DISTRIBUTION OF NITROGEN IN COMMON BEAN (PHASEOLUS-VULGARIS L) GENOTYPES SELECTED FOR DIFFERENCES IN NITROGEN-FIXATION ABILITY

The improvement of N2 fixation in legumes may lead to increased yields and reduced fertilizer requirement. Levels of N2 fixation were determined for three cultivars and nine progeny lines from two inbred backcross common bean (Phaseolus vulgaris L.) populations that were grown at Hancock, Wisconsin in 1984 and 1985 using N-15-depleted (NH4)2SO4. The high N2-fixing line 'Puebla 152' was the donor parent for both inbred backcross populations and the cultivars 'Porrillo Sintetico' and 'Sanilac' were the recurrent parents for populations 21 and 24, respectively. Total N yield, fixed N2 and % N derived from the atmosphere were determined for whole plants and plant parts at the R3 (50% bloom) and R9 (maturity) growth stages. Significant year-by-line interactions were found for N2 fixation traits among the population 21 lines and parents, but not for population 24 lines and their parents. Measures of N2 fixation at R3 were inadequate to predict N2 fixation at R9. Population 24 lines and parents differed for N2 fixation ability at R9, and fixed N2 was correlated with maturity. The recovery of an inbred backcross progeny line, 24-21, which matured earlier and fixed more N2 than the recurrent parent 'Sanilac' indicated that N2 fixation was heritable and that favorable alleles, independent of maturity, were recovered from a late-maturing, high N2-fixing donor parent by utilizing the inbred backcross breeding method. Since most fixed N2 and non-fixed N (> 80%) was found in the seeds at maturity, and most lines did not vary for the distribution of nitrogen throughout the plant, selection for improved remobilization of nitrogen to the seed to increase yield is impractical in this genetic material. The highest N2-fixing lines tended to have high and similar % Ndfa in all plant parts.