TETCYCLACIS SENSITIVITY OF NO3- UPTAKE, NO3--DEPENDENT EXTRACELLULAR FERRICYANIDE REDUCTION AND INTRACELLULAR NO3- REDUCTASE IN ROOTS, LEAVES AND SUSPENSION CELLS OF HIGHER-PLANTS

Tetcyclacis, a growth retarding substance, known to interfere with cyt ochrome P-450-dependent processes, was found to selectively inhibit pl ant NO3- uptake. The effect was studied with roots of Zea mays, Limnob ium stolonifenrm, photosynthesizing leaves and shoots of Egeria densa and with heterotrophically grown suspension cells of Catharanthus rose us. The inhibition was strongest when the NO3--uptake system, but not the NO, reductase (EC 1.6.6.2), was induced by NO2-. Tetcyclacis impai red the nitrate reductase only indirectly by inhibiting NO3- uptake. P hosphate and K+ uptake were not affected either. In addition, tetcycla cis interfered with the ferricyanide-triggered, plasma membrane-locate d, redox system. This inhibition was most pronounced after preincubati on of the roots with NO3-. The results suggest a common intermediate o f the high affinity nitrate uptake system and the redox system at the plasma-membrane level of plant cells. Since preincubation of the tissu e with tetcyclacis was necessary for inhibition and since C-14-labeled tetcyclacis only slowly accumulated within the tissue with saturation after one hour, tetcyclacis must affect some reaction at the inner si de of the plasma membrane and not from outside.