Selection for increased nitrogen fixation in red clover

Plants were selected and bred from the modal yield class of the population and from the class showing the largest yield (and nitrogen content), under conditions of test-tube culture using a single strain of rhizobia as inoculum. Progeny tests made on about 60 crosses within each group over two generations showed larger yields in families raised from the most effective parent plants, but the average effect of selection was small (about 5% dry weight increase). When the same material was tested with other strains of bacteria the effect of selection was less, but did not entirely disappear. In these conditions, selection was partly for increased symbiotic nitrogen fixation and partly for tolerance of the restricted conditions of growth. RESP-6536