Abstract
The influence of UVB irradiation on the enzymes of nitrogen metabolism (nitrate reductase, EC 1.6.6.1, nitrite reductase, EC 1.7.7.1, alanine aminotransferase, EC 2.6.1.2, aspartate aminotransferase, EC 2.6.1.1, glutamine synthetase, EC 6.3.1.2) and the uptake of ammonium and nitrate in turions of Spirodela polyrhiza was investigated. UVB irradiation was started either immediately after transferring the turions from 5 °C (after-ripening conditions) to 25 °C (germination conditions) or after a storage period at 25 °C in darkness for 72 h. The activities of all enzymes investigated were decreased by UVB irradiation (12 or 24 h). The activity of nitrate reductase was more strongly inhibited than that of nitrite reductase, and the activity of alanine aminotransferase was more strongly decreased than that of aspartate aminotransferase. The inhibitory effect of UVB on glutamine synthetase was only small. Within a period of 5 h neither the uptake of ammonium nor that of nitrate was influenced by UVB irradiation. Because UVB irradiation did not influence the light-dependent germination of turions, the results presented support the conclusion that the regulation of germination and the regulation of nitrate assimilation are unrelated phenomena in turions
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