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

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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.

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