Abstract
The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean (Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N2 fixation. Shoot CER, root + nodule respiration, and apparent N2 fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N2 fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N2 fixation was measured within 4 hours, and decreasing CO2 concentration from 320 to 90 microliters CO2 per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO2 concentration around the shoot from 320 to 1,000 microliters CO2 per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO2-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO2 enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N2 fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N2-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known.
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Selected References
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