Quantification of temperature effects on germination and induction of secondary dormancy in barley cultivars

Introduction Temperature stress during germination and emergence is one of the most important environmental stresses during plant life cycle. All growth stages of a plant, including seed germination and seedlings establishment, are affected by temperature. High temperatures during germination can cause numerous problems for germination or seedling growth. Temperature regulates the germination and seed emergence in the field in three ways: (1) effect on the germination rate and percentage of non-dormant seeds; (2) elimination of the primary and secondary dormancy; and (3) induction secondary dormancy. One of the most important planting problems is thermoinhibition and thermodormancy induction in seeds during warm season. Studies has shown that barley seeds have different levels of primary and secondary dormancy. So paying attention to the reaction of the seeds to the temperature sensitivity during germination is very important. In present study it has been attempted to study the behavior of germination and the induction of secondary dormancy of barley cultivars at various temperatures, especially at high temperatures. Materials and methods This study was carried out at Seed Research Laboratory of Gorgan University of Agricultural Sciences and Natural Resources. The factors included 15 levels of barley cultivars (Aras, Dasht, Eram, Fajr 30, Jonoob, Karoon, Nike, Nimrooz, Nosrat, Zarjoo, Sahra, Sina, Torkaman, Valfajr and Zahak) and eight levels of germination temperature (5, 10, 15, 20, 25, 30, 35 and 40 ºϹ). Germination percentage by average comparison and germination rate was described by fitted segmented model. Seeds germination, death and dormancy induction behavior at high temperature (40ºϹ) was described by placing seeds in different duration of imbibition at 40ºϹ, then transfer them to optimum temperature (20ºϹ) for one week and finally tetrazolium test. Results and discussion The results showed that there was a significant difference between the cultivars in terms of germination percentage at different temperatures and the highest differences were observed at high temperatures (especially at 35°C). Also germination cardinal temperatures were different between various cultivars. The highest difference was found in the base temperature and optimum temperature of germination, and there was no significant difference between the cultivars in ceiling temperature. Non-germination at high temperature (40ºϹ) was partly due to the induction of secondary dormancy and at some extent related to the seed death after being exposed to high temperatures. The germinability and seed deaths respectively increased and decreased exponentially as duration of seed imbibition at high temperatures increased. On the other hand, induction of secondary seed dormancy followed a normal distribution, so that as imbibition duration at high temperatures increased, the capacity for dormancy induction first increased and then decreased. Of course, these trends differed depending on the cultivar. According to the recent findings, thermodormancy induction in barley seeds is highly dependent on the sensory pathways and biosynthesis of the hormone regulating dormancy and germination, including gibberellin, ethylene, cytokinin and abscisic acid. Conclusions In this study, the reaction of germination to temperature in the most important and common cultivars of barley in iran was investigated. There were significant differences between cultivars in terms of reaction to temperature in the germination stage, and each cultivar showed a unique behavior. The temperature sensitivity pattern was different in the cultivars, and the germination reaction of seeds at high temperature had high variance. Present study shows, the cause of non-germination of barley seeds at high temperatures was related to thermoinbition of germination, seed mortality, and dormancy induction. Therefore, in future studies, it is suggestion that particular attention should be paid to the hormonal balance pathways associated with induction of dormancy in the seeds, as well as the gene expression.