FERRIC LEGHEMOGLOBIN IN PLANT-ATTACHED LEGUMINOUS NODULES

Leghemoglobin (Lb) is essential for nitrogen fixation by intact legumi nous nodules. To determine whether ferric Lb (Lb(3+)) was detectable i n nodules under normal or stressed conditions, we monitored the status of Lb in intact nodules attached to sweet clover (Melilotus officinal is) and soybean (Glycine max [L.] Merr.) roots exposed to various cond itions. The effects of N-2 and O-2 streams and elevated nicotinate lev els on root-attached nodules were tested to determine whether the spec trophotometric technique was showing the predicted responses of Lb. Th e soybean and sweet clover nodules' Lb spectra indicated predominantly ferrous Lb and LbO(2) in young (34 d) plants. As the nodule aged beyo nd 45 d, it was possible to induce Lb(3+) with a 100% O-2 stream (15 m in). At 65 d without inducement, the nodule Lb status indicated the pr esence of some Lb(3+) along with ferrous Lb and oxyferrous Lb. Nicotin ate and fluoride were used as ligands to identify Lb(3+). Computer-cal culated difference spectra were used to demonstrate the changes in Lb spectra under different conditions. Some conditions that increased abs orbance in the 626 nm region (indicating Lb(3+) accumulation) were roo t-fed ascorbate and dehydroascorbate, plant exposure to darkness, and nodule water immersion.