ASSIMILATION OF (NO3)-N-15- TAKEN UP BY PLANTS IN THE LIGHT AND IN THE DARK

An experiment was conducted to determine the extent that NO3- taken up in the dark was assimilated and utilized differently by plants from NO3- taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, ''Ransom'') were exposed to 15NO3- throughout light (9 h) or dark (15 h) phases of the photoperiod and then returned to solutions containing 14NO3-, with plants sampled subsequently at each light/dark transition over 3 days. The rates of 15NO3- absorption were nearly equal in the light and dark (8.42 and 7.93 .mu.mol/h, respectively); however, the whole-plant rate of 15NO3- reduction during the dark uptake period (2.58 .mu.mol/h) was 46% of that in the light (5.63 .mu.mol/h). The lower rate of reduction in the dark was associated with both substantial retention of absorbed 15NO3- in roots and decreased efficiency of reduction of 15NO3- in the shoot. The rate of incorporation of 15N into the insoluble reduced-N fraction of roots in darkness (1.10 .mu.mol/h) was somewhat greater than that in the light (0.92 .mu.mol/h), despite the lower rate of whole-plant 15NO3- reduction in the darkness. A large portion of the 15NO3- retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of 15NO3- acquired during the light period. Taking into account reduction of NO3- from all endogenous pools, it was appparent that plant reduction in a given light period (.apprx. 13.21 .mu.mol/h) exceeded considerably the rate of acquisition of exogenous NO3- (8.42 .mu.mol/h) during that period. The primary source of substrate for NO3- reduction in the dark was exogenous NO3- being concurrently absorbed. In general, a relatively small portion (< 20%) of the whole-plant reduction of NO3- in the light occurred in the root system.