Effects of salt stress on physiological characteristics and yield of different salt-tolerant wheat varieties

Soil salinity is a global issue that affects wheat production, and it is of great interest to improve the production efficiency of wheat in saline fields. A comprehensive understanding of salt-tolerance mechanisms and the selection of reliable screening indices are crucial for breeding salt-tolerant wheat cultivars. Previous studies have reported the performance of wheat under salt stress and controlled experimental conditions, such as potted plants, seawater (saltwater) irrigation, hydroponics, and salt ponds, but could not simulate the actual production environment in the field and reflect the law of crop growth in a natural state. How salinity stress affects wheat yield, and the physiological indicators that contribute to yield formation under saline field conditions are not yet to be established. Five spring wheat varieties with significant differences in salt tolerance (salt-tolerant varieties: ‘NM21’ ‘YM20’ ‘YFM4’; salt-sensitive varieties: ‘YM23’ ‘AN1124’) screened in a previous experiment were grown at two sites with significantly different soil salinity, namely: non-saline (control, soil salinity before sowing was 0.770±0.062 g∙kg⁻¹) and saline (soil salinity before sowing was 3.294±0.198 g∙kg⁻¹) fields, in Dafeng, Jiangsu, China. The yield and its components, post-anthesis chlorophyll content, chlorophyll fluorescence F_v/F_m, malondialdehyde content, and proline content were measured. The results showed that the leaf area index, dry matter accumulation, and tillers number decreased significantly in saline field. Moreover, compared with the control, wheat yield in saline field decreased significantly and was only 26.2% of the control. The number of spikes, kernels per spike, and 1000-grain weight also decreased significantly. The number of spikes, which decreased by 60.7%, was the main constraint on yield production, followed by the 1000-grain weight, which also decreased. Salt stress also caused a significant decrease in chlorophyll relative content (SPAD value) and chlorophyll fluorescence F_v/F_m but significantly increased the malondialdehyde and proline contents; the range of change differed among varieties. Salt-tolerant varieties had a lower decrease in chlorophyll content and chlorophyll fluorescence F_v/F_m and a lower increase in malondialdehyde content but a higher increase in proline content, therefore, there was a lower decrease in yield. Correlation analysis was carried out for the physiological characteristics at the flowering stage, yield, and its constituent factors in wheat with different salt tolerances. The results showed that there was a significant positive correlation between chlorophyll fluorescence F_v/F_m and the number of spikes, 1000-grain weight, and yield, and SPAD value was positively correlated with dry matter accumulation. A significant positive correlation was observed between dry matter accumulation and the number of spikes, 1000-grain weight, and yield, indicating that salt stress inhibits photosynthesis in wheat by reducing chlorophyll content and chlorophyll fluorescence F_v/F_m, reducing the production of photosynthetic products, consequently resulting in a final yield reduction. Hence, in field identification, SPAD value and chlorophyll fluorescence F_v/F_m at the flowering stage can be used as fast and reliable indices for salt tolerance in wheat. Furthermore, ‘YM20’ had the lowest yield reduction rate and better overall performance, making it suitable for planting in Dafeng saline land.