| The category of seed-borne virus is increasing gradually and the virosis endanger acutely, that cause the yield and quality If crops to descend. The Welsh onion virosis occur widespreadly and endanger seriously too . This experiments regard Welsh onion as experiment material,probe into the way to rapid propagation of virus-free seedlings which belong to sexual propagation plant and can spread virus by seeds, establish the seed-breeding system and provide the foundation of scientific theories and the key technology.On the basis of the identification of virus pathogen and initial identification of seed virus towards Zhangqiu Welsh onion in Heilongjiang province .this experiments regard stem apex as explant .study virus-free seedlings .culture and propagation .and agronomical character of Welsh onion derived from virus-free seed .conclusion as follows:1 Occurrence of Welsh onion virosis is widespread in Heilongjiang province ,it is the main reason to cause the yield and the quality to descend; According to the determined result of host range .appearance of virus particles and pathological ultramicroscopic structure etc.on Welsh onion virosis ,it can ascertain initially that the virus pathogen which infects Welsh onion belongs to the family of potato Y virus (Potyviridae) it is one of provisional members in the genus of potato Y virus (Potyvina) .and it can spread virus by its seeds; After being infected virus, it will produce pinwheel-like and strand inclusion in cytoplasm of mesophyll of Welsh onion .organelle such as chloroplast and nucleus etc. will be destroyed seriously.2 The optimum medium for meristem tip to start culture is MS+NAA 0. 1mg/L+2ip 0.4 mg/L, its rate of shoot induction is 100%,cauloid width is 0202cm; the optimum medium for root induction is 1/2MS+NAA 0.01mg/L+IBA 1.5 mg/L. Both rate of rooting and survival rate after habituation are 100%; After heat treatment at 75 C for 10 min .shell the stem apex between 0.3 and 0.5mm from the cauloid of diseased plant for culture ,and survival rate is 98.3%; the result of symptom observation and electron mircroscopy inspection show that the rate of virus-free is 100%; The optimum medium for meristem tip to differentiate rank seedlings isMS+NAA0. lmg/L+2ip0.6mg/L, it can realize differentiation of rank seedlings by subculture once every 25d,the average differentiate of each meristem tip is 6.8,and the rate of rosette is 100%; The optimum medium for disc stem to induce callus is MS+KT0.5mg/L +2.4-D11.5 mg/L, the rate of callus induction is 97.5% .the weight of callus increases 14.54 times. The optimum medium forcallus to differentiate regeneration plant is MS+NAA0.2mg/L+ 2ip 0.6 mg/1, the rate of differentiation is 91.7%,the differentiation is 10.62 per callus on the average; The number of chromosome, which is from die cultured seedlings of meristem tip and the rank seedlings of meristem tip ,is not altered ,both the number are 2n=16,the hereditary stability of callus and regeneration plant is poor; There are 3 ways to realize rapid propagation (1) induce seedlings from the explant of stem apex directly (2)differentiate rank seedlings from the explant of stem apex directly (3)induce callus from disc stem ,and induce regeneration plant from callus ,at last .extend propagation with test-tube plantlets.3 The Welsh onion derived from virus-free seed is lower than the Welsh onion derived from viruliferous seed on rate of exosmosis electrical conductivity ,higher on the content of chloro- plast .stronger on the activity of roots ,and larger on the absorption area of roots .The Welsh onion derived from virus-free seed is superior to the Welsh onion derived from viruliferous seed on many indexes ,such as plant height ,the number of leaves ,cauloid width, the length of maximum leaf, the weight of single plant and yield , and the former's yield is 30% above than the latter;There is significance of difference between the Welsh onion derived from virus-free seed and the Welsh onion derived from viruliferous seed on many indexes , such as Vc, prot...
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