Introduction: Wheat (Triticum aestivum L.) is the main staple food in many parts of the world including Iran. This crop is one of the most cold stress tolerant crops amongst domesticated plants. Still, Severe freezing in winter and early spring may influence the growth and survival of wheat plants and decrease its growth and seed yield. The absolute minimum temperature in North Khorasan reach to -20°C in many years and if a cold tolerant cultivar is not choose or if wheat field in not hardened properly then the freezing stress might be injurious. Therefore each new cultivar that introduced to the region should firstly test to chilling and freezing stress. This experiment was carried out to explore freezing tolerance of two wheat cultivars (Mihan and Pishtaz) under controlled and field conditions with spraying paclobutrazol (PBZ) and non-spray.
Material and Methods:  Seeds of two wheat cultivar Mihan and Pishtaz were sown in plastic pots in 20^th of September in Bojnord and kept them in open space until third leaf stage in order to grow in the same climatic conditions as they will grow in the future. All plants could receive the enough temperature for winter hardening then they transfer to Mashhad to apply freezing stress in controllable freezer. After applying freezing stress all pots transferred into research greenhouse at Ferdowsi University of Mashhad with light density of 500±50 μmol m^-2 s^-1 and light/dark cycle of 14/10.   Sampling and measurement were arranged after four weeks that plants grown in the greenhouse.  Measured parameters included electrolyte leakage, plant height, green leaf area, plant survival rate and number of tiller and leaves per plant. Field experiment was arranged in the Kohnekand Agricultural research center in Bojnord. Both experiments conducted as a split-split plots based on a completely randomized design with three replications. The main plots in green house were six freezing temperatures (zero,-4, -8, -12, -16, and -20°C), the sub-plots were three paclobutrazol (PBZ) concentrations (foliar application of zero as control, 30 and 60 mg L^-1) and the sub-sub-plots were wheat cultivars (‘Mihan’ and ‘Pishtaz’) with three replication. In the field experiment main plots were three sowing dates (16^th Oct., 13^th Nov. and 5^th Dec.), and the sub-plots and sub-sub-plots were the same as controlled experiment. Data were statistically analyzed using the SAS software (SAS version 9.2). The LSD test (p=0.05) was used to determine which treatment is statistically different from the others.
Results and Discussion: The results showed that, Mihan cultivar produced more leaves but lower tiller per plant compare to pishtaz, Up to -16°C but at and -20°C the tiller number was also not significant. But reduced to -20°C, the electrolyte leakage of membrane increased significantly by 71.5%. The electrolyte leakage in Mihan cultivar was significantly lower than that of Pishtaz cultivar when freezing temperature was decreased lower than -16°C.  PBZ also caused shorter plants but less leaf lost observed due to freezing stress in both cultivars. Delay in sowing date from 16^th Oct. to 13^th Nov. and to 5^th of Dec. caused 19% and 31% seed yield reduction in two wheat cultivars, respectively. While the average seed yield of Mihan was 17% higher than pishtaz cultivar. Biological yield also followed the same trend of grain yield. PBZ application did not show any significant effect on seed yield, this result might be due to ABA production after PBZ spraying, which can negatively affect yield component formation.  Based on our results temperatures below -12°C is much more injurious that higher temperatures, and the advantage of freezing tolerance wheat cultivars appears in lower temperatures.
Conclusions: Based on our results, Mihan cultivar has higher freezing tolerance capacity and in temperature below -12°C and PBZ application this cultivar can alleviate negative effects of freezing stress. Therefore, introduction and extension of this cultivar in mountain regions and those areas that the risk of freezing stress is high might reduce the risk of freezing damage in Bojnord area and at the same time produce reliable grain yield.