The effect of ecotype and planting date on freezing tolerance in garlic medicinal plant (Allium sativum L.) under controlled conditions

Garlic is a plant with a relatively high tolerance to cold winter but extreme autumn and winter in cold regions are among the factors that affect the growth and survival of plants. So it seems that one of the critical success factors for fall planting is identification of cold tolerant plants in these areas. To evaluate and recognition tolerant plants to cold stress and to avoid some inevitable limitations in field evaluation, different types of artificial freezing tests have been developed. Electrolyte leakage test is one of this methods that is simple, repeatable and cheap and relatively fast which determines the degree of damage to cell membranes caused by the stress. When the plant tissues damage by cold, membrane activity decreases and electrolytes inside the cell leaks to the outside its. It has been reported by many researcher that cold stress increases electrolyte leakage and this trait can be used as a criterion for evaluation of freezing tolerance. Also the freezing temperature in which 50% of ions are leaked from the cells has been proposed to be used as the 50% threshold of damage of freezing stress.
In order to evaluate freezing tolerance of garlic a factorial experiment was conducted in 2012 year based on completely randomized design with four replications under controlled conditions in faculty of agriculture, Ferdowsi University of Mashhad. Experimental factors consisted of two planting dates (19 September and 22 October) and four ecotypes of garlic (Bojnurd, Torbat- Heydarye, Neyshabur and Khaf). Plant after growth and hardening under natural environment exposed to eight freezing temperatures (0, -3, -6, -9, -12, -15, -18 and -21 degree ºC). After freezing, electrolyte leakagepercentage and lethal temperature for %50 of plants according to electrolyte leakage percentage (LT50el) were determined. After 21 days survival percentage was measured in the greenhouse.
Electrolyte leakage percentage decreased in four ecotypes of garlic in the second compared to the first planting, but the greatest reduction (11.3%) was observed in the Khaf ecotype, while this reduction was 1.1% in the Torbat-Heydariye. In both planting dates electrolyte leakage was roughly stable until -15 degree ºC and it was increased at lower temperatures but increasing in the first was more than the second planting, So that by reducing the temperature from -15 °C to -21 °C electrolyte leakage increased in first and second planting by about 34 and 24 percent, respectively. Also, lethal temperature 50% of plants according to the electrolyte leakage percentage in the second planting date was about 1° C lower than the first planting. At the time of freezing stress, most plants were planted in September were at four-leaf stage and the plants were cultivated in October at the two-leaf stage and although the first planting plants were more time under normal conditions and thus acclimation, But electrolyte leakage was more in them. This case may be due to more food reserve in second planting plants. In the both planting date in the Bojnurd and Torbat-Heydariye ecotypes electrolyte leakage percentage was stable until -15oC and then increased with decreasing of temperatue, While in Neyshabur and Khaf ecotypes in the first planting the temperature reduction from –6 and -12 oC was increased electrolyte leakage. However, in the last two ecotypes (Khaf and Neyshabur) in the second planting date electrolyte leakage was relatively stable up to temperatures of -15 oC and at lower temperatures electrolyte leakage increased. It seems that more plant growth and development toward the higher growth leads to less tolerance in the first planting of plants. Lower values of lethal temperature 50% are indicating greater tolerance to cold and amount of this index was lower in October planting, so it can be said that early planting garlic (28 September) is likely leads to increase plant sensitivity to cold. There was high and negative correlation between electrolyte leakage% and survival percentage (r=-0.70*) in this survey. In other words, by increasing the survival percentage electrolyte leakage decreased.
In the first planting electrolyte leakage from Khaf and Neyshabur ecotypes increased with exposure to temperatures of -6 and -9 ° C, while in two other ecotypes this situation stared from -15° C temperature. While the in second planting in four ecotypes began the process of temperature -15 ° C. Different reactions of ecotypes to freezing stress, especially in the first planting is probably due to genetic characteristics geographical origin. Also, growth stage of plants were effective in the cold tolerance of them, So that Khaf and Neyshabur ecotypes in lower growth stages (second planting plants) compared to the advanced growth stages showed more freezing tolerance and it seems that the two listed ecotypes in the later planting have freezing tolerance more than the first planting. Torbat- Heydariye and Bojnurd ecotypes had more membrane stability and they showed better reaction in both fall planting under cold stress, despite the continuation of these studies under controlled and field conditions will provide more information about cold tolerance mechanism of this plant in real winter conditions.