Study on Cold Tolerance Potential of Newly Improved Bread Wheat Lines and Cultivars in Cold Observation Nursery in Field

Freezing stress (FS) is one of the limiting factors causing loss of yield in wheat in cold regions of Iran. The objective of this study was to determine the genotype x environment interactions for LT50 and some morpho-physiological characters associated with FR in bread wheat (Triticum aestivum L. em. Thell) genotypes grown in both temperate and cold regions. Randomized Complete Block Design (RCBD) was set up for 30 diverse wheat genotypes from Iran, Canada, USA and Russia in three replications under field conditions of Karaj temperate region and Firoozkooh cold area of Iran in 2007-08 cropping season. Freezing resistance as measured by LT50 (temperature at which 50% of plants are killed at that temperature) method was determined with a programmable freezer for 8 and 10 weeks cold acclimated plants at Karaj and Firoozkooh Agricultural Research Station, in the fall and winter seasons of in 2007-08. Morpho-physiological traits such as width, length or leaf area of leaves no 3 and 5 on main stem and dry matter weight of total plant were measured on plants collected from the field during acclimation periods. Transition from the vegetative to the reproductive stage was determined by dissecting shoot apices and monitoring double ridge formation under a binocular dissecting microscope. Guard cell length (GCL) of leaf no 4 on the main stem of plants was measured with a 'Zeiss' microscope Chlorophyll content of leaf no 4 was recorded with an "Opti OS30 Model" chlorophyll-meter. The simple ANOVA results for all morpho-physiological traits showed significant differences (P0.01) among genotypes in both locations The combined ANOVA results of characters showed that differences due to genotypes was significantly different (P0.01) for LT50, width and leaf length characters. Means of LT50 determination showed that average LT50 for all genotypes was -4.3oC more freezing tolerant at Firoozkooh cold region than plants grown at Karaj with temperate climate. Iranian wheat genotypes had an intermediate level of FR with average LT50 between –7oC and -14 oC. Spring wheat genotypes reached their maximum LT50 between -2 and -7 oC. 'Norstar', 'CDC Ospray' originating from Canada and 'Mirnovoskaya808' from Russia, very hardy winter wheat genotypes, had the greatest FR with the average LT50 of between -18 oC and -22oC. Results of morpho-physiological characters showed that non-hardy genotypes had wider (5-7 mm), or longer leaves (15-20 cm) and high leaf (leaves no 3 and 5) areas (4.5-8.5 cm2) than hardy and semi-hardy genotypes. The average dry matter weight of hardy genotypes varied between 4 and 10 gr, while it was between 9.5 and 11 gr in non-hardy genotypes sampled in the fall. The most LT tolerant genotypes had the smallest GCL(51.8-60T), while the genotypes with the largest GCL(64- 65T) were spring types. Most facultative wheat genotypes had the intermediate GCL. Estimates of crown LT50 gave the highest correlation with leaf length (-0.81**), width (- 0.79**), or leaf area (r