SYMBIOTIC CHARACTERISTICS OF TRIFOLIUM AMBIGUUM SEEDLINGS GROWN IN TUBE CULTURE AS AFFECTING SUBSEQUENT SYMBIOTIC VIGOR

A hexaploid line of Caucasian clover (T. ambiguum), inoculated with 2 strains of Rhizobium trifolii, was grown in tubes of N-free medium under bacteriologically-controlled conditions. Observations were made on various symbiotic characteristics of the seedlings. After 30 days, they were transplanted into pots of sand and vermiculite watered with nutrient solution lacking N and grown for another 100 days. Uninoculated plants supplied with various levels of combined N were used for comparison. R. trifolii strain CC283b was classified as highly effective and was greatly superior in N-fixing ability to strain CC231a which had been used routinely in Australia for many years for inoculation of experimental sowings of Caucasian clover. In this line of T. ambiguum, the inherent plant-to-plant variation was accentuated in plants relying on symbiotically-fixed N. This effect was strong in plants inoculated with the intermediately-effective strain CC231a but mild in plants inoculated with the highly-effective strain CC283b, i.e., a larger component of the variability among CC231a plants was due to the symbiosis than was the case for CC283b plants. Seedlings inoculated with the intermediately-effective strain and selected on the basis of within-tube characteristics such as time to initial nodulation, symbiotic effectiveness rating, and nodule size produced more dry matter when grown in pots than the unselected population and were as productive as plants inoculated with the highly-effective strain. In contrast with the highly-effective strain, selected plants grown in pots did not produce much more dry matter than the unselected population. The use of within-tube symbiotic characteristics as a means of selection for improved production will only be successful when a substantial component of the plant-to-plant variability is due to the symbiosis.