SHOOT CONTROL OF NITRATE UPTAKE RATES BY ROOTS OF BRASSICA-NAPUS L - EFFECTS OF LOCALIZED NITRATE SUPPLY

In order to study the regulation of NO3- absorption, experiments were performed with Brassica napus plants submitted to a localized supply o f NO3- by means of a split root system. When about 50% of roots were d eprived of KNO3, net uptake rates by KNO3-supplied split roots increas ed by about two-fold within less than 24 h, Analysis of N and NO3- con tents of root and shoot tissues showed that this increased uptake was sufficient to satisfy shoot growth requirements. This enhancement of u ptake rate, resulting from higher NO3- influx, was inhibited by about 75% by actinomycin D and more strongly by cycloheximide and p-fluoroph enylalanine. The use of NO3- pulses at low concentration (20 mu M) ind icates that NO3- itself induced this increase in net uptake rate. This regulation seems to occur at the translational and/or transcriptional levels. Highly significant negative correlations were found between s hoot N or NO3- contents and NO3- uptake rates achieved by NO3--fed roo ts, but pools of free amino acids in roots did not seem to be involved in the control of root NO3- uptake.