چكيده لاتين :
Introduction: In order for wheat to transit from vegetative to reproductive stage, it requires experiencing a cold period. This phenomenon is known as vernalization. Buds are the sites where vernalizaton takes place. Therefore, wet seeds, young seedlings, developing and immature seeds on maternal plant and even callus from embryonic tissue culture are capable of responding to vernalization (Cao and Moss, 1991). Bread wheat cultivars are considerably varied depending on the developmental or flowering stages. This variability is mostly influenced by a group of genes that include vernalization requirements, photoperiod sensitivity and intrinsic early maturity. The factors that influence vernalization include plant age, intensity of temperature, cold duration, genotype, developmental stage and growth hormones (Rawson et al., 1998). Classifying cultivars as spring and winter based on their response has been prevalent from the past and no classification more reliable than winter, facultative, and spring was provided yet for wheat, barley and rye. According to Wang et al. (1995) among the methods employed to determine biological and phenological stages of cereals, only the final leaf number can clearly exhibit essential biological changes. Vernalization requirement and photoperiod response of bread wheat cultivars are often determined under controlled environments. However, the use of controlled conditions may reduce the applicability of results for predicting field performance and also can lead to increased cost of conducting evaluations. This study was aimed at determining a combination of different vernalization regimes and planting date for 104 bread wheat cultivars in the field trials. This was followed by evaluating the effects of the combination on the final number of leaves and days till ear emergence of the wheat cultivars in re-sponse to vernalizaton and photoperiod sensitivity. Also, the research provided the opportunity to survey some of the improved bread wheat cultivars of Iran in terms of growth habits and vernalization requirements. Materials and Methods: The study was conducted at the farm of Agriculturral and Natural Resources Research and Training Center of South Khorasan in the spring of the year 2014. The seeds of wheat genotypes obtained from breeding programs were first disinfected for 2 min with 1 % sodium hypochlorite and then were rinsed with distilled water. Next, the seeds were placed in 10-cm petri dishes where they were watered with sterile water twice a week to allow them to maintain 50 % of their moisture content (dry weight basis). The pre-treatment procedure included the placement of the germinated seeds in a refrigerator at 2-4°C for 60 days under dark conditions. Following vernalization, seedlings that were similar in length growth were transferred to the field. Lines and cultivars were subjected to two photoperiod and temperature regimes through the use of two planting dates in the field. Planting dates were on April 3 and May 6, which were selected according to the field based evaluation method proposed by Herndl et al. (2008). This was in relation to photoperiod and temperature variations resulting from the two planting dates (Table 2). To remove the cofounding effects of low temperatures, tempera-tures throughout the growing season were constantly above 14 °C, therefore, it was expected that the first planting date could be used for applying photoperiod treatments and the second planting date had the least effect on the photoperiod. The experiment consisted of planting dates (at two levels), vernalization (at two levels of fully vernalized and non-vernalized) and 104 lines and cultivars of bread wheat. To quantify the response to vernalization and photoperiod, two traits were measured: date of end of ear emergence and final number of leaves on the main stem. Also the traits days till ear emergence, ear length during maturity and plant growth habit at seedling stage (Prostrate(1), semi-prostrate(2), average(3), semi-erect(4), erect(5)) were recorded. SAS software package was used to analyze data and classify the genotypes. The genotypes were classified based on the measured traits and the difference in final leaf number under vernalized and non-vernalized treatments using cluster analysis. Results and Discussion: There were statistically significant differences among the winter and spring genotypes in the final number of leaves on the main stem under vernalization and non vernzlization treatments (Figs 3 and 4). Non-vernalized winter genotypes produced more leaves than the vernalized ones, indicating that when vernalization requirement of the genotypes is fulfilled, their final leaf number decreases as a result of response to long photoperiod and ear emergence. Some genotypes subjected to non-vernalization treatment, which could not fulfill their vernalization requirements, did not enter the reproductive stage and consequently generated more leaves. Growth habit at seedling stage was not markedly affected by the treatments (Table 1). This shows that the changes in seedling growth habit were similar for the vernalized and non-vernzlized genotypes, indicating that this trait is influenced by the plant genes rather than the environmental cues. As shown in Figure 5, winter wheat genotypes exhibited more prostrate growth habit as compared to spring genotypes. Days till ear emergence under non-vernalization treatment was significantly different from those under vernalization treatment (Table 1). This indicates that when vernalization requirements of the genotypes are met, their growth period declines in respone to long photoperiods (Van Beem et al., 2005). Vernalization requirement is one of the adaptation mechanisms of plants by which they can adjust their growth and reproductive periods to growing season conditions. This enables the fully vernalized plants to accelerate their reproductive phase, particularly in the Mediterranean climate, to avoid late season unfavorable conditions such as heat and drought. Planting date had no significant effect on the meatured traits except for days till ear emergence. Based on cluster analysis, the genotypes such as Gaspard, Baran, C-86-5, Zare, Rasad, Sabalan, Sardari, Mihan, Soisons, Gaskogen, Homa, Sween 220, Omid, Karim, Pishgam, MV-17 and C-85- 3 belonged to winter group. The genotypes like Bezostaya, Ohadi, Karaj3, Tabasi, Navid, Mahooti, Shahriar, Orom, C-86-6, Zarin, Toos, Alvand, Rooshan, C-85-D8 and C-85-6 were classified as facultative group. The others were spring. According to these findings, majority of the Iranian bread wheat cultivars belonged to class of spring habit (69%) with minimal vernalization requirement. Almost 16.5% were winter types and about 14.5% were facultative.
