Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Abstract

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day−1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3‐ to 4‐week‐old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g−1 root dry weight h−1, whereas the total N flux was 21.8 ± 0.25 μmol g−1 root dry weight h−1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot.The apparent Vmax for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased 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 nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression 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 nitrate‐specific responses from the nutritional effect of nitrate.