A field evaluation using the (15)N isotope dilution method of lines of Phaseolus vulgaris L. bred for increased nitrogen fixation
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N2 fixation in lines of Phaseolus vulgaris was measured by 15N-isotope dilution to determine whether a programme of crossing and recurrent selection had resulted in enhanced nitrogen fixation. In field experiments on an isohyperthermic Aquic Hapludoll soil the amount of N2 fixed by the different lines ranged from 18 to 36 kg ha?1 (32 to 47% of plant N) in 56 days. The quantity of N2 fixed and the proportion of plant N derived from fixation was not significantly greater in the lines selected for N2 fixation (RIZ lines) than parental lines. Total shoot N ranged from 53 to 77 kg ha?1 and partitioning of N to pods differed from 28 to 52% among the lines which all had similar growth habit and duration. Nodulation patterns were also distinct. Nodules formed early (10 to 15 plant?1 at 13 days) in many lines, and smallest amounts of fixation were observed in those lines which nodulated slowly and did not form substantial nodule mass until after 40 days. The screening criteria used in the selection of the RIZ lines had been largely indirect with other factors such as disease resistance also being included. Progress for increasing N2 fixation over good-fixing parental lines such as BAT76 was not significant and it is recommended that more attention be paid to early nodulation, to the use of soils with lower available N and to inter-crossing of lines having different good N2 fixation traits in order to further enhance the potential for N2 fixation in beans.
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