| Alfalfa (Medicago sativa L.) is the most important and widely cultivated forage legume in the world, and it plays an important role on livestock husbandry in China. However, lack of winter hardiness is a major limitation to the reliable use of alfalfa. Fall dormancy has traditionally been the primary component used to predict winter hardiness in alfalfa because of its association with winter survival. The important point of selection for winter hardiness is that the germplasm selected must also be high yielding, thus winter hardiness and less fall dormancy are being selected concurrently. Fall dormancy is playing an irreplaceable role on evaluation of winter hardiness in alfalfa varieties and lines and also in selection in alfalfa breeding. Alfalfa varieties with contrasting fall dormancy class were use in this research, to test the responses of seed germination to constant temperature, and compare the germination traits and parameters of thermal time model of different varieties and test the relationships between these traits and fall dormancy class. Freezing tolerance of alfalfa seedlings with different growth stage was studied and the freezing temperature that kills 50% of seedlings (LT50) for each variety of alfalfa was evaluated as well. Local alfalfa varieties and Medicago falcata germplasm were used with other alfalfa varieties with contrasting fall dormancy class to study the growth traits of seedling with different growth stages under different sowing dates, and the correlated relationships between the growth traits and fall dormancy were tested as well. Research on the appropriate defoliating time of the determination for fall dormancy class of alfalfa varieties was conducted. The root and crown traits of different alfalfa varieties in the establishment year were also studied. In addition, experiment was conducted to study the emergence and seedling growth of five forage legume species at various burial depth and two light levels.The results of alfalfa seed germination response to temperature indicated that there was no significant liner regression relationship between the final germination percentage and fall dormancy class. However, there were significant or extremely significant liner regression relationship between the germination rate, Tb,θand fall dormancy class, especially forθ(r=-0.99,p<0.00). Germination rate and base temperature decreased with the fall dormancy class of alfalfa varieties whileθincreased with the fall dormancy class. The high correlated between fall dormancy and germination rate, Tb andθindicated that measuring fall dormancy class of alfalfa through the equations using those traits will make the evaluation of FD of the experimental lines and varieties much more convenient, and also shorten the screening and breeding cycle. Experiments were conducted in the growth chamber to determine freezing tolerance of 3 alfalfa varieties at four growth stages. Tolerance to freezing temperature varies among varieties and seedling growth stages. There was a significant interaction between seedling growth stage and freezing temperature, the differences of freezing tolerance among seedling growth stages were mainly determined by freezing temperature. Seedling freezing tolerance increased with seedling age when the freezing temperature was higher than LT50. However, freezing tolerance decreased with seedling age when the freezing temperature was lower than LT50. -9℃was suitable to distinguish different varieties, because the differences in freezing tolerance of seedling of all growth stages of different varieties were significant. The LT50 were -6.8 to -8.1℃for alfalfa varieties, the relative ranking among varieties was the same with their fall dormancy class. The prediction equation for each variety have potential for predicting seedling stand loss at given freezing temperatures.According to the response of alfalfa seedling morphological traits and biomass accumulation to different sowing date and its relationship with fall dormancy class, when sowing in autumn, the correlations between growth traits of 4 weeks old seedlings and fall dormancy class of varieties were the greatest. Seedling height, seedling biomass, unifoliate internode length, unifoliate leaf petiole length, unifoliate leaf length and wide of alfalfa seedlings sowing in autumn and fall dormancy classes of varieties were significantly highly correlated, and the correlated coefficient were all higher than 0.9. The regression equations according to these traits could predict the fall dormancy to a certain extent. The results of hierarchical clustering indicated that, according to the seedling growth traits sowing in autumn, alfalfa varieties could be classified to three groups: fall dormant group, semi-fall dormant group and non-dormant group, which was similar with that predicting by fall dormancy class. Plants of the three fall dormant types could be distinguished by the seedling growth traits sowing in autumn.The comparation about the differences among alfalfa varieties under different defoliating time indicated that defoliationg on August 25 and determining the regrowth height after 25days or more could discriminate varieties with different fall dormancy. And the result under solid planted condition is also reliable. The determined fall dormancy class of Ximeng was 1 in this study, which indicated the extremly fall dormancy of M.falcata.There were significantly differences between root and crown traits of different alfalfa varieties in the establishment year. GongNong No.2, CW402 and Golden Empress had better root and crown development abilities, for their taproot diameter, taproot mass, taproot length, total root mass, lateral root number, crown diameter, crown branch number and crown bud number were higher than other varieties. The taproot diameter and root biomass of all alfalfa varieties were decreased from soil surface layer to deep layer. Distribution of alfalfa root systems in the establishment year were mainly in the underground 40 cm. Lateral root of all varieties which grew from the taproot were in underground 0～30 cm. Few lateral roots were found below 30 cm. In this study, Medicago falcata from XiMeng had thinner taproot, lower total root mass, fewer lateral root and shallower crown depth, which indicated that the growth rate of M. falcata was slow and it developed incompletely in the establishment year. However, most characteristics of local alfalfa variety GongNong No.2 were better than those of other cultivars, which meant that root and crown traits were related to variety adaption. Correlation analysis indicated that there was no significant relationship between fall dormancy and root or crown traits. However, there were significantly or extremely significantly correlation between taproot diameter, total root mass, taproot mass, taproot length, lateral root number, crown diameter, crown depth and basal bud number.The results of the seedling emergence and structure of five forage legumes at five planting depths and two light levels indicated that the optimal planting depth for all the forage legumes in this study was 1 to 2 cm. Increasing depth lowered and slowed seedling emergence, and reduced the seedling size. Shade condition improved seedling emergence but was unfavorable for seedling growth. Seedling emergence and growth were usually better for species with bigger seed than those with small seed. M. falcata, M. suaveolens and M. sativa had higher percent emergence and better growth from deeper depth or under shade condition.In conclusion, we found some important traits that are valuable on predicting fall dormancy and adaption of alfalfa varieties through the studies on alfalfa biological traits under different environment conditions. The prediction equations of fall dormancy class were obtained according to these traits. It is a quick and effective way on evaluation of fall dormancy of alfalfa varieties and lines and also can shorten the selection of winter hardiness in alfalfa breeding. |
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