NITRATE REGULATION OF NITRATE UPTAKE AND NITRATE REDUCTASE EXPRESSIONIN BARLEY GROWN AT DIFFERENT NITRATE AMMONIUM RATIOS AT CONSTANT RELATIVE NITROGEN ADDITION RATE/

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative a ddition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nit rogen addition was 0.09 day(-1) (growth limiting by 35%), while the ni trate addition was varied by means of different nitrate: ammonium rati os. In 3- to 4-week-old plants, these ratios of nitrate to ammonium su pported nitrate fluxes ranging from 0 to 22 mu mol g(-1) root dry weig ht h(-1), whereas the total N flux was 21.8 +/- 0.25 mu mol g(-1) root dry weight h(-1) for all treatments. The external nitrate concentrati ons Varied between 0.18 and 1.5 mu M. The relative growth rate, root t o total biomass dry weight ratios, as well as Kjeldahl nitrogen in roo ts and shoots were unaffected by the nitrate:ammonium ratio. Tissue ni trate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indica ting increased translocation of nitrate to the shoot. The apparent V-m ax for net nitrate uptake increased with increased nitrate fluxes. Upt ake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitr ate concentration in the medium under these conditions. In contrast, n itrate reductase (NR) activity in roots was unaffected by different ni trate fluxes, whereas NR activity in the shoot increased with increase d nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of ni trate is required to elicit expression of NR and uptake activity. Howe ver, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR e xpression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/ or plant. The method used in this study may facilitate separation of n itrate-specific responses from the nutritional effect of nitrate.