Variation in the relationship between nitrogen fixation, leghemoglobin, nodule numbers and plant biomass in alfalfa (Medicago sativa) caused by treatment with arsenate, heavy metals and fluoride

Potential relationships between alfalfa (Medicago sativa L. cv. Vernal) acetylene reduction activity (ARA) and leghemoglobin content, nodule numbers, shoot biomass, root biomass, or total plant biomass were estimated using linear regression analysis after treatment with a range of concentrations of arsenate, Cd2+, Cu2+, Pb2+, Zn2+, and F–. There were highly significant positive linear regressions between ARA and all the other plant values in the control plants, but this linear relationship was significantly altered between ARA and one or more of the other parameters in all treatments. There was also evidence for an alteration of the slope in some of the treatments when compared to control treatments even though a linear model was still applicable.