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Photosynthesis and Nitrogen Fixation in Soybean Exposed to Nitrogen Dioxide and Carbon Dioxide

Dep. of Agricultural Sciences, Univ. of Maryland Eastern Shore, Princess Anne, MD 21853.

^* Corresponding author.

ABSTRACT

‘Williams-82’ Soybean [Glycine max (L.) Merr.] plants, at the flowering stage, were exposed to NO₂ (0.1 µmol mol^–1, 7 h d^–1) and CO₂ (450 µmol mol^–1, 12 h d^–1) for 5 d singly and in combination in open-top field chambers. The rate of photosynthesis (PN), specific root nodule nitrogenase activity (SNA), and leaf N were measured after exposure using the LICOR 6200 Portable Photosynthesis system, the acetylene-ethylene reduction method, and the Kjeldahl digestion, respectively. The plants exposed to NO₂ (with 350 µmol mol^–1 CO₂) showed significantly higher PN, SNA, and N compared with the control plants. The SNA and N were reduced, with no change in PN on exposing the plants to CO₂ (in the absence of NO₂). When the plants were exposed to a combination of these two gases, SNA was increased but the PN and N did not change. The stimulatory effect of NO₂ was reduced by the inhibitory impact of CO₂ on the parameters studied.