Water Uptake by Seeds in Yellow-seeded Soybean (Glycine max (L.) Merrill) Cultivars with Contrasting Imbibition Behaviors

Abstract

The rate of water uptake by seeds is assumed to be an important factor affecting the susceptibility of seeds to flooding injury, but the traits which could contribute to restricting rapid imbibition by seeds remain undetermined in yellow-seeded soybeans. This study was conducted to determine the possible factors controlling the rate of imbibition in soybeans with yellow seed coats. The imbibition behavior of two yellow-seeded cultivars, Tamahomare and Tachinagaha, with different flooding susceptibilities was examined in relation to initial seed moisture content, the hydration location of the seed surface during imbibition, and the state of the seed coat. Low seed moisture increased the water uptake by Tamahomare intact seeds and even allowed substantially faster imbibition than occurred in its embryos, while in Tachinagaha seed moisture content had little influence on seed imbibition. This rapid imbibition by low-moisture Tamahomare seeds was not due to alterations in the permeability of the seed coat nor in water uptake by the embryo, but rather to increased movement of water along the abaxial sides of the cotyledons. The seed coat of Tamahomare loosened readily upon submergence whilst that of Tachinagaha rather tightly adhered to the embryo, suggesting that the seed coat of Tamahomare adheres very loosely to the embryo and may aid in conducting water laterally between the seed coat and embryo. The degree of adherence of the coat to the embryo and/or the ease with which the coat can be loosened upon hydration appear to play a crucial role in determining the rate of water uptake in yellow-seeded soybeans.     

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

Abstract

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.