ISSN:
1399-3054
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
In order to examine the light-modulation of nitrate reductase (NR: EC 1. 6.6.1) activity in tissues of maize (Zea mays L. cv. Plauto). the enzyme was assayed under both selective (5 mM, Mg2+) and unselective conditions (2 mM EDTA; Vmax). in leaf and root extracts from seedlings transferred from high irradiance (HI: 450 μmol m−2s−1 PAR) to shade (200 μmol m−2 s−1) or to darkness. The corresponding changes in tissue nitrate were also evaluated. The results show that maize leaf NR activity (NRA) is subjected to reversible light-modulation involving rapid changes in the sensitivity of the enzyme to inhibition by Mg2+, In fact, NR assayed in the presence of Mg2+ lost about 60% and 20% of its activity after 30 min in the dark and in the shade, respectively. Vmax started to decrease only after 1–3 h. depending on the extent of light reduction. Deactivation in the shade was reversed in vivo by returning the plants to HI. the kinetics of reactivation being similar to those of deactivation. Following dark-deactivation in vivo, spontaneous reactivation of leaf NR occurred during in vitro incubation at 25°C. and the process was accelerated by AMP. The NR from HI-leaves was deactivated (half-time 2 min) by incubation with MgATP. but the extent of the in vitro deactivation was small. Maize root NR was more sensitive to Mg2+ than leaf NR, with 5 mM Mg2+ causing a 50% inhibition of the activity in HI-roots. In contrast to leaves, marked changes in the Mg2+-sensitivity of maize root NRA became apparent only after 3 h in the dark or in the shade, when Vmax had also declined significantly.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1399-3054.1995.tb05316.x
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