Host Plant and Rhizobium Effects on Acetylene Reduction in Alfalfa during Regrowth 1

Abstract

The effects of host plant and Rhizobium strain variation on apparent N2 fixation (C2H2 reduction) and total nonstructural carbohydrate (TNC) of the root system during regrowth of alfalfa (Medicago sativa L.) were studied to determine the prevalence of the known decline in C2H2-reduction activity following shoot harvest. Harvest at 10% bloom in ramets of a ‘Dohfari’ clone inoculated with Rhizobium meliloti strain 102F28 decreased shoot carbon exchange rate (CER) 99% immediately, but C2H2 reduction declined only 75% in the following 3 days. Thereafter, CER and C2H2 reduction increased in parallel during shoot regeneration. Those data provided circumstantial evidence that carbohydrates stored in roots or root nodules were used to maintain low rates of C2H2 reduction during the intial period of regrowth. In experiments with seedlings of 10 alfalfa cultivars inoculated with R. meliloti 102F28 and in trials with Dohfari and ‘Vernal’ clones inoculated with one of 10 different R. meliloti strains, shoot harvest produced a significant (p ≤0.001) decline in C2H2 reduction within 3 days to values ranging from 4-30% of the original rate. Root system TNC generally declined over the same period, but in only two of the 30 host-symbiont combinations examined was that change significant (p ≤0.05). No significant correlation was observed between C2H2-reduction rate and root system TNC in the same plants at harvest (day 0) or on day 3 in any host-symbiont combination. Results from this study suggest that although nonstructural carbohydrates are consumed in alfalfa roots during defoliation, the), are not used to maintain maximum (day 0) rates of N2 fixation during the early stages of regrowth in widely different host-symbiont combinations. This evidence is consistent with the concept that the recovery of N2 fixation following shoot harvest depends upon the regrowth of new shoots.