Proline Accumulation Is Not Correlated with Saline-Alkaline Stress Tolerance in Rice Seedlings

Pro line accumulation is a common physiological response of plants to various environmental stresses. However, the physiological importance of proline accumulation in stress tolerance remains controversial. Here, we examined whether proline accumulation in rice (Oryza saliva L.) seedlings is correlated with tolerance to saline alkaline stress factors, namely, salt, alkaline and osmotic stresses, by using 25 japonica cultivars. The tolerance of seedlings to each stress treatment varied significantly among the cultivars as evaluated by membership degree (MD) of fuzzy set theory on the basis of changes in shoot length (SL), total root length (TRL), total root surface area (RSA), average root diameter (RD), total root volume (RV), and root numbers (RN), after 6 d of stress treatment. The proline levels in all the japonica cultivars increased significantly in response to the investigated stresses. However, partial correlation analysis showed no significant correlations between the basal or elevated proline levels and tolerance to any of the investigated stress factors. These findings suggest that praline accumulation levels do not serve as reliable parameters for assessing saline alkaline stress tolerance in rice. On the other hand, several root growth indices of rice seedlings, namely TRL, RSA, RD, RV, and RN, showed good correlations with the investigated stress tolerance, and therefore, represent a useful set of growth parameters for assessing saline-alkaline stress tolerance in rice.