EFFECTS OF PROLONGED FLOODING ON SOYBEAN AT THE R2 GROWTH STAGE .2. N AND P UPTAKE AND TRANSLOCATION

Knowledge of N and P uptake and translocation is essential for a greater understnding of soybean [Glycine max (L.) Merrill] nutrient relations under flooding conditions. A field experiment was conducted on a Crowley silt loam (Typic Albaqualf) to determine N and P uptake and translocation rates in soybean under flooded and non-flooded conditions. Forrest soybeans were flooded at the R2 growth stage for 7 consecutive days at a flood height of 2.5 cm. The soybeans were partitioned into stems, branches, leaves, and pods at 0, 7, 14, 21, 36, and 62 days after flooding. Dry matter, N and P concentrations, and total amounts of N and P accumulated for each compartment were determined at each sampling time. Calculations of whole plant uptake and translocation rates of N and P were made of the plant parts and of the nodes during the five intervals. The leaves and then the branches were the first plant parts to respond to the prolonged flooding. Leaves of the flooded plants had a greater decrease in N fluxes than the other plant parts. During the flood, N fluxes were negative in the middle nodes and positive in the upper nodes which indicated that N was translocated from the middle to the upper canopy without substantial replenishment from the flooded soil. Higher P fluxes were found in the pods and whole plants in flooded soybean as compared to non-flooded controls. In both treatments and for both elements, the highest elemental fluxes occurred between the stems and pods which indicated that the pods were the main sink for both N and P. The magnitude of transport coefficients for both elements and in both treatments indicated that the main transport pathway during early reproductive growth was from stems to branches. During late reproductive growth the main transport pathway was from the stems to the pods. The redistribution of N within soybean plant parts was greater than that of P.