SOIL OXYGEN EFFECTS ON TWO DETERMINATE SOYBEAN ISOLINES1

Because a large portion of southern determinate soybean is grown on land frequently subject to poor soil aeration, an understanding of soybean responses to this condition is needed. It was hypothesized that soybean nutrient status, growth, and stomata would respond in a manner similar to that observed in many other species and, to test this hypothesis, grew soybean [Glycine max (L.) Merr., cv. Lee] in the greenhouse in sealed root chambers in equal volumes of soil and perlite. The soil was a Norfolk loamy sand (fine-loamy, siliceous, thermic, Typic Paleudult). Soil water was kept near field capacity; N, P, and K were added at equivalent field recommendation rates. Humidified gas containing 21, 4, 2, or 0% oxygen was passed over the soil at 500 ml/min beginning 46 d after planting. When soil-O2 diffusion rate (ODR) fell below 40 x 10-8 g cm-2 min-1, leaf diffusive resistance (R8) increased sharply. Plants were harvested 14, 22, and 29 d after treatment initiation. Adaxial stomatal density (SD) of 10/mm2 was unaffected by soil O2. Abaxial SD was inversely related to soil O2, rising from 55/mm2 at 21% O2 to 90/mm2 at 0%. All growth parameters were negatively affected by reduction in soil O2. The increase in SD was approximately proportional to the loss in leaf area in the absence of soil O2, indicating that changes in aperture rather than SD account for the rise in R, In general, concentrations of K and Ca declined in all tissue with declining soil O2. Magnesium concentrations were not consistently affected by O2 treatment.