Evaluation of genotypes of navy and culinary bean (Phaseolus vulgaris L.) selected for superior growth and nitrogen fixation

Levels of nitrogen fixation by navy and culinary beans (Phaseolus vulgaris L.) in Australia are low and contribute little to the N economies of the cr ops. As a consequence, they must be grown in highly fertile soils or fertil ised with N to obtain economic yields. Eliminating the need for fertiliser nitrogen would save growers A$1 million annually. Following a 10-year progr am in which almost 1500 genotypes of P. vulgaris were screened for superior nodulation and nitrogen fixation, we conducted experiments at the Southedg e Research Station, Mareeba, during 1995-97 to identify elite genotype(s), which could either be released as cultivar(s) or used as donor parent(s) in a breeding program. Selection criteria were plant biomass, nitrogen fixati on activity assessed using the ureide method and grain yield. The best-performing genotypes were ICA20667 and ICA21573. They produced abo ut 20% more shoot biomass than the commercial check cultivars, Spearfelt, G allaroy and Rainbird, and had Pfix (percentage of plant nitrogen derived fr om nitrogen fixation) values that were consistently about 30% higher. Howev er, both genotypes responded strongly to fertiliser nitrogen (>200% increas e in shoot nitrogen and >100% increase in grain yield at rate of 150 kg nit rogen/ha), suggesting that their nitrogen fixation capacity was inadequate. This study reinforced current recommendations that commercial crops of P, vulgaris be fertilised with nitrogen and indicated a low likelihood of rele ase of high nitrogen-fixing cultivars to growers in the immediate future.