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Articles

Effect of Altered pO₂ in the Aerial Part of Soybean on Symbiotic N₂ Fixation ¹

^a Central Research and Development Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898

Dry matter accumulation, nitrogen content and N₂ fixation rates of soybean (Glycine max [L.] Merr. cv. Wye) plants grown in chambers in which the aerial portion was exposed to a pO₂ of 5, 10, 21, or 30% and a pCO₂ of 300 µl CO₂/l or a pO₂ of 21% and a pCO₂ of 1200 µl CO₂/l during the complete growth cycle were measured. Total N₂[C₂H₂] fixed was increased by CO₂/O₂ ratios greater than those in air and was decreased by ratios smaller than those in air; the effects on N₂ fixation of decreased pO₂ or elevated pCO₂ were quantitatively similar during the period of vegetative growth. Decreased pO₂ produced a smaller increase then elevated pCO₂ during the reproductive period, presumably because of the decreased sink activity of the arrested reproductive growth under subambient pO₂. At a pO₂ of 5% and a pCO₂ of 300 µl CO₂/l total N₂ fixed was increased 125% and per cent nitrogen content in the vegetative parts was increased relative to air while that in the seed was decreased. Dry matter production was increased and reproductive growth was arrested as previously reported for plants receiving only fertilizer nitrogen. At a pO₂ of 30% and a pCO₂ of 300 µl CO₂/l total N₂ fixed was decreased 50% and per cent nitrogen content in the vegetative part was increased relative to air while that in the reproductive structures was unaffected. Dry matter production was similarly decreased in both vegetative and reproductive structures. These effects of altered pO₂ in the aerial part on N₂ fixation are consistent with the hypothesis that the amount of photosynthate available to the nodule may be the most significant primary factor limiting N₂ fixation while sink activity of the reproductive structures may be a secondary factor.