Winter wheat varieties require an extended low temperature period to initiate or accelerate the flowering process (vernalization). Spring wheat varieties do not have the vernalization requirement, and they would be exposed to a high risk of cold injury if sowed in low temperature. Correct evaluation of the winterness is the prerequisite to study and utilization of wheat germplasm, wheat production, and minimizing the cold injury. It also has certain theoretical significance when explaining the ecotype of wheat. Therefore, it's of great importance to build an accurate method to evaluate vernalization characteristic of different wheat varieties. To address this issue, we investigated the growth characters of 12 wheat varieties under natural or artificial vernalization conditions, and proposed two reliable standards to evaluate the winterness of wheat.
Twelve varieties, including Shannong8355, Jimai20, Jimai22, Shangnong14, Shannong15, Taishan008, Shannong664, Jimai19, Shannong16, Zimai12, Yannong19 and Shannong2413, were sowed every month from October to the following February. They experienced the winter and acquired vernalization naturally. For comparison, these twelve varieties were sowed every month from March to May after an artificial low temperature treatment was appllied from March to May next year. The temperature of artificial vernalization was set to 3℃, and the durations were 0d, 10d, 20d, 30d, 40d, 50d, and 60d, respectively. The experiment was repeated twice in two years.
The characters related to vernalization, such as number of spikes, spikelets and stem tillers, and thousand seed weight, were investigated. The datasets obtained from wheat that have been naturally or artificially vernalized were statistically compared. In addition, the growth cones of vernalized wheat planted in May were dissected and checked on the 20th day after emergence.
The data was normalized first, followed by variance test and cluster analysis. The results showed, that the character of number of stem tillers, spikelets and thousand seed weight, were overlapping among different kinds of cultivars under natural and artificial vernalization, therefore the indexes above are not suitable to evaluate the strength of winterness of wheat. Neither can the number of wheat spikes under condition of natural vernalization, because it was affected by many factors, such as planting time and seeding rate. By contrast, the number of wheat spikes, under artificial vernalization that the effect of natural low temperature was eliminated, can serve as a good indicator to evaluate the strength of winterness. In addition, the shape of growth cone, which grows up about 20 days after emergence, is another indicator as good as the numbers of spikes.
Based on the cluster analysis of the numbers of spikes under the conditions of artificial vernalization, the twelve wheat varieties surveyed here were divided into three categories, namely weak winterness, winterness and strong winterness. Shannong8355, Jimai20, Jimai22 and Shangnong14 belonged to weak winterness cultivars. Shannong15, Taishan008, Shannong664 and Jimai19 were winterness cultivars, and the strong winterness cultivars included Shannong16, Zimai12, Yannong19 and Shannong2413. This result is largely consistent with that obtained from observation of morphology characteristics, but the former gives more accurate evaluation for some varieties.
As a conclusion, the number of spikes and shape of growth cone under artificial vernalization are two indicators of wheat winterness. When using this method, to avoid influence of natural low temperature, the daily maximum temperature should be higher than 12℃ at sowing stage. Meanwhile, the days of daily minimum temperature below 3℃ should be less than 4 days after planting. For evaluating vernalization characteristic of wheat in Shandong province, the temperature in April is suitable to sow after different time of artificial low temperature treatment. In other area, optimum planting date can be determined to satisfy the above temperature requirements.