Effect of various salt–alkaline mixed stress conditions on sunflower seedlings and analysis of their stress factors

Abstract

Sunflower seedlings were treated under 30 different conditions of alkalinity and salinity, which were established by mixing NaCl, NaHCO 3, Na 2SO 4, and Na 2CO 3, at various proportions. The treatments included a salt concentration range of 50–250 mmol and pH values from 7.12 to 10.72. Several physiological indices of seedlings stressed—including relative growth rate (RGR), leaf area, electrolyte leakage rate, proline content, citric acid content, and contents of Na + and K +—were determined to analyze the characteristics of the stresses due to the salt–alkali mixes and their main stress factors. The results showed that the physiological responses of sunflower closely correlated not only with salinity (the total concentration of stress salt) but also with the pH (or alkalinity) of the treatment solution. RGR, leaf area, and of K + content decreased with increasing salinity and pH. Electrolyte leakage rate, proline content, citric acid content, and Na + content increased with increasing salinity and pH. The deleterious effects of a high pH value or salinity alone were significantly less than those of high pH in combination with salinity. This result suggested that for a salt–alkali mix stress, a reciprocal enhancement between salt stress and alkali stress was a characteristic feature. The buffer capacity of the treatment solution was taken as a stress factor in order to simplify the stress factor analysis. The results of the statistical analysis showed that for the stress factors of the salt–alkali mix stress, [CO 3 2−] and [HCO 3 −] could be fully represented by the buffer capacity; [Na +] could be fully represented by salinity; whereas [SO 4 2−] was negligible. Therefore, four factors, salinity, buffer capacity, pH and [Cl −], could reflect all of stress factors. Perfect linear correlations were observed between all strain indices and the four stress factors. However, the effects of the four stress factors on the strain indices were significantly different in magnitude. Buffer capacity and salinity were dominant factors for all strain indices. Thus, it is reasonable to consider the sum of salinity plus buffer capacity as the strength value of salt–alkali mix stress. Furthermore, the relationships between different strain indices and various stress factors were shown to be different.