Genetic improvement and related activities. Improvement of individual characters. N-fixation

In the 1986 breeding program for enhanced N fixation, emphasis was placed on small seeded indeterminate beans; however, at the beginning of 1986 it was decided that some priority should be given to materials for Africa, including larger seeded determinate and indeterminate bush types. In Popayan, Colombia, some lines such as Canadian Wonder, Catrachita, and RIZ 30 demonstrated good early nodulation, but by mid-flowering the nodules were already starting to senesce. In collaboration with Rothamsted Exptl. Station, United Kingdom, investigations into obtaining nonnodulating and supernodulating bean lines (lines which continue to nodulate in the presence of nitrate and which nodulate more than usual under all conditions) by mutation were carried out. Nodule mass is 1 component which has been identified in beans as contributing to improved fixation. Expt. carried out under field and glasshouse conditions to measure N fixation of different bean genotypes showed similar differences in N fixation. In general, the RIZ lines and A 268 fixed more N than the BAT lines. Data on differences between Rhizobium strains in tolerance and growth at low and high temp. are presented. (CIAT)En el programa de mejoramiento de 1986 para incrementar la fijacion de N, se dio enfasis al frijol indeterminado de semilla pequena; sin embargo, a comienzos de 1986 se decidio que se daria alguna prioridad a materiales para Africa, incluyendo los tipos arbustivos determinados e indeterminados de semilla mas grande. En Popayan, Colombia, algunas lineas como Canadian Wonder, Catrachita y RIZ 30 demostraron buena nodulacion temprana, pero hacia la mitad de la etapa de floracion, los nodulos ya empezaban a envejecer. Se realizaron investigaciones para la obtencion de lineas de frijol que no nodulaban y otras con nodulacion abundante (lineas que continuan su nodulacion en presencia de nitrato y que nodulan mas de lo usual en cualquier condicion) por mutacion, en colaboracion con Rothamsted Exptl. Station, Reino Unido. La masa de nodulos es un componente que se ha identificado en frijol como contribuyente a una mejor fijacion. Los expt. realizados en condiciones de campo e invernadero para medir la fijacion de N de diferentes genotipos de frijol mostraron diferencias similares en cuanto a fijacion de N. En general, las lineas RIZ y A 268 fijaron mayor cantidad de N que las lineas BAT. Se presentan los datos sobre las diferencias entre cepas de Rhizobium en cuanto a tolerancia y crecimiento a bajas y altas temp. (CIAT