Assimilation of 15NO3− Taken Up by Plants in the Light and in the Dark

An experiment was conducted to determine the extent that NO(3) (-) taken up in the dark was assimilated and utilized differently by plants than NO(3) (-) taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, ;Ransom') were exposed to (15)NO(3) (-) throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing (14)NO(3) (-), with plants sampled subsequently at each light/dark transition over 3 days. The rates of (15)NO(3) (-) absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of (15)NO(3) (-) 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 (15)NO(3) (-) in roots and decreased efficiency of reduction of (15)NO(3) (-) in the shoot. The rate of incorporation of (15)N 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 (15)NO(3) (-) reduction in darkness.A large portion of the (15)NO(3) (-) 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 (15)NO(3) (-) acquired during the light period. Taking into account reduction of NO(3) (-) from all endogenous pools, it was apparent that plant reduction in a given light period ( approximately 13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO(3) (-) (8.42 micromoles per hour) during that period. The primary source of substrate for NO(3) (-) reduction in the dark was exogenous NO(3) (-) being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO(3) (-) in the light occurred in the root system.