Secondary Aerenchyma Formation and its Relation to Nitrogen Fixation in Root Nodules of Soybean Plants (Glycine max) Grown under Flooded Conditions

AbstractSoybean (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 ni...