Effect of Altered pO(2) in the Aerial Part of Soybean on Symbiotic N(2) Fixation

Abstract

Dry matter accumulation, nitrogen content and N(2) fixation rates of soybean (Glycine max [L.] Merr. cv. Wye) plants grown in chambers in which the aerial portion was exposed to a pO(2) of 5, 10, 21, or 30% and a pCO(2) of 300 mul CO(2)/l or a pO(2) of 21% and a pCO(2) of 1200 mul CO(2)/l during the complete growth cycle were measured. Total N(2)[C(2)H(2)] fixed was increased by CO(2)/O(2) ratios greater than those in air and was decreased by ratios smaller than those in air; the effects on N(2) fixation of decreased pO(2) or elevated pCO(2) were quantitatively similar during the period of vegetative growth. Decreased pO(2) produced a smaller increase then elevated pCO(2) during the reproductive period, presumably because of the decreased sink activity of the arrested reproductive growth under subambient pO(2). At a pO(2) of 5% and a pCO(2) of 300 mul CO(2)/l total N(2) 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 pO(2) of 30% and a pCO(2) of 300 mul CO(2)/l total N(2) 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 pO(2) in the aerial part on N(2) fixation are consistent with the hypothesis that the amount of photosynthate available to the nodule may be the most significant primary factor limiting N(2) fixation while sink activity of the reproductive structures may be a secondary factor.