Nodulation failure in Trifolium subterraneum L. CV. Woogenellup (Sum. Marrar)

Abstract

The symbiotic relationship between Trifolium subterraneum L. cv. Woogenellup and a number of strains of Rhizobium trifolii was examined in tube and pot culture experiments. Plants grown in agar culture with the roots darkened and at 22°C either failed to nodulate or formed nodules slowly when inoculated with a number of strains; with other strains nodulation was prompt and the effectiveness levels were similar to those achieved with other subterranean clover cultivars. Exposure of the roots to light in tube culture experiments promoted nodulation by the "slowly nodulating" strains; higher root temperatures (28°) also promoted nodulation. Nine samples of the cultivar from widely separated sources behaved in a similar manner. Approximately 50% of 287 field isolates of R. trifolii from three localities were slow to nodulate this host, even when the roots were exposed to light. The proportion of field isolates nodulating the host promptly, and the effectiveness of nodulation achieved varied within and between the localities from which the isolations were made. R. trifolii strain TA1 failed to nodulate Woogenellup adequately in a pot culture experiment with two soils differing in the size of their natural population of R. trifolii (< 2/g, 104/g). Under the same conditions, three other strains formed a high proportion (> 80%) of the nodules in both soils, as determined by serological analysis of isolates from the nodules. A competition study with mixed inoculants showed that strain CC2480a formed a higher proportion of the nodules on cv. Woogenellup than strains WA67 or WU290, the present commercial inoculant strains. The results are discussed in relation to conflicting reports of poor nodulation of Woogenellup in the field, and of satisfactory nodulation in the laboratory and the glasshouse.