A FIELD-EVALUATION USING THE N-15 ISOTOPE-DILUTION METHOD OF LINES OFPHASEOLUS-VULGARIS L BRED FOR INCREASED NITROGEN-FIXATION

N2 fixation in lines of Phaseolus vulgaris was measured by N-15-isotop e dilution to determine whether a programme of crossing and recurrent selection had resulted in enhanced nitrogen fixation. In field experim ents 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 pla nt 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 se lected 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 fr om 28 to 52% among the lines which all had similar growth habit and du ration. Nodulation patterns were also distinct. Nodules formed early ( 10 to 15 plant-1 at 13 days) in many lines, and smallest amounts of fi xation were observed in those lines which nodulated slowly and did not form substantial nodule mass until after 40 days. The screening crite ria used in the selection of the RIZ lines had been largely indirect w ith other factors such as disease resistance also being included. Prog ress for increasing N2 fixation over good-fixing parental lines such a s 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 tra its in order to further enhance the potential for N2 fixation in beans .