Study On Cold Tolerance Characteristics And Estabilishment Of Cold Tolerance Evaluation System In Chrysanthemum

Chrysanthemum, as one of the famous traditional flowers in China, was widely used as cut flower, potted plant and garden ornamental flowers. Cell membrane stability of leaf and petal, cold tolerance and the recovery of emergent rhizomes were investigated in chrysanthemum under cold acclimation or cold treatment via LSD and correlation analysis. The changes of SOD and CAT activity, MDA content, soluble sugar and soluble protein content in leaves under cold treatment were also investigated, the correlation and hierarchical cluster analysis were conducted subsequently. In order to establish the cold resistance evaluation system of chrysanthemum, cold indexes were searched by correlation analysis, hierarchical cluster analysis, main component analysis, regression analysis and path analysis. The main results are as follows:(1) The traditional method of testing relative electric conductivity was not suitable for testing the relative electric conductivity of chrysanthemum leaves. Our study showed that soaking the entire leaves for 15h can get accurate results; the LT50 temperature of 20 chrysanthemum cultivars ranged from-3.55℃to-10.92℃, cultivar 03 (6)-12 is the most non-hardy cultivar, while 'Jinlinghuanghe' is the most hardy cultivar. The results of cold resistance judged by the LT₅₀ temperature of leaves and petals separately are different.(2) Natural low temperature during autumn and winter enhanced the cold resistance of chrysanthemum obviously. The LT₅₀ temperature decreased with the drop in temperature, the decrease of LT₅₀ are cultivar dependent, varied from 4.0℃to 9.4℃. Among 8 cultivars, 'Yinxing', 'Jinlinghuanghe', 'Aoyunwanxia' and 'Jinling-zhiguang' were more tolerant to cold, whereas 'Aoyunhuoju', 03 (6)-12, 03 (6)-16 and 'Aoyunjinyun' are less tolerant. Emergent rhizomes recovering test showed a similar result to the freezing test indicating that LT₅₀ temperature was credible to evaluate the cold tolerance of chrysanthemum. No emergent rhizomes recovered after lethal low temperature of-14℃treatment. (3) Correlation between SOD activity and cold resistance was significant. So it's a reliable index to evaluate the cold resistance. The correlation relationship is much more obviously under lower temperature. When subjected to low temperature treatment of-14℃～-17℃, the SOD activity decreased in of non-hardy cultivars while increased in of hardy cultivars.No significant correlation between CAT activity and cold resistance was observed. The change of CAT activity showed similar 'â…¤' trends in 8 cultivars, i.e., decreased in the earlier stages, then increased subsequently. Under two gradient temperature conditions of control temperature to-8℃or-11℃to-14℃, the CAT activity in hardy cultivars decreased or increased more greatly than non-hardy cultivars. CAT can't be a valid index of the cold resistance of chrysanthemum.MDA content is related to the cold resistance, although no significant relationship was observed between them. It inferred that MDA can't be used to evaluate the cold resistance of chrysanthemum solely. When the treatment temperature lower to-8℃, the MDA content of 8 cultivars began to reduce and then increased during the continuing lower temperature treatment. The change ratio of MDA content during-11℃～-14℃showed a close relationship with cold resistance, MDA content of non-hardy cultivars increased less than those in hardy cultivars.Soluble sugar is a good cold torelance index, the content of soluble sugar showed close relationship with cold resistance. The soluble sugar content changed undulately under low temperature. During CK (room temperature)～-8℃or-11℃～-14℃, the soluble sugar content in hardy cultivars reduced less or increased more than those in non-hardy cultivars.Soluble protein content is closely related with cold resistance suggesting that it can be used as a cold resistance index. The soluble protein content in 8 cultivars all decreased under cold treatment. Under CK～-8℃treatment, the content in hardy cultivars reduced less than non-hardy cultivars.Six leaf anatomical structure index, Layer of palisade tissue, thickness of palisade tissue, thickness of upper epidermis cells, palisade tissue against spongy tissue, cell tense ratio and spongy ratio, were closely related to cold resistance. Hardy cultivars have thick and close palisade tissue, thick upper epidermis cells, and big value of palisade tissue to spongy tissue; non-hardy cultivars have thinner epidermis cells, thinner palisade tissue, and low ratio of palisade tissue to spongy tissue. Four out of 14 parameters were screened as cold tolerance indexes by main component analysis and correlation analysis, cold resistance evaluation system was established based on these four indexes. The evaluation model formula is Y=16.370-0.029SS₁-0.075L₅-0.314L₆-12.250L₄ (SS₁, the ratio of soluble sugar content changes from-11℃to-14℃; L₄, the ratio of palisade tissue against spongy tissue;L₅, Cell tense/density ratio; L₆, the density of palisade tissue.; R²=1.000).The model proved to be of high predicting evaluation accuracy more than 94%.Six cultivars were identified into three clusters by hierarchical cluster analysis. Hardy cultivars were characterized by lower LT50 temperature, increased soluble sugar content under LT₅₀ temperature stress, high value of ratio of palisade tissue against spongy tissue, thick and close palisade tissue.Path analysis was employed to figure out the relationships between cold indexes and to find out how they affect each other. The results showed that leaf anatomical structure determined the cold resistance directly. Besides this, the changes of soluble sugar content also played an important role in cold tolerance of chrysanthemum.