Freezing Tolerance of Garlic Ecotypes (Allium sativum L.) under Controlled Conditions

Garlic is a medicinal and cool season plant, but there is not much information on its level of cold tolerance. Khorasan province is one of the suitable areas for growing garlic. The conventional planting date of garlic in this region takes place between 6 October and 5 December. Available evidences shows that garlic plants have been affected by cold stress due to inappropriate planting date. Some researchers have recommended the use of controlled freezing tests as a suitable approach to assess cold tolerance of plants. In this regard, it is stated that LT50su in wheat had the highest correlation with field survival index when the crown was exposed to cold stress. The present experiment was conducted to study cold tolerance and understanding the most critical growth stages of garlic ecotypes to cold stress under controlled conditions.

This experiment was conducted as a factorial based on completely randomized design with four replications in Agricultural Faculty of Ferdowsi University of Mashhad during 2012. The experimental factors included two levels of planting date (18 September and 21 October), garlic ecotype in four levels (Bojnurd, Torbat-e Heydarieh, Neyshabur and Khaf) and eight levels of freezing temperature (0, -3, -6, -9, -12, -15, -18 and -21°C). Plants that grown in outside conditions and acclimated to cold temperatures were transferred to the thermogradient freezer to apply cold stress. Survival percentage, Lethal Temperature 50% mortality according to the survival percentage (LT50su), plant height, leaf area, reduced leaf area temperature 50 (RLAT50), dry weight and reduced dry matter temperature 50 (RDMT50) were determined at three weeks after applying the stress.

In the first planting date, the survival of Bojnurd and Torbat-e Heydarieh ecotypes decreased 20% and 26% at -15 °C compared with zero temperature, respectively, while two other ecotypes disappeared at this temperature. In the second planting date, Khaf and Bojnurd ecotypes had a good survival at -15 °C while Neyshabur and Torbat-e Heydarieh ecotypes disappeared 60 and 67% at this temperature, respectively. Since the growth stage has an effect on cold acclimation and stress tolerance, this response can be due to difference in the growth stage of plants. However, this reaction was different depending on the ecotypes. With changing planting date from September to October, LT50su decreased 9.6 and 6 °C in Khaf and Neysabur ecotypes, and increased 3.9 and 2 °C in Bojnurd and Torbat-e Heydarieh ecotypes, respectively. In the first planting date with decreasing temperature from zero to -18 °C, the maximum and minimum decrease in height (90 and 27%) were observed in Torbat-e Heydarieh and Bojnurd ecotypes, and Neyshabur and Khaf ecotypes disappeared at this temperature. Also, in the second planting date with decreasing temperature from zero to -15 °C, plant height decreased 47, 52 and 30% in Khaf, Torbat-e Heydarieh and Neyshabur ecotypes, respectively. However, plants height of Bojnurd ecotype decreased only 4% under similar conditions. In the first planting date Bojnurd and Torbat-e Heydarieh ecotypes had the lowest decrease in leaf area (about 3 and 17%, respectively) at 12 °C compared with zero temperature, while Khaf and Neyshabur ecotypes assigned to themselves the highest decrease in leaf area (about 85 and 58%, respectively). Also, in the second planting date leaf area of Bojnurd ecotype decreased 18% at -15 °C compared with zero temperature, while in similar conditions Torbat-e Heydarieh, Khaf and Neyshabur ecotypes had 56, 38 and 58% decrease in leaf area, respectively. The lowest decrease in dry matter percentage was observed in Neyshabur (23%) and Bojnurd (22%) ecotypes in the first and second planting date, respectively, following decreasing temperature to -12 °C. Studies have shown that plants with faster growth before the frost were more sensitive to cold temperatures and as a result, damaged more. So, it seems that increasing growth of Khaf and Neyshabur ecotypes and their progress towards more advanced growth stages reduced cold tolerance of these plants in the first planting date.
 

Freezing stress decreased survival percentage and recovery of garlic ecotypes. In the first planting date Bojnurd and Torbat-e Heydarieh ecotypes had a better survival percentage and Khaf and Neyshabur ecotypes had lower survival percentage while in the second planting date Khaf and Neyshabur ecotypes had higher survival percentage than Bojnurd and Torbat-e Heydarieh ecotypes. Also, results of LT50su indicated that Bojnurd and Khaf ecotypes had higher freezing tolerance in the first and second planting date, respectively. Different reactions of garlic ecotypes at various stages of stress may be due to their genetic characteristics and geographic origin. In terms of recovery (height, leaf area and dry weight), the plants of the second planting date had more height and dry matter, as well. Based on the RDMT50 and RLAT50, moreover, Bojnurd ecotype in both planting date and Khaf ecotype in the first planting date were recognized as most tolerant and sensitive ecotypes, respectively.