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Influence of Phosphorus Nutrition on Phosporus and Nitrogen Utilization Effeciencies and Associated Physiological Responses in Soybean

USDA-ARS, Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC 27695-7619
USDA-ARS, Oxford, NC 27565
Dep. of Crop Sci., North Carolina State Univ., Raleigh, NC 27695-7620

^* Corresponding author.

Since nutrient concentrations in plant tissues are diluted by growth when the supplies in the root zone are limited, it has been difficult to evaluate the effects of nutrient stress on the efficiency of nutrient utilization in dry matter production by plants. Therefore, this study was initiated to apply the nutrient utilization efficiency concept, (whole plant biomass)/(whole plant nutrient concentration), in evaluating the effect of variable P supply on the utilization of P and N in dry matter production by soybean [Glyclne max (L) Merr.] plants and to assess physiological responses associated with changes in nutrient utilization efficiency. Soybean plants inoculated with Bradyrhizobium japonicum USDA 110 were grown in outdoor pot culture and supplied daily with either minus N or 20 mM NO^–₃ solutions containing 0 to 2.0 mM KH₂PO₄. Phosphorus utilization efficiencies and biomass accumulation for plants in both N regimes increased with increased P supply up to external concentrations of 0.25 to 0.50 raM. At the external P concentrations > 0.50 raM, P utilization efficiencies declined as P concentrations in the tissue increased without any additional growth. Maximal N utilization efficiencies in both N regimes were also attained at external P concentrations of 0.25 to 0.50 raM; however, they did not decrease at higher external P concentrations. Increases in P and N utilization efficiencies were associated with (i) increases in whole plant leaf areas and leaf area ratio, (ii) decreases in morning starch concentrations in leaves, (iii) increases in the relative proportions of dry matter, P, and N in shoot tissues, (iv) decreases in concentrations of soluble reduced-N in tissues of NO^–₃-assimilating plants, and (v) increases in concentrations of soluble reduced-N in tissues of N₂-fixing plants.

Received for publication January 4, 1988.