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

Abstract

Soybean (Glycine max [L.] Merr.) is generally considered sensitive to flooding stress. Data on relative sensitivities of biomass accumulation and N2 fixation to flooding stress, however, are limited. Additionally, it is not clear why plants dependent on N2 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 pO2. Dry matter and N were determined at various times following the initiation of flood or low O2 treatment. In all experiments, N2 fixation was more sensitive to flooding than was biomass accumulation. The decrease in N2 fixation occurred faster (within 7 d of flooding) than the decrease in biomass (within 14–21 d), and the decrease in N2 fixation was more pronounced than the decrease in biomass. Addition of nitrate decreased flood sensitivity relative to plants dependent on N2 fixation. Plant response to hypoxia was similar to flooding. Biomass of plants with roots exposed to 1.5 kPa pO2 was decreased by 34% when dependent on N2 fixation and 12% when supplemented with nitrate. Collectively, the data indicate that decreased soybean growth under flooding is a result of decreased N2 fixation and that supplementation of soybean plants with nitrate may improve their tolerance to flooding relative to those relying on N2 fixation.