The effects of drought on the activity of nitrate reductase (NR) were studied in Helianthus
annuus L. plants subjected to soil drying and subsequent rewatering. Drought did not
negatively affect the activation state of NR, but resulted in linearly correlated decreases in the
activity of the unphosphorilated active form and the total activity of NR, both in roots and
leaves. The concentration of nitrate in roots, xylem and leaves also decreased in waterstressed
plants, whereas the concentration of total amino acids was only transiently depressed
at the leaf level. In contrast, soluble sugars accumulated both in roots and leaves of waterstressed
plants. Drought-induced decrease in root NR activity was correlated with the
observed changes in root nitrate concentration. A higher percentage of the decrease in foliar
NR activity could be explained by the decline in nitrate flux to the leaves than by leaf nitrate
content. Following rewatering, the extent of recovery of NR activity was higher in roots than
in leaves. The delay in the recovery of foliar NR activity did not result from the persistence of
reduced flux of nitrate through the xylem. Several hypotheses to explain the after-effect of
soil drying on foliar NR activity are discussed
