Influence of UVB irradiation on nitrate and ammonium assimilating enzymes in Spirodela polyrhiza

The influence of UVB radiation on nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), NADH-dependent glutamate synthase (NADH- GOGAT) and ferredoxin-dependent glutamate synthase (Fd-GOGAT) was investiga ted in fronds and turions of Spirodela polyrhiza. When white light (4.3 W m (-2)) was supplemented with UVB (0.55 W m(-2) lambda(max) = 311 nm), a 40% transient decrease in the growth rate was observed after 12 h UVB irradiati on. In addition, the activities of all enzymes investigated, except for the cytoplasmic isoform of GS, were substantially inhibited (NR, 80%; NiR, 32% ; GS, 30%; NADH-GOGAT, 36%; Fd-GOGAT, 54% in green fronds). The extent of t he inhibition in fronds as well as in turions is in accordance with the ass umption that the light-induced increases of enzyme activities are prevented by simultaneous application of UVB. The uptake of nitrate was not influenc ed by white light or UVB, As shown by western analysis, a correlation betwe en the enzyme activities and their protein amounts was found. Northern anal ysis revealed that UVB influenced the transcript accumulation of NR but not of Fd-GOGAT. No obvious correlation was observed between the mRNA and the protein amount. This suggests that WE damage occurs at the transcriptional and posttranscriptional levels as well as at the protein level, (C) 1999 El sevier Science B.V. All rights reserved.