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Nitrogen Nutrition and Temporal Effects of Enhanced Carbon Dioxide on Soybean Growth

Dep. of Plant Sci., Univ. of Manitoba, Winnipeg, MB, R3T 2N2, Canada
Dep. of Forestry, North Carolina State Univ., Raleigh, NC 27695-8002
Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC 27695-7619

^* Corresponding author.

Plants grown on porous media at elevated CO₂ levels generally have low concentrations of tissue N and often appear to require increased levels of external N to maximize growth response. This study determines if soybean [Glycine max (L.) Merr. ‘Ransom’] grown hydroponically at elevated CO₂ requires increases in external NO^–₃ concentrations beyond levels that are optimal at ambient CO₂ to maintain tissue N concentrations and maximize the growth response. This study also investigates temporal influences of elevated CO₂ on growth responses by soybean. Plants were grown vegetatively for 34 d in hydroponic culture at atmospheric CO₂ concentrations of 400, 650, and 900 µL L^–1 and during the final 18 d at NO^–₃ concentrations of 0.5, 1.0, 5.0 and 10.0 mM in the culture solution. At 650 and 900 µL L^–1 CO₂, plants had maximum increases of 31 and 45% in dry weight during the experimental period. Plant growth at 900 µL L^–1 CO₂ was stimulated earlier than at 650 µL L^–1. During the final 18 d of the experiment, the relative growth rates (RGR) of plants grown at elevated CO₂ declined. Elevated CO₂ caused increases in total N and total NO^–₃-N content and leaf area but not leaf number. Enhancing CO₂ levels also caused a decrease in root:shoot ratios. Stomatal resistance increased by 2.1- and 2.8- fold for plants at the 650 and 900 µL L^–1 CO₂, respectively. Nitrate level in the culture solutions had no effect on growth or on C:N ratios of tissues, nor did increases in CO₂ levels cause a decrease in N concentration of plant tissues. Hence, increases in NO^–₃ concentration of the hydroponic solution were not necessary to maintain the N status of the plants or to maximize the growth response to elevated CO₂.

Received for publication April 24, 1989.