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

Abstract

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.

The Physiological Response of Different Brook Willow (Salix acmophylla Boiss.) Ecotypes to Salinity

“…The SOS1 [ 54 , 55 ] and HKT [ 56 ] gene families have been reported to be involved in the extrusion of Na to the external media and removal of Na from the transpiration stream in the xylem, respectively. By preventing the translocation of Na + to the shoots, K + /Na + homeostasis in the leaves can be maintained, as was recently demonstrated in S. alba [ 43 ]. Expression levels of these genes increase over time with exposure to salt stress, and variations in expression levels between ecotypes coincide with varying degrees of salt tolerance.…”

“…The phenotypic response observed here with Darom and Mea She’arim further supports the hypothesis that S. alba is a parent of these two ecotypes, which may explain their observed phenotypic tolerance to the higher salinity solution. S. alba was recently found to be salt tolerant up to 342 mM NaCl (or 34.2 dS m −1 ) through an enhanced ability to regulate Na translocation from the roots to the shoots [ 43 ].…”

“…Several crop species upregulate key ion transporters for K + in the roots when under sustained exposure to salt. This response helps overcome K + /Na + imbalance in tissues, as maintenance of high cytosolic K + /Na + is considered a key salt tolerance trait [ 43 , 53 ]. The SOS1 [ 54 , 55 ] and HKT [ 56 ] gene families have been reported to be involved in the extrusion of Na to the external media and removal of Na from the transpiration stream in the xylem, respectively.…”

“…Not all willow species are suitable for phytoremediation of landfill leachates, as some previous studies involving willow species have shown lower salinity tolerance compared to the S. acmophylla ecotypes tested here. For example, the growth of Salix amygdalina L. was negatively affected by leachate solutions with electrical conductivity (EC) values higher than 3.0 dS m −1 [ 43 ]. In another recent study screening three native North American willow species ( S. discolor , S. eriocephala and S. interior ), only the latter was able to grow when subjected to a salinity level of 3.0 dS m −1 [ 65 ].…”

“…The relatively high salt tolerance of Darom and Mea She’arim may therefore be the result of hybrid vigor. Both S. acmophylla and S. alba have demonstrated previous tolerance to increased EC [ 34 , 43 ]. Given the widespread use of willow species in phytoremediation, these two ecotypes show promise for use under saline conditions.…”