Delayed inoculation and competition of nitrogen-fixing strains in Medicago noeana (Boiss.) and Medicago polymorpha (L.)

Abstract

Seed inoculation is frequently essential for annual Medicago establishment in dryland farming systems, particularly in Mediterranean-type environments. As post-planting soil inoculation is often practised when seed inoculation fails, the effect of delayed inoculation was investigated. Roots of Medicago noeana ICARDA sel. 1938, and Medicago polymorpha cv. Circle Valley, were pre-exposed to Rhizobium meliloti strains. Subsequently, roots were exposed to a secondary inoculum after 6, 48 and 168 h to simulate delayed inoculation, and subsequent establishment of other strains in the nodules were investigated. Combinations of highly effective and host compatible strains (M29 and M15) and effective–ineffective strains (M29 and M33) were used to evaluate proportional nodulation responses. Plants were harvested after 6 weeks of growth under environmentally controlled conditions. Nodules were assessed for distribution in the root system and for occupancy based on their differential resistance to spectinomycin and streptomycin, and, in the case of M33, on nodule characteristics. The strain M29 was a better competitor than M15 when applied in equal density to M. polymorpha, at zero time. When forming nodules with M. noeana, M15 was equally competitive under the same conditions. With M29 as the primary inoculum, and M15 inoculation delayed for 6, 48 and 168 h, the incidence of M29 nodules increased on M. noeana from 55% (at zero time) to 83, 80, and 95% and, on M. polymorpha, from 72% (at zero time) to 80, 90, and 97% for the three inoculation time delays, respectively. Conversely, strain M15 dominated nodule production at all time intervals when used as primary inoculant on both hosts. The percentage of total nodulation by M33, applied at the three later inoculation times, was markedly lower (21, 2 and 0%, respectively) when M. polymorpha was pre-exposed to M29. This suggested a host preference for M29, even if applied as a late inoculum. Pre-exposure of 2-day-old M. noeana seedlings to the ineffective strain M33 as the primary inoculant resulted in nodule number increases ( P≤0.01) as compared with M29. Nonetheless, when M29 was the primary inoculum, M33 was able to produce significantly fewer nodules than its competitor when applied at the 6 and 48 h time delays. Results indicate that the early events in the nodulation process of annual medics coupled with host-specificity factors are perhaps the most critical for competition among R. meliloti strains for nodule formation. Therefore, remedial inoculation after the seed has been planted may be of little benefit.