Effect of Changes in Shoot Carbon-Exchange Rate on Soybean Root Nodule Activity

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Abstract
The effect of short-and long-term changes in shoot C-exchange rate (CER) on soybean (G. max [L.] Merr. [inoculoted with Rhizobium japonicum]) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N2 fixation. Shoot CER, root plus nodule respiration and apparent N2 fixation (acetylene reduction) were measured on intact soybean plants grown at 700 .mu. einsteins/m/s, with constant root temperature and a 14/10-h 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. If plants remained in darkness following their normal dark period, a significant decline in apparent N2 fixation was measured within 4 h and decreasing C02 concentration from 320-90 .mu.C02 per l produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84 and 76% in 2-, 3- and 4-wk-old plants, respectively, by raising the CO2 concentration around the shoot from 320 to 1,000 .mu. CO2 per l, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 h of the increased CER treatment. When the CO2-enrichment treatment was extended in 3-wk-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO2 enrichment, root plus nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. On a 24-h 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.