, ion absorption, transport and distribution in the roots and leaves, and the changes in the photosynthetic fluorescence parameters were studied after 20 days under hydroponics. Results The results show that The germination and elongation of roots are promoted in the presence of 171mM NaCl, but root growth is comprehensively inhibited under increasing salt stress. Under salt stress, Na+ accumulates significantly in the roots and leaves, and the Na+ content and the Na+/K+ and Na+/Ca2+ root ratios are significantly greater than those in the leaves. When the NaCl concentration is ≤ 342mM, Salix alba can maintain relatively stable K+ and Ca2+ contents in its leaves by improving the selective absorption and accumulation of K+ and Ca2+ and adjusting the transport capacity of mineral ions to aboveground parts, while K+ and Ca2+ levels are clearly decreased under high salt stress. With increasing salt concentrations, the net photosynthetic rate (Pn), transpiration rate (E) and stomatal conductance (gs) of leaves decrease gradually overall, and the intercellular CO2 concentration (Ci) first decreases and then increases. When the NaCl concentration is < 342mM, the decrease in leaf Pn is primarily restricted by the stomata. When the NaCl concentration is > 342mM, the decrease in the Pn is largely inhibited by non-stomatal factors. Due to the salt stress environment, the OJIP curve (Rapid chlorophyll fluorescence) of Salix alba turns into an OKJIP curve. When the NaCl concentration is > 171mM, the fluorescence values of points I and P decrease significantly, which is accompanied by a clear inflection point (K). The quantum yield and energy distribution ratio of the PSⅡ reaction center change significantly (φPo, Ψo and φEo show an overall downward trend while φDo is promoted). The performance index and driving force (PIABS, PICSm and DFCSm) decrease significantly when the NaCl concentration is > 171mM, indicating that salt stress causes a partial inactivation of the PSII reaction center, and the functions of the donor side and the recipient side are damaged. Conclusion The above results indicate that Salix alba can respond to salt stress by intercepting Na+ in the roots, improving the selective absorption of K+ and Ca2+ and the transport capacity to the above ground parts of the plant, and increasing φDo, thus shows an ability to self-regulate and adapt.