Recovery of Bradyrhizobium cells and effects on the physiological quality of soybean seeds sown in dry soil

Abstract

Farmers sometimes sow soybean (Glycine max (L.) Merrill) in dry soil in the expectation of rain in the short time. However, the forecast may not confirm, letting the inoculated seeds in the dry soil indefinitely. We assessed the survival of inoculated Bradyrhizobium and physiological quality of soybean seeds sown in dry soil. In the first experiment, irrigation was applied with 2 h, 1, 4, 11, 18, or 21 days after sowing; in the second experiment, sowing was carried out 2 h, 1, 5, 12, 14, or 20 days before irrigation. Each time represented a treatment in a completely randomized design. Bradyrhizobium cells dropped from ~8-9 × 104 colony forming units per seed soon after inoculation to -60% at 2 h after sowing in dry soil, and decreased to close to zero with time in both experiments. Although there was no effect on germination (59% and 81% in the first and second experiments, respectively), the exposure to dry soil reduced the emergence speed index from 19.5 (2 h) to 12.0 (21 days) in the first experiment and from 37.8 (2 h) to 13.8 (21 days) in the second. In the first experiment, the number of abnormal seedlings increased from 7% (2 h) to 24% (21 days); in the second, cotyledons showed cracks, which increased from 1% (2 h) to ~50% (≥ 5 days). Sowing in dry soil negatively affects not only the inoculated Bradyrhizobium, but also the physiological quality of soybean seeds.