Relationship of Tissue Water Relations to Asparagine Uptake in Developing Soybean Seeds1

Amino acids play an important role in the N nutrition of developing soybean [Glycine max(L.) Merr.] seeds. Factors affecting cotyledonary amino acid uptake within those seeds are not well understood. The objectives of this study were to investigate the effects of altered soybean cotyledon water relations on in vitro asparagine uptake by those cotyledons. Two experimental plant introduction (PI) lines with genotypic differences in seed growth rate and final seed weight were grown in the field. They were PI 416.845 (heavy seeded), and PI 361.058 (light seeded). Cotyledons were sampled at 32 days after anthesis for the light seeded line, and 32 and 45 days after anthesis for the heavy seeded line. Cultivar Evans was used for the studies involving plants grown in the growth chamber. Cotyledons were incubated in uptake buffer solutions containing 10 mmol kg‐114C‐asparagine and adjusted to various water potential levels with mannitol (a slowly penetrating osmoticum) or ethylene glycol (a rapidly penetrating osmoticum). Solution water potential levels were approximately 0, −0.5, −1.0, −1.5, and −2.0 MPa. The rate of asparagine uptake for the heavy seeded genotype was greater than that for the light seeded genotype. Uptake was greater at −0.5 and −1.0 MPa than at 0 and −2.0 MPa in the experiments using plants grown in the growth chamber. In the study using field‐grown plants, asparagine uptake was similar in mannitol solutions of 0, −0.5, and −1.0 MPa, but declined in mannitol solutions of −1.5 and −2.0 MPa. Results for ethylene glycol as osmoticum were similar at 32 days after anthesis although reductions of uptake at −1.5 and −2.0 MPa were not as great. Our results indicate that asparagine uptake by excised cotyledons is influenced by the water potential components of their cells, specifically the cell turgor.