| Persistent modifications and their genetic importance
for spring wheat breeding Part I I.E. GLOUSHCHENKO Laboratory of Experimental Biology of Lenin All-Union Academy of Agricultural Sciences, Leningrad, U.S.S.R. In biology there circulates an opinion that modificational variability does not affect genetical characters and properties of organisms (I). Carrying out a number of experiments some scientists have come across the facts that made them introduce amendments into the notion about the modificational variability. The term "persistent modifications" has been firmly established in science for a long time. This term designates the fact of retaining during a number of generations the changes induced by the environment. These changes are retained when the influencing agent is eliminated and they are inherited by the maternal lines. For a long time many investigators have been interested in the following problem : can persistent modifications turn into hereditary changes and is it possible by having studied this phenomenon to make it solve the problems of breeding? As far back as 1932 N.I. VAVILOV dwelled at length on the problem of theoretical and practical importance of studying the phenomenon of persistent modifications, of elucidating their nature and types, and the factors inducing these modifications. N.I. VAVILOV wrote : "Of great theoretical interest is the problem of persistent modifications changing into mutations, the influence of persistent modifications on the mutational ability of the organism . . . . ". However the phenamenon of persistent modifications has been studied very poorly up to now and it occupies rather vague position in the hereditary science. Experimental material on this problem, accumulated by microbiologists, protistologists, entomologists and plant breeders, deserves thorough study. During many years of research (1965-1975) in spring wheat we have obtained evidence that persistent modifications are not only observed in seberal generations, but also can turn into hereditary changes. It has been shown that the variability of the character "winter habit - spring habit", induced by partial vernalization and spring sowing of winter varieties under uncontrolled (field) conditions, is expressed in two directions - as common modifications and as persistent modifications. The latter are consolidated from the third to the fifth generations and become stable later on. The influence of extreme conditions at the earlier stages of plant development both during the year of vernalization and during the growth of successive generations has been the decisive factor of this complicated process. The influence of unusually high temperatures, long day and other factors at the early stages of organogenesis disturbs the normal reaction of the organism as a result of which a homogeneous variety turns into a heterogeneous population in which there a re created preconditions for the genetical change of the character "winter habit" into "spring habit". This shows that the cropped up modifications can autoreproduce, intensify, and serve as a basis for a hereditary change of a certain character and property, which is of great interest for breeders. The problems associated with the genetical transformation of the character "winter habit - spring habit" in a number of wheat varieties are stated below. For the experiment there were taken varieties with different length of the vernalization stage (Kavkaz and Bezostaya 1, 40 days ; PPG 186, 55 days ; Mironovskaya 808, 60 days ; IGEN 3, 80 days). The seeds of these winter varieties were vernalized during different number of days (from 5 days to complete vernalization) and sown at the beginning of May. The number of shoots was counted. On all heading plants of the first, second, and often third generations paper bags were put. The numbers of heading and shooting plants were counted once a month. In successive years the seeds of heading plants were sown during the first ten days of May by families without extra vernalization. We have shown that the dynamics of the vernalization stage has its turning points, at which the influence causes the highest effect. The highest possible reformation of developmental rhythms is observed, as a rule, in plants which were vernalized lesser number of days when compared with the date that caused mass plant heading. |
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