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

Low winter temperature in Mediterranean rangelands delays the growth of mos t pasture and forage plants and, in particular, annual Medicago, (medics). Finding proper associations of medic- Rhizobium, capable to grow and fix ni trogen at low temperature, is necessary to extend the grazing period. In th is paper, we studied the performance of three Medicago cultivars: M. aculea ta 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 in oculation with different strains of Rhizobium meliloti (M620, M508 and BZI) . A third treatment (C) was used as a control, neither inoculated nor ferti lized. The trial was repeated for 2 consecutive years (1998 and 1999) at IC ARDA research station (Aleppo, Syria). Plant development, dry matter produc tion and nodule initiation were investigated. The percentage of fixation -d erived shoot nitrogen was also quantified using N-15 dilution technique. Ou r results indicate a genotypic variation in low-temperature tolerance of an nual medics. Accession A had a slow growth pattern with low dry matter prod uction compared with B and R over the two seasons. In the two seasons and a t the early growth stages, Rhizobium strain played a key role in plant grow th, where both inoculated and fertilized plants of accession R had a compar able dry matter. Furthermore, the proportion of nitrogen derived from fixat ion 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 r esponse to low winter temperature. Among those we studied, M. rigidula acce ssion 716, inoculated with the strain BZI, proved to be the most adapted as sociation to our environment. In addition, M. aculeata accession 5099 seems to be promising for winter growth, but it needs to be inoculated with a st rain more adapted to low temperature. (C) 2001 Elsevier Science B.V. All ri ghts reserved.