| Verticillium wilt is a worldwide destructive cotton disease. Recently, more and more cotton fields in China have been severely damaged by the disease, being recognized as a serious threat to cotton production. It has been confirmed that breeding for highly tolerant or resistant varieties to Verticillium wilt is the most effective and economical disease control method. However, the progress for development of Verticillium wilt resistant varieties still have some problems, and there is a far degree of difference between resistant level in cotton varieties and the production demands. Such problems may be directly caused by lacking the systematic understandings to some related questions, such as pathogenicity differentiation of Verticillium dahliae, inheritance and mechanism of cotton resistance to Verticillium wilt etc. Another reason is .lack of resistant resources and effective judging methods for plant resistance in cotton resistant breeding program. Therefore, further studies about the inheritance and mechanism of cotton resistance to Verticillium wilt and the molecular markers linking to Verticillium wilt resistance are needed urgently.Thirteen varieties of four Gossypium species, includes Gossypium barbadense L, G. hirsutum L, G. arboreum L and G. thurberi Todro were used as host plants, and the Verticillium dahliae Anyang strain with intermediately virulent have been used as pathogenic fungus in this experiment to study the mechanism and inheritance of cotton resistance to Vertivcillium dahliae and to look for the molecular markers linking to wilt resistant gene, though histological anatomy, physiology and biochemistry, exudates of cotton root sytem etc. The main results are as follows.1. Mechanism of cotton resistance to Verticillium dahliaeThe mechanism of cotton resistance to Verticillium dahliae have been studied through the compared analysis on the tissue structure, antifungal substance and root exudates of 13 cotton varieties in 4 Gossypium species before and after inoculation with Verticillium dahliae Anyang strain, theintermediately virulent strain.It was observed that there were more pith rays, more quantity of xylem parenchymacell and more thickness of vessel cell in stem, taproots and primary lateral roots in the resistant varieties, comparing with the susceptible ones. The stems and taproots ofresistant varieties had more vessels in number but less in vessel diameter. tThe results showed that the POD activity was different among the Gossypium species before inoculation, and two species G. thurberi, resistant species to Verticillium wilt, and G. arboreum, the susceptible species to Verticillium wilt, were the highest in POD activity, which was not any difference between them. The relationship between the activity of POD and resistance to Verticillium wilt of cotton varieties was not obvious before inoculation. However, the POD activity of all varieties was increased after inoculation and reached the highest in the 3rd day after inoculation.The PAL activity in Gossypium barbadense was highest. Among G. arboreum, G. thurberi and G. hirsutum resistant varieties, there were no difference in PAL activity before inoculation. The PAL activity of all varieties were increased after inoculation and reached the highest in the 1st day after inoculation, then declined. However, the PAL activity for all entry resistant varieties was higher than that of susceptible ones.The content of hemigossypol in cotton leaf was not the key factor for the cotton resistance to Verticillium wilt, although it had some affect on the host rsistance to Verticillium wilt.The kinds and contents of amino acids in the root exudates of resistant varieties were much less than that of susceptible ones. Compared with susceptible varieties, the root exudates of resistant varieties were lack in threonine, glutamic acid, phenylalanine, lysine and proline. The kinds of sugar in the root exudates were no difference between resistant varieties and susceptible ones. The contents of glucose, fructose and sa...
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