Assessment of Reaction of Some Iranian Hazelnut (Corylus avellana L.) Cultivars to Freezing Stress

.Background and objectives: Freezing stress is a serious threat to the sustainable production of crops that affects plant development and causes a series of morphological, physiological, biochemical and molecular changes. Freezing damage to hazelnut flowers is one of the most limiting factors of its planting in the world. This research was conducted to evaluate the response of some Iranian hazelnut cultivars to freezing stress at flowering time, also to find the relationship between flowering time, the germination rate of pollen, the amount of proline amino acid and hydrogen peroxide enzyme with the degree of susceptibility and tolerance of local hazelnut cultivars against freezing stress.
Materials and methods: Eight Iranian hazelnut cultivars including Asle Gharebagh, Tabestaneh, Jonoube Gharebagh, Khandan, Rasmi, Gerde Eshkaverat, Mish Pestan and Nakhon Rood, due to the relative superiority of their quantitative and qualitative traits compared to other cultivars, were selected. Four trees from each cultivar were chosen and the emergence time of male and female flowers, emergence time of red stigma and flowering type were recorded. The pollen germination rate in media containing 10 and 15% sucrose was examined. In order to evaluation of freezing effect, unopened male and fertilized female flowers of eight hazelnut cultivars were exposed to 10 levels of freezing treatment and after 24 hours, the freezing damage was evaluated and based on the type and the amount of morphological changes was scored. The activity of hydrogen peroxide enzyme and proline amino acid was measured before and after freezing treatment. The factorial experiment based on a completely randomized design with three replications was performed. The data analyzed using SAS software and Duncan‘s multiple range test were applied to test for significant differences between the treatments.
Results: The male and female flowers appeared on July-September. Tabestaneh and Mish Pestan cultivars had the highest germination rate of pollen in media containing 10 and 15% sucrose. Rasmi, Tabestaneh and Mish Pestan cultivars were showed the highest yield. Morphological assessments were indicated that the starting of freezing of fertilized female flowers and unopened male flowers occurred at -3 °C and -5 °C, respectively; and complete freezing and tissue destruction of fertilized female flowers and unopened male flowers were observed at -5°C and -11 °C, respectively. In this regard, the male flowers of Mish Pestan cultivar and the female flowers of Tabestaneh cultivar had the highest resistance to freezing stress. The results revealed that there was no significant relationship between the activity of hydrogen peroxide and the coldness. Also, the tolerant cultivars had a lower content of this enzyme compared to the control. Our results revealed that the percentage of the freezing damage depends on the proline content of hazelnut flowers. Mish Pestan and Tabestaneh cultivars had the highest proline content.
Conclusion: According to our results, Mish Pestan and Tabestaneh cultivars, those had the highest yield and tolerance against freezing stress, are recommended to cultivate in regions with a high risk of spring freezing that the temperature at the emergence time of male and female flowers are not below -9 and -3 °C, respectively.