Description
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
5
sugars and amino acids to explain the observed variations in NR activity are
discussed.
; Fonseca, Filomena 
