DIURNAL CHANGES IN NET UPTAKE RATE OF NITRATE ARE ASSOCIATED WITH CHANGES IN ESTIMATED EXPORT OF CARBOHYDRATES TO ROOTS

The rate of NO3- uptake by soybean (Glycine max [L.] Merrill) roots ge nerally declines during the night in association with progressive depl etion of the nonstructural carbohydrate pool in the shoot as well as t he concentration of carbohydrates in roots. To determine if NO3- uptak e rate changes in response to variations in translocation rate of carb ohydrates from shoot to roots per se or to carbohydrate status of the roots, the night period was interrupted with a low light level from in candescent lamps to alter the diurnal pattern of NO3- uptake by roots and export of carbohydrate from shoots of nonnodulated soybean. Deplet ion of NO3- from replenished, complete nutrient solutions containing 1 mM NO3- was measured by ion chromatography and rates of NO, uptake we re calculated. Changes in export of carbohydrates from shoot to roots during intervals of the night period were calculated as the difference s between rates of disappearance in contents of nonstructural carbohyd rates and their estimated rates of utilization in shoot respiration an d growth. A positive, significant correlation occurred between changes in calculated rates of carbohydrate export from shoots and NO3- uptak e rates. Conversely, there was no significant correlation between conc entrations of nonstructural carbohydrates in roots and NO3- uptake rat es. These results support the hypothesis that carbohydrate flux from s hoot to roots has a direct role in regulation of nitrogen uptake by th e whole plant.