| White clover (Trifolium repens L.), which is leguminous forage grass,was mainly used for interplanting in woodland, green cover, green manure with high quality, nectariferous plant, medicinal plant, and forage, etc, has an very important biological significance and socialeconomic benefits. The viability of white clover seeds is better, and the plants have strong ability of occupation. But its ecological range is not wide, and especially, the natural survival rates of some white clover varieties in north China are very lower. Therefore, the demonstration areas of white clover were restricted in north China. Based on this situation,10white clover varieties (Rivendel, Durana, Tribute, Tahora Ⅱ, Prop, Barbzan, Haifa, Alice, Dutch White, and Huia) were chosen in this experiment, and the seed germination characteristics, the cold tolerance, and the relationship of the cold tolerance with the leaf morphological and anatomical structure were studied in order to assess the cold tolerance of10white clover, and select the suitable varieties with higher over-winter rates. The results showed that:(1) The seeds of ten white clover varieties (Rivendel, Durana, Tribute, Tahora Ⅱ, Prop, Barbzan, Haifa, Alice, Dutch White, and Huia) were respectively exposed to30℃,25℃,20℃,15℃and10℃. To understand the characteristics of white clover germination, the seed germination energy, germination rate, germination index, and root length were investigated. Results showed that the seed germination energy, germination rate and germination index of Rivendel, Tahora II and Dutch White followed reduction trends with the reduction of temperature. The seed germination energy, germination rate and germination index of Durana, Prop, Haifa and Alice at25℃were the maximum value, but Barbzan and Huia at20℃showed the best. The germination rate and germination index of Tribute were the best at25℃. The seed germination energy, germination rate and germination index of10varieties of white clover at15℃were decreased, and the seed germination index and root length were obviously decreased comparing with them of at20℃(P<0.05), especially, they were decreased significantly at10℃. Therefore, both15℃and10℃was not the suitable germination temperature for all of varieties.(2)The plant leaves of ten Trifolium repens varieties were respectively exposed to10℃,5℃,0℃and -5℃(25℃is as control) with the method of indoor artificial simulating low temperature treatments, and the relative electrical conductivity (REC), malondialdehyde (MDA), free proline (PRO), superoxide dismutase (SOD) and peroxidase (POD) of their leaves were measured respectively in order to assess the cold tolerance of10varieties of T. repens at the seedling stage. Results showed that the REC, MDA, PRO and SOD followed increase trends with the reduction of temperature, and the POD contents exhibited the "N" type trends of increase-reduction-increase. According to the results of the furthest neighbor cluster analysis of the5physiological indexes for10T. repens varieties at-5℃,10varieties could be divided into three groups, e.g., the stronger cold tolerance (Durana, Barbzan and Dutch White), the medium (Alice, Rivendel, Tahora Ⅱ and Huia) and the weaker cold tolerance (Haifa, Tribute and Prop).(3)The order (from higher to lower) of the winter natural survival rates of10Trifolium repens varieties in the field, were Barbzan>Durana>Dutch White>Rivendel-Tahora Ⅱ>Alice>Tribute≈Huia>Prop>Haifa. The results were basically identical with the above results using the furthest neighbor cluster analysis of the5physiological indexes for10T. repens varieties at-5℃.(4)Two stronger cold tolerant varieties (such as Barbzan and Durana), and Two weaker cold tolerant varieties (such as Haifa and Prop) were selected to observe the morphological and anatomical structure of leaf epidermal cells at-5℃. The results showed that the fine hairy warts were attached with the leaf upper and lower epidermis cells of4varieties, and the stomatal apparatus were sunken into the epidermal cells. The stomatal length (14.79μm、8.34μm), width(3.71μm、1.87μm), stomatal area (54.87μm2、15.55μm2), and stomatal density (121.33/mm2、221.25/mm2) of the stronger cold tolerant varieties were less than the ones of the weaker cold tolerant varieties (16.15μm、10.08μm)、(4.61μm、2.79μm)、(74.38μm2、28.14μm2)、(154.64/mm2、238.27/mm2) in upper and lower epidermis, respectively. But the ratio of the stomatal length and width of the stronger cold tolerant varieties (4.01、4.48) were larger than the ones of the weaker cold tolerant varieties (3.51、3.61). The cells density (1582.98/mm2) of the stronger cold tolerant varieties were more than the ones of the weaker cold tolerant varieties (889.28/mm2), but the areas of the long cells and short cells (1117.33μm2、710.27μm2) of the stronger cold tolerant varieties were less than the ones of the weaker cold tolerant varieties (1569.72μm2、975.65μm2)... |
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