| Chrysanthemum (Chrysanthemum morifolium) is one of the ten traditional Chinese famous flowers and of four important cut flowers all over the world, which is widely used as its high ornamental value and unique culture, and plays an important role in flower producing. Chrysanthemum is very sensitive to waterlogging. Waterlogging inhibits crop growth and yield, primarily through initiation of hypoxia conditions. The serious impact of waterlogging on chrysanthemum production and quality has made breeding new tolerant varieties as an important goal. Wild species of Chrysanthemum and its relative genera often carry important resistance genes and are important resource for resistant breeding materials. To identify the sources of waterlogging tolerant germplasm in Chrysanthemum and explore the mechanism of waterlogging, to promote innovation and laid the foundation for breeding new varieties, the waterlogging tolerance was identified in 57 taxa of Chrysanthemum and its related genera. The main results are as follows:1. An evaluation system for waterlogging-tolerance of chrysanthemum was established. Pot plants were exposed to mimicked waterlogging treatment. In terms of the morphological changes under the waterlogging treatment, the level of waterlogging damage was classified into 7 grades. Four morphological criteria i.e., leaf color, leaf morphology, stem color and stem morphology, were quantified via grading and scoring. The waterlogging tolerance was identified by the comprehensive scores of the four criteria, then the evaluation system was established. And the waterlogging tolerance was identified in 57 taxa of Chrysanthemum and its related genera as well. The results showed that the 6th day after submergence (waterlogging) was critical day to identify the waterlogging tolerance. The most of the related genera of Chrysanthemum are more susceptible to waterlogging, which belongs to less tolerant class. Different tolerance to waterlogging was observed in different populations of the same taxa. Eight materials showing highly waterlogging tolerance were screened out.2. The Effects of waterlogging on root vigor, chlorophyll content, photosynthetic characteristics and morphological changes in five related species of Chrysanthemum were studied. The waterlogging was mimicked by treating pot plants with flooding (2-3 cm in depth). Waterlogging tolerance of five species was evaluated. The results showed that the root vigor of five species all declined significantly under waterlogging, except that Chrysanthemum zawadskii showed high root vigor in the early stage of waterlogging stress.The chlorophyll contents of the five species increased during the early stages of stress and then decreased afterwards; After flooding treatment for 15d, in Ajania shiwogiku Kitam. var. kinokuniense and Chrysanthemum boreale, the net photosynthetic rate declined significantly, CO2 assimilation declined as well and even decreased to negative value which suggests that photosynthetic capacity was lost. Obvious decrease in photosynthetic rate was also observed in Chrysanthemum crassum and Chrysanthemum yoshinaganthum. However, Chrysanthemum zawadskii maintained higher net photosynthetic rate under the stress.3. Responses to waterlogging of a tolerant chrysanthemum cultivar (’05 (53)-4’) were compared with those of a susceptible one (’05 (13)-13’). Just four days of waterlogging were enough to induce wilting and leaf chlorosis in’05 (13)-13’, but there was no visual damage to the leaves of’05 (53)-4’after eight days of treatment. After 20 days, only a small number of adventitious roots had emerged from’05 (13)-13’stems, but many vigorous adventitious roots had formed in’05 (53)-4’. Waterlogging induced increases in the activity of alcohol dehydrogenase (EC 1.1.1.1), pyruvate decarboxylase (EC 4.1.1.1) and lactate dehydrogenase (EC 1.1.1.27) in both cultivars, but the increases in’05 (13)-13’were more pronounced than in’05 (53)-4’. On the other hand, the activities of superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and catalase (EC 1.11.1.6) were higher in’05 (53)-4’ thanin ’05 (13)-13’. Leaves of’05 (13)-13’had a higher content of malondialdehyde, and the amount of this stress indicator in’05 (53)-4’was stable throughout the waterlogging period. Ethylene production was enhanced by waterlogging in both cultivars, but peak ethylene production occurred two days earlier in the tolerant cultivar, and was three fold higher than in the susceptible one.4. Sensitive (C. nankingense) and tolerant (C. zawadskii) wild species were compared to determine the mechanism of waterlogging tolerance. Although the stress induced wilting and leaf chlorosis in both species, symptoms were more apparent and appeared earlier in the intolerant C. nankingense. In the more tolerant C. zawadskii, adventitious roots formed above the flooding level, and aerenchyma developed in the root, stem and leaf. The C. zawadskii palisade parenchyma was thicker, and larger intercellular spaces developed in the spongy mesophyll. The activities of alcohol dehydrogenase, pyruvate decarboxylase and lactate dehydrogenase were enhanced in roots of both species following the imposition of stress. In C. zawadskii the rise in ADH activity was more pronounced, while C. nankingense showed a significantly higher LDH activity. The activities of superoxide dismutase, ascorbate peroxidase and catalase were all higher in the leaves of C. zawadskii than in C. nankingense, and the leaves of C. nankingense showed a higher content of malondialdehyde throughout the period of waterlogging. Photosynthesis was decreased in both species, and there was a significant fall in the intercellular CO2 concentration in C. zawadskii. These data suggested that the greater relative waterlogging tolerance of C. zawadskii appears to depend on a combination of metabolic and morpho-anatomical responses. Ethylene as a gaseous hormone, its role in hypoxic signaling remains controversial and is uncertain in non-wetland species. Chrysanthemum species contrasting for their hypoxia tolerance were treated with ethylene and inhibitor of ethylene action to determine the ethylene signalling in hypoxia-induced aerenchyma formation and ethanolic fermentation. Ethylene appears to signal an acceleration of PCD and aerenchyma formation, and to alleviate ethanolic fermentation in tolerant species, while in sensitive one it activates fermentation and increases the toxic fermentation by-products responsible for hypoxia injury. |
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