Symbiotic characteristics of Trifolium ambiguum seedlings grown in tube culture as affecting subsequent symbiotic vigour

Abstract

A hexaploid line of Caucasian clover (Trifolium ambiguum), inoculated with two strains of Rhizobium trifolii, was grown in tubes of nitrogen-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 nitrogen and grown for another 100 days. Uninoculated plants supplied with various levels of combined nitrogen were used for comparison. R, trifolii strain CC283b was classified as highly effective and was greatly superior in nitrogen-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 nitrogen. This effect was strong in plants inoculated with the intermediately-effective strain CC231 a but mild in plants inoculated with the highly-effective strain CC283b, i.e., a larger component of the variability amongst 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 produced very little more dry matter than the unselected population. It was concluded that 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.