Investigation of some biochemical responses to salt stress in edible onion (Allium cepa L.) cultivars

Onion is considered as a salt-sensitive plant, however, tolerance to salt seems to be cultivar-dependent and the genotypic responses have not been extensively investigated. In order to evaluate some biochemical responses of six cultivars of onions with different salinity tolerance, a factorial experiment in completely randomized design with two factors include: cultivars (six cultivars of onions) and different levels of salinity (1.2, 3.7, 6.1, 8.5 and 11 dS/m) with three replications carried out in a water culture system. The results showed that the type of physiological response to salinity was cultivar-dependent. In all cultivars, the chlorophyll a, b, total chlorophyll and total carotenoids significantly decreased and proline significantly increased under salt stress. Reductions in photosynthetic pigmentation were observed among all cultivars, but the decrease was higher in sensitive cultivars (Behbahan and Karaj). Susceptible cultivars showed more increase in proline content than tolerant cultivars in stress conditions. The amount of soluble sugars and leaf protein in tolerant cultivars increased, whereas in other cultivars decreased significantly. Total phenol increased in low salinity and decreased with increasing salinity while this reduction in tolerant cultivars (Isfahan and Azarshahr) was less than semi-tolerant (Harsin and P.S) and semi-tolerant was less than susceptible ones. Correlation between stress tolerance index and yield and biochemical indices showed a positive and very significant correlation between salt tolerance index with yield, chlorophyll a, chlorophyll b, total chlorophyll, carotenoids and soluble sugars, and a very significant negative correlation with proline. In general, the results showed that the physiological responses could be used to select more tolerant cultivars.