Growth and nitrogen fixation of annual Medicago-Rhizobium associations during winter in Mediterranean region

Abstract

Low winter temperature in Mediterranean rangelands delays the growth of most pasture and forage plants and, in particular, annual Medicago (medics). Finding proper associations of medic- Rhizobium, capable to grow and fix nitrogen at low temperature, is necessary to extend the grazing period. In this paper, we studied the performance of three Medicago cultivars: M. aculeata accessions 80 (A) and 5099 (B) and M. rigidula accession 716 (R), grown in the field with three different nitrogen sources. The first treatment (N) was obtained through fertilization, while the second (I) was ensured by inoculation with different strains of Rhizobium meliloti (M620, M508 and BZI). A third treatment (C) was used as a control, neither inoculated nor fertilized. The trial was repeated for 2 consecutive years (1998 and 1999) at ICARDA research station (Aleppo, Syria). Plant development, dry matter production and nodule initiation were investigated. The percentage of fixation-derived shoot nitrogen was also quantified using 15N dilution technique. Our results indicate a genotypic variation in low-temperature tolerance of annual medics. Accession A had a slow growth pattern with low dry matter production compared with B and R over the two seasons. In the two seasons and at the early growth stages, Rhizobium strain played a key role in plant growth, where both inoculated and fertilized plants of accession R had a comparable dry matter. Furthermore, the proportion of nitrogen derived from fixation in A was very low, which was reflected, in turn, on the total amount of fixed nitrogen. The proportion of fixed nitrogen of the association R-BZI was high for the 2 years, where it reached 72 and 87% of the total N in the plant, respectively. It is suggested that associations differed in their response to low winter temperature. Among those we studied, M. rigidula accession 716, inoculated with the strain BZI, proved to be the most adapted association to our environment. In addition, M. aculeata accession 5099 seems to be promising for winter growth, but it needs to be inoculated with a strain more adapted to low temperature.