“…In most types of stress, the initial decrease of nitrogenase activity is associated with a decline in the O 2 concentration reaching the infected cells and bacteroids (Witty et al, 1986;Carroll et al, 1987;Layzell et al, 1990;Escuredo et al, 1996). Prolongation of stress induces premature nodule senescence, which shares some features with natural senescence (nodule aging), such as the loss of N 2 fixation, the increase in lytic activities, and the formation of green pigments from leghemoglobin (Lb) (Pfeiffer et al, 1983;Sarath et al, 1986).…”