Variation in the abundance of 13C and biologicalnitrogen fixation in legume plants such as alfalfa (Medicago sativa L.)can be used in cultivar selection programs allowing a better understanding ofthe physiological processes related to water use, photosynthesis and biologicalnitrogen fixation and their interdependence. This would supply data for selectionprograms for the improvement of dry matter yield (DM), water use efficiency(WUE) and biomass quality. Thus evaluation of the plant under contrasting conditionsbecomes necessary, therefore in this study, the plants were evaluated underrainfall and irrigated conditions in lysimeters. In this study, 12 and 6 alfalfacultivars was evaluated respectively in two experiments installed in lysimetersby split-plot statistical design under constant irrigation and regime of rains.The results showed the existence of a complex interdependence between the agronomiccharacteristics of the alfalfa cultivars. Under controlled conditions the alfalfaresponded positively to irrigation, in relation to both quantitative and qualitativecharacteristics. The cultivars Crioula Chilena and Crioula Brasileira showedthe best results. In the first evaluation (after 6 months) correlations betweencarbon isotopic discrimination (CID), biological nitrogen fixation (BNF), drymatter yield (DM) and water use efficiency (WUE) were not significant. In thesecond evaluation (after 1 year), under irrigated conditions CID was negativelycorrelated with DM (-0.92**), WUE (-0.94**) and BNF (-0.79*). BNF was positivelycorrelated with DM (0.93**) and WUE (0.87*). Under non-irrigated conditionscorrelation between these parameters was not significant. There exists a tendencyfor a positive correlation between DM and CID in studies with C3plants that can be altered when water is limiting, root development is limitedor when contrasting cultivars are used. If a positive correlation is maintained,the possibility of increasing the WUE is minimal, since the increase of CIDis a result of the increase in transpiration.
