Reversible dark-induced senescence of soybean root nodules.

Nodule senescence was induced in intact soybean [Glycine max. (L.) Merr., cv Woodworth] plants by an 8-day dark treatment. Dark-induced senescence resulted in the complete loss of acetylene reduction activity, a 67% loss of total soluble protein, and an almost complete loss in total leghemoglobin of nodule extracts. Isoelectric focusing gels demonstrated a preferential loss of certain proteins, which was correlated with an increase in endoprotease specific activity toward azocasein. Nodules were completely green after the 8-day dark treatment. If plants were returned to a normal photoperiod after 8 days in the dark, nodules recovered from the dark treatment in 12 to 16 days. Acetylene reduction activity returned to normal, and both total soluble protein and leghemoglobin were resynthesized while protease activity against azocasein decreased to the level of control nodules. The nodule population that had turned green after 8 days in the dark exhibited a progressive increase in red color starting nearest the exterior of the nodule, and after 16 days of recovery nodules were indistinguishable from control nodules maintained under a normal photoperiod.