EVIDENCE FOR SUBSTRATE INDUCTION OF A NITRATE EFFLUX SYSTEM IN BARLEYROOTS

Induction of an NO3- efflux system in intact barley (Hordeum vulgare L .) roots was demonstrated. Since the measurement of NO3- efflux is dep endent on its accumulation, experiments were devised to facilitate acc umulation under noninducing conditions. This was accomplished by incub ating seedlings in 10 mM NO3- in the presence of RNA and protein synth esis inhibitors. Under these conditions NO3- uptake is mediated by con stitutive high- and low-affinity transport systems. Control roots we'r e incubated with 1.0 mM NO3-. This resulted in the accumulation of sim ilar levels of NO3- in both treated and control roots; however, cytopl asmic NO3- efflux from inhibitor-treated roots was much lower than fro m control roots. Following a brief lag period, efflux rates increased rapidly in the presence of NO3- for 8 to 12 h. The NO3- efflux system was also induced by ambient NO2-. After induction the efflux system wa s relatively stable in the presence of RNA and protein synthesis inhib itors as long as NO3- or NO2- was present. These results suggest that NO,- efflux may be an inducible system requiring both RNA and protein synthesis, as does induction of the uptake system. The efflux system, however, has a much slower turnover rate than the uptake system.