3 research outputs found
Effects of soil drying and subsequent re-watering on the activity of nitrate reductase in roots and leaves of Helianthus annuus
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
Effects of water deficit on the activity of nitrate reductase and content of sugars, nitrate and free amino acids in the leaves and roots of sunflower and white lupin plants growing under two nutrient supply regimes
The effects of a two weeks soil drying period on the activity of nitrate reductase (NR;
EC 1.6.6.6) were studied on Helianthus annuus L. and non-nodulated Lupinus albus
L. plants, growing under two nutrient supply regimes. NR activity was assessed in
leaf and root extracts, by measuring the activity of the unphosphorylated active form
(NRact), the maximal extractable activity (NRmax) and the activation state. To get
insight into potential signalling compounds, nitrate, amino acids and soluble sugars
concentrations were also quantified. On both species, foliar NRact and NRmax were
negatively affected by soil drying and reduced supply of nutrients, the observed
changes in NR activity being linearly-correlated with the depletion of nitrate. Similar
results were obtained in the roots of sunflower. Conversely, in white lupin roots
NRmax was found to be independent of tissue nitrate concentration. Regardless of the
species and organ, the activation state of the enzyme was unaffected by the nutrient
supply regime. In well-watered sunflower roots only about 50% of the existing NR
was unphosphorylated, but the activation state increased significantly in response to
drought. In contrast, lupin roots always exhibited NR activation state values close to
80% or even higher. At the leaf level, NR activation state was hardly changed in
response to soil drying. The contribution of changes in the concentrations of soluble
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sugars and amino acids to explain the observed variations in NR activity are
discussed