UV-B radiation mediated alterations in the nitrate assimilation pathway of crop plants 1. Kinetic characteristics of nitrate reductase

Abstract

The kinetics and other characteristics of nitrate reductase (NR, EC 1.6.6.1) in cowpea [Vigna unguiculata (L.) Walp.] seedlings irradiated with biologically effective UV-B radiation (280-320 nm, 3.2 W m-2 s-1) were recorded. The in vivo and in vitro NR activities were inhibited by 34 and 41 % under UV-B treatment, respectively. Both V(max) and K(m) for the substrate were enhanced by UV-B radiation. The K(m) for nitrate increased from 1.2 to 1.7 mM after the UV-B irradiation. The change in K(m) for NADH was from 0.12 to 0.17 mM. The increases in K(m) indicate that UV-B radiation seriously changes the topology of NR, particularly with respect to the nitrate and NADH binding sites. The rate of NR turnover indicates the extent of damage inflicted by UV-B radiation on the nitrate metabolism. The half-life (t(1/2)) of NR was reduced from 7 to 4 h in the UV-B treated seedlings. UV-B also inhibited the kinetics of nitrate uptake by plants: its K(m) increased from 0.08 to 0.12 mM.