Soybean dry matter and N accumulation responses to flooding stress, N sources and hypoxia

Soybean (Glycine max [L.] Merr.) is generally considered sensitive to flooding stress. Data on relative sensitivities of biomass accumulation and N-2 fixation to flooding stress, however, are limited, Additionally, it is not clear why plants dependent on N-2 fixation appear to be more flood-sensitive than plants supplemented with inorganic N. This study evaluated the response to flooding and N source of biomass and N accumulation in various soybean genotypes. Soybean plants were grown in a potting mixture in a greenhouse and flooded for 21 d in degassed nutrient solution. An additional experiment evaluated root hypoxia by exposing roots of plants to a gas mixture supplying 1.5 kPa pO(2). Dry matter and N were determined at various times following the initiation of flood or low O-2 treatment. In all experiments, N, fixation was more sensitive to flooding than was biomass accumulation. The decrease in N-2 fixation occurred faster (within 7d of flooding) than the decrease in biomass (within 14-21 d), and the decrease in N-2 fixation was more pronounced than the decrease in biomass, Addition of nitrate decreased flood sensitivity relative to plants dependent on N-2 fixation. Plant response to hypoxia was similar to flooding. Biomass of plants with roots exposed to 1.5 kPa pO(2) was decreased by 34% when dependent on N-2 fixation and 12% when supplemented with nitrate. Collectively, the data indicate that decreased soybean growth under flooding is a result of decreased N-2 fixation and that supplementation of soybean plants with nitrate may improve their tolerance to flooding relative to those relying on N-2 fixation.