Nitrate reductase in Zea mays L. under salinity

Young maize seedlings (Zea mays L. cv.Giza 2) were exposed to moderate salinity in hydroponic culture. NADH-nitrate reductase (NR) activity (E.C. 1.6.6.1), NR activation state, NR-mRNA-steady state levels and major solute contents in leaves and roots were investigated. With increasing external salt concentration, Na+, Cl-, sugars, amino acids and quarternary ammonium compounds accumulated in leaves and roots, with concentrations in leaves exceeding those in roots. The nitrate content of leaves decreased, but increased in roots. The diurnal pattern of NR activity and of NR-mRNA was also changed under salinity, but the NR activation state was not affected, In the first light phase, maximum NR activity increased rapidly in leaves of control plants, but was much slower in leaves from salinized plants. Thus, integrated over the whole day, the NR activity of salt-stressed plants was lower than in control plants. NR transcript levels of control plants were low in the early night phase, started to increase in the second night phase, followed by a distinct peak at 2 h into the light period, This large 'early morning peak' of NR-mRNA was hardly affected by salinity, whereas the initial slow increase of m-RNA levels in the early night phase was almost absent in salinized plants. This is considered as one reason for the low NR activity of salinized plants in the first half of the day. It is also suggested that nitrate is a major signal affecting NR expression and activity under salinity. Sugars and amino acids appeared less important.