EVALUATION OF NITRATE NUTRITION INDEXES IN MAIZE LEAVES - METABOLIC NITRATE, TOTAL NITRATE CONTENT AND NITRATE REDUCTASE-ACTIVITY

The Metabolic Nitrate Pool (MNP) was measured in leaves by using the i n situ nitrate reductase (NR) activity assay, as described Robin et al . [Physiol. Veg. 1983, 21:155-122]. Total nitrate (NO3- content and in vitro NR activity were also determined as indexes of the NO3- nutriti on status of pure hydroponic grown maize plants. Two experiments were designed: i) increasing concentrations of NO3- in the nutrient solutio n, and ii) deprivation of NO3- in the nutrient solution. Although a ti me course accumulation of nitrite (NO2-) occured after the incubation of maize leaves in anoxia and darkness in the in situ NR activity assa y, the MNP determination was no consistent with the NO3- treatments. T he MNP was small in excess of NO3- (30 mM), whereas the total NO3- con tent and the in vitro NR activity were the highest. Moreover, in the N O3- depletion experiment, the MNP increased after 6 h deprivation, whi le the total NO3- concentration and the in vitro NR activity decreased . The Metabolic Nitrate Corrected Pool (MNCP) was calculated from the results of the MNP and the in vitro NR activity, under the assumption that Part of the NO2- accumulated under anoxia and darkness was reduce d by a putative residual nitrite reductase (NiR) activity. MNCP showed a better correlationship with the total content of NO3- in shoot than MNP did in both experiments. Therefore, our results suggest that the MNCP could provide useful information about the physiology of the NO3- assimilation, but it should be cautiously used as a tool for the diag nosis of the NO3- nutrition status of aerial parts of maize plants.