Ion response of sunflower at sprouting stage to mixed salt stress

Research on the mechanism of response of sunflower to salt stress can provide scientific basis for rapid screening of salt resistant varieties, a critical element in the exploitation of saline-alkali lands. In this study, salt-sensitive variety 'YK18', moderately salt-tolerant variety 'YK06' and highly salt-tolerant variety 'GF01' of sunflower were used to analyze seed germination, ion accumulation and distribution in seedlings of different varieties of sunflower under mixed salt (NaCl/Na₂SO₄ of 9/1 mol) concentrations of 0 mmol·L^-1, 50 mmol·L^-1, 100 mmol·L^-1, 150 mmol·L^-1, 200 mmol·L^-1 and 250 mmol·L^-1. Scanning ion-selective electrode technique (SIET) was used to determine K⁺, Na⁺ and Ca²⁺ fluxes in roots after 24 h mixed salt stress. The results showed that seed germination, germination rate and germination index decreased under salt stress and the average germination time of oil sunflower extended. Under salt-stress condition, there was an obvious K⁺ efflux in roots. Compared with high salt-tolerant variety 'GF01', the roots of salt-sensitive variety 'YK18' and moderate salt-tolerant variety 'YK06' had higher K⁺ extrude capacity. Salt stress led to a net Na⁺ influx in the range of 0-100 mmol·L^-1 in mixed salt concentrations, and was highest for salt-sensitive variety 'YK18'. The pattern of Na⁺ flux in roots changed significantly under higher mixed salt concentrations (150-200 mmol·L^-1) and there was a clear efflux of Na⁺ in seedlings, which was highest for salt-tolerant variety 'GF01'. After mixed salt stress, Na⁺ content increased while K⁺ content decreased, resulting in a decrease in K⁺/Na⁺ ratio in the whole plant. Salt-tolerant variety 'GF01' had the lowest K⁺/Na⁺ ratio, had the potential to intercept Na⁺ in stems under low salt concentration ( < 150 mmol·L^-1). The sunflower variety 'GF01' had higher capacity to extrude Na⁺. As a result, 'GF01' had the least Na⁺ content (for the whole plant) and had higher K⁺/Na⁺ ratio in leaves under high salt stress (≥150 mmol·L^-1). SIET data also showed that after 24 h exposure to mixed salt stress, a clear Ca²⁺ influx in salt stressed seedlings that was proportional to the mixed salt concentration developed. The Ca²⁺ absorption rate of 'GF01' was higher than that of 'YK18'. In conclusion, different degrees of salt tolerance in different sunflower varieties were regulated by Na⁺ and K⁺ absorption and efflux as a mode of adaption to the salt stress environment. Strong salt tolerant variety had stronger ability to protect K⁺, but K⁺ can also be protected by regional Na⁺ application (under low salt concentration), salt rejection mechanisms to enhance salt tolerance and then by maintaining reasonable K⁺/Na⁺ ratio in leaves. In addition, accelerated absorption of Ca²⁺ by plants alleviated salt damage in plants after salt stress. The results of the study provided theoretical basis for the selection and cultivation of salt tolerant varieties. And the established ion flux detection technique was a reliable screening method for salt tolerant varieties selection in plant breeding.