Carbon and Nitrogen Limitations on Soybean Seedling Development 1

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Carbon and nitrogen limitations on symbiotically grown soybean seedlings (Glycine max [L.] Merr.) were assessed by providing 0.0, 1.0, or 8.0 millimolar NH4NO3 and 320 or 1,000 microliters CO2/liter for 22 days after planting. Maximum development of the Rhizobium-soybean symbiosis, as determined by acetylene reduction, was measured in the presence of 1.0 millimolar NH4NO3 under both levels of CO2. Raising NH4NO3 from 0.0 to 8.0 millimolar under 320 microliters CO2/liter increased plant dry weight by 251% and Kjeldahl N content by 287% at 22 days after planting. Increasing NH4NO3 from 1.0 to 8.0 millimolar under 320 microliters CO2/liter increased total dry weight and Kjeldahl N by 100 and 168%, respectively, on day 22. Raising CO2 from 320 to 1,000 microliters CO2/liter during the same period had no significant effect on Kjeldahl N content of plants grown with 0.0 or 1.0 millimolar NH4NO3. The maximum CO2 treatment effects were observed in plants supplied with 8.0 millimolar NH4NO3, where dry weight and Kjeldahl N content were increased 64% and 20%, respectively. An increase in shoot CO2-exchange rate associated with the CO2-enrichment treatment was reflected in a significant increase in leaf dry weight and starch content for plants grown with 1,000 microliters CO2/liter under all combined N treatments. These data show directly that seedling growth in symbiotically grown soybeans was limited primarily by N availability. The failure of the CO2-enrichment treatment to increase total plant N significantly in Rhizobium-dependent plants indicates that root nodule development and functioning in such plants was not limited by photosynthate production.

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