Abstract
An experiment was conducted to determine the extent that NO3− taken up in the dark was assimilated and utilized differently by plants than NO3− taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, `Ransom') were exposed to 15NO3− throughout light (9 hours) or dark (15 hours) 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 micromoles per hour, respectively); however, the whole-plant rate of 15NO3− reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). 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 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant 15NO3− reduction in 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 apparent that plant reduction in a given light period (∼13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO3− (8.42 micromoles per hour) during that period. The primary source of substrate for NO3− reduction in the dark was exogenous NO3− being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO3− in the light occurred in the root system.
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