Pratt, David : David Pratt, Professor, Bacteriology, U.C., Davis.

Thomas, Bruce R. : Bruce R. Thomas, formerly graduate student, U.C., Davis.

Soybean yield trials use as a control a line genetically incapable of being infected by nitrogen - fixingbacteria to measure soil nitrogen availability ( pale green plants ) . Other plants are nodulated and use both soil and atmospheric nitrogen . organisms are being studied . One concrete example of enhancing nitro - gen fixation and agronomic yield through genetic alteration of Rhizobiurn has been of the demonstrated in soybeans . A mutant commercial R.japonicurn strain USDA 110 was selected for high nitrogen - fixation fertility . New callus cultures initiated from capacity in pure culture in R . C . Valentineâ??s the regenerated plants typically had at least a laboratory . Under microaerophilic condi - 30 - fold increase in paraquat tolerance rela - tions , similar to those encountered in the tive to normal callus of strain L2 . Paraquat plant root nodule , the mutant strain C33 had tolerance was transmitted to sexual progeny Herbicide tolerance 94 percent more activity than the parent in each of the three clones tested , confirming strain 110 . In subsequent tests with C33 on the hypothesis that the paraquat tolerance re - Bruce R . Thomas soybean plants grown under controlled en - sulted from a mutation . David Pratt 40 vironments , we measured approximately a Limited tolerance was expressed at the percent increase in dry matter and nitrogen plant level in regenerated plants from two out content relative to plants infected with strain of seven clones tested . Although some were 110 . killed and the rest were damaged by para - Isolation of mutants tolerant or resistant to Those results stimulated field trials run at quat , plants deriving from paraquat - tolerant herbicides may become a valuable applica - Davis in 1979 and 1980 . Seed yield and total clones PQT â?? ) and PQTZ2were significantly tion of cell culture techniques . Every herbi - seed nitrogen ( protein ) content were in - less damaged than plants of parent strain L2 . cide is restricted in use by the number of creased significantly in â?? Clark â?? soybean crops it damages or kills . Tolerant mutants of Each of these mutant plants is probably plants treated with mutant strain C33 , which heterozygous for one or more alleles that various plant species could broaden the use - yielded 39.2 bushels per acre in 1979and 44.6 fulness of currently available herbicides . The confer the paraquat tolerance . The tolerance in 1980 , compared with 36 and 38.3 bushels , of different mutants may arise through more advantages of searching for this kind of mu - 110 . Seed nitrogen respectively , with USDA tant using cell cultures are ( 1 ) accuracy and than one pathway . Thus , highly resistant content with C33 was 146 pounds per acre in uniformity of herbicide exposure in culture , plants might be achieved through inbreeding 1979 and 186pounds in 1980 , compared with ( 2 ) the ease with which billions of cultured to produce homozygous tolerant genotypes , 136and 159 pounds , respectively , with USDA cells may be screened for ability to grow in through hybridization to combine different 110 . During both field seasons , more than 80 of the herbicide , and ( 3 ) the po - the presence tolerance allelesin a single strain , or through percent of the rhizobia reisolated from root isolation of new and better mutants . tential ( as yet unrealized for most crop spe - nodules of field - grown plants were identified cies ) for easy isolation of recessive mutants Eventually , herbicide - tolerant tomato as the applied 110or C33 strain on the basis of using haploid cell cultures . mutants are likely to have agronomic ap - genetic markers . plications . The greatest weed problems of Paraquat - tolerant mutants of tomato were A genetically similar , but nonnodulating , isolated using an interspecific hybrid strain tomato in California are the closely related line of â?? Clark â?? soybean yielded 27.1 and 17.7 called L2 . This plant had 25 percent culti - nightshades , which are sensitive to the same bushels per acre containing 73.6 and 44.8 vated and 75 percent wild tomato ancestry herbicidesas are tomato cultivars . Paraquat - pounds of nitrogen per acre in 1979and 1980 , tolerant tomato mutants , however , may be and was selected for the wild tomato traits of respectively . That decline in available soil ni - useful only as a model system , because the rapid callusgrowth and efficient plant regen - trogen associated with different field sites eration . When callus cells of this hybrid were high human toxicity of this herbicide will was responsible for the more obvious promo - restrict the times at which it can be safely ap - placed on agar medium containing a lethal tive effect of strain C33 in 1980 when com - concentration of paraquat , spontaneously plied , even to a tolerant crop of a species pared with 1979 . Thus the genetically altered occurring presumptive mutant callus clones grown for edible fruits or foliage . Further ex - 110 under the strain C33 was as good as strain that could grow under these conditions ap - periments may produce tomato plants that moderate soil nitrogen conditions used in peared at a frequency of one out of four bil - tolerate other herbicides , perhaps achieving 1979and clearlysuperior under the greater ni - lion cells tested . effective weed control together with the flex - trogen stress experienced in 1980 . 10of Diploid plants were regenerated from ibility to choose compounds that pose the least risk to the environment . the 22 clones isolated . Although some of Donald A . Phillips , Professor , Agronomy and U.C . , Davis , and Larry E . Range Science , these plants were normal in appearance , Bruce R . Thomas , formerly graduate student , Williams , former Graduate Research Associate , others were abnormal ( leaflets smaller and U.C . , and David Pratt , Professor , Bacteriology , now Assistant Professor , Agronomy , Louisiana Davis . State University , Baton Rouge . thicker than normal ) with reduced vigor and CALIFORNIA AGRICULTURE . AUGUST 1982 33