Dynamic of ionic absorption and salt tolerance screening in wheat seedling under salt stress

Salt tolerance evaluation is the basis of wheat introduction, screening, and breeding. Salt stress damages plants mainly through osmotic stress, ion toxicity, and other processes. In this study, salt-sensitive varieties 'Liaochun10' and 'Jing411', moderately salt-tolerant breeding material 3D232, and highly salt-tolerant varieties 'DK961' and 'Xuezao' of common wheat were used to analyze ion absorption, growth, and ion concentrations in seedlings under salt concentrations of 0 mmol·L^-1 (CK) and 250 mmol·L^-1 sodium chloride (NaCl). The ion flux and direction of potassium (K⁺), sodium (Na⁺), and chlorine (Cl^-) around the roots of five wheat lines with different salt tolerances were measured by applying dynamic ion detection techniques. The relationship between ion flux and wheat salt tolerance was established by studying the mechanism of wheat salt tolerance, which provided a scientific basis for the rapid selection of salt-tolerant varieties using dynamic ion detection technology. The main results were: 1) the detection of dynamic flux demonstrated that the circulation of K⁺ changed from influx to efflux in the salt-sensitive wheat varieties ('Liaochun10' and 'Jing411') under salt treatment, whereas in the medium salt-tolerant variety (3D232), the efflux of K⁺ decreased. For the salt-resistant varieties, K⁺ efflux changed to influx ('Xuezao') or maintained K⁺ influx ('DK961'). Na⁺ efflux increased after stress, and the velocity range changed from 23-47 pmol·cm^-2·s^-1 to 61-150 pmol·cm^-2·s^-1. Compared with Na⁺, the Cl^- efflux increased, and 'Liaochun10' showed the largest change; the efflux was 10 times higher than that under CK. Na⁺ and Cl^- efflux were not significantly correlated with salt tolerance. 2) Under salt stress, the root-seedling ratio of salt-sensitive wheat decreased, whereas that of salt-tolerant and mid-salt-tolerant wheat increased. The fresh weight of salt-sensitive wheat decreased significantly compared to CK, but the changes in salt-tolerant and mid-salt-tolerant wheat were not significant. 3) After salt stress, the content of K⁺ in the roots and shoots of salt-tolerant and mid-salt-tolerant wheat increased by 57%-88% and 18%-112%, respectively, whereas in salt-sensitive wheat, it decreased by 40%-44% and 24%-42%, respectively. However, the Na⁺ increase in the shoots of salt-tolerant wheat was less than that of salt-sensitive wheat, and more Na⁺ was blocked in the roots. Salt-tolerant wheat was better able to separate Na⁺. 4) Under salt stress, the K⁺ flux was highly correlated with changes in the root-seedling ratio and the rate change of fresh weight, with correlation coefficients of 0.972 and 0.832, respectively. In conclusion, under high salt environments, salt-tolerant lines have a stronger ability to protect K⁺, but K⁺ can also be protected by regional Na⁺ application and salt rejection mechanisms to enhance salt tolerance. The results of this study showed that K⁺ flux in the mature zone of wheat roots can be used as a biomarker for wheat salt tolerance screening after 24 h of 250 mmol·L^-1 NaCl stress.