Screening Pomegranate Cultivars for Freezing Tolerance by Reliable Methods

Freezing damage is a significant environmental challenge that limits both the geographic distribution and production of pomegranates in the world. The objective of this research was to search for useful correlations between freezing tolerance and soluble carbohydrate and proline content, as well as stomatal density in different parts of the leaves, to allow indirect selection of winter hardiness in pomegranate cultivars. Our results showed that freezing tolerance at the non-acclimated stage (August) was not strongly correlated with freezing tolerance during deep winter dormancy in January. Cold tolerance in summer was strongly correlated with leaf stomatal density; however, a moderate correlation was observed between cold tolerance in winter and leaf stomatal density. The results showed that 'Alak', the most cold-tolerant cultivar in summer and winter, had the highest leaf stomatal density. 'Agha Mohammad Ali' cultivar had the lowest leaf stomatal density and cold tolerance in summer; however, this cultivar showed considerable cold tolerance in winter. Moreover, with the except of 'Agha Mohammad Ali', stomatal density in summer was related to soluble carbohydrate concentration in the stems, which could justify the correlation between stomatal density and winter hardiness in pomegranate trees. It was found that LT50 values in January were negatively related to soluble carbohydrate concentration in stems. However, there was no statistical correlation between winter hardiness in January and proline content in the stems. These results suggest that soluble carbohydrates and stomatal density are suitable indices for predicting freezing tolerance of pomegranate cultivars throughout the year and growing season, respectively.