RELATIONSHIPS BETWEEN EXTERNAL NITRATE AVAILABILITY, NITRATE UPTAKE AND EXPRESSION OF NITRATE REDUCTASE IN ROOTS OF BARLEY GROWN IN N-LIMITED SPLIT-ROOT CULTURES

Despite the large number of studies of nitrate metabolism in plants, i t remains undetermined to what extent this key plant system is control led by overall plant N nutrition on the one hand, and by the nitrate i on itself on the other hand. To investigate these questions, V-max for nitrate uptake (high-affinity range), and nitrate reductase (NR) mRNA and activity, were measured in roots of N-limited barley (Hordeum vul gare L. cv. Golf) grown under conditions of constant relative addition of nitrate, with the seminal roots split between two culture compartm ents. The total amount of nitrate added per unit time (0.09 . d(-1)) w as distributed between the two root parts (subroots) in ratios of 100: 0, 98:2, 95:5, 90:10, 80:20, and 50:50. These nitrate-addition ratios resulted in nitrate fluxes ranging from 0 to 23 mu mol nitrate . g(-1) DW root . h(-1), while the external nitrate concentrations varied bet ween 0 and 1.2 mu M. The apparent V-max for net nitrate uptake showed saturation-type responses to nitrate flux maintained during preceding growth. The flux resulting in half-maximal ''induction'' of nitrate up take was approximately 4 mu mol nitrate . g(-1) DW root . h(-1), corre sponding to an external nitrate concentration of 0.7 mu M. The activit y of NR and levels of NR mRNA did not saturate within the range of nit rate fluxes studied. None of the parameters studied saturated with res pect to the steady-state external nitrate concentration. At the zero n itrate addition - the 0%-root - initial uptake activity as determined in shortterm N-15-labelling experiments was insignificant, and NR acti vity and NR mRNA were not detectable. However, nitrate uptake was rapi dly induced, showing that the 0%-root had retained the capacity to res pond to nitrate. These results suggest that local nitrate availability has a significant impact on the nitrate uptake and reducing systems o f a split-root part when the total plant nitrate nutrition is held con stant and limiting.