Secondary aerenchyma formation and its relation to nitrogen fixation in root nodules of soybean plants (Glycine max) grown under flooded conditions

Soybean (Glycine max (L.) Merr.) is considered to be susceptible to flooding, a major agronomic problem in the world, and nitrogenase activity rapidly declines due to oxygen deficiency in root nodules. We investigated nodule acclimation to flooding at the morphological level using a soybean cultivar possessing the ability to form secondary aerenchyma. After 1 week of treatment, lenticels were formed on the surface of the root nodules, and secondary aerenchyma were observed through the lenticels under both irrigated and flooded conditions. As the plant grew, the nodule epidermis came off, and well-developed secondary aerenchyma covered the nodule surface. The secondary aerenchyma originated from the secondary meristem (phellogen) girdling the sclerenchyma, and the degree of development was greater in flooded nodules than in irrigated ones. Although root nodulation and total nitrogenase activity (TNA) decreased under flooded conditions, there were no differences in shoot N concentration, specific nitrogenase activity (SNA) and relative ureide-N in the xylem bleeding sap between plants in the irrigated and flooded conditions. Under flooded conditions, however, when the entry of oxygen into the secondary aerenchyma formed in the hypocotyl. was inhibited by vaseline treatment (pasting on the surface of the hypocotyl), the shoot N concentration, TNA, SNA, the ureide-N concentration and the relative ureide-N in the sap declined remarkably. These results suggested that secondary aerenchyma formation in soybean plants is a morphological acclimation response to flooding stress, and that one of the functions is to supply atmospheric oxygen to root nodules, which consequently enables nodule activity to be maintained.