APPEARANCE OF NITRATE REDUCTASE, NITRITE REDUCTASE AND GLUTAMINE-SYNTHETASE IN DIFFERENT ORGANS OF THE SCOTS PINE (PINUS-SYLVESTRIS) SEEDLING AS AFFECTED BY LIGHT, NITRATE AND AMMONIUM

Appearance of nitrate reductase (NR, EC 1.6.6.1-3), nitrite reductase (NIR, EC 1.7.7.1) and glutamine synthetase (GS, EC 6.3.1.2) under the control of nitrate, ammonium and light was studied in roots, hypocotyl s and needles (cotyledonary whorl) of the Scots pine (Pinus sylvestris L.) seedling. It was found that appearance of NiR was mainly controll ed by nitrate whereas appearance of GS was strongly controlled by ligh t. In principle, the NR activity level showed the same dependency on n itrate and light as that of NiR. In the root, both nitrate and ammoniu m had a stimulatory effect on GS activity whereas in the whorl the ind uction was minor. The level of NIR (NR) activity is high in the root a nd hypocotyl and low in the cotyledonary whorl, whereas the GS activit y level per organ increases strongly from the root to the whorl. Thus, in any particular the operation of the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle is not closely connected to the operation o f the nitrate reduction pathway. The strong control of GS/GOGAT by lig ht and the minor sensitivity to induction by nitrate or ammonium indic ate a major role of the GS/GOGAT cycle in reassimilation of endogenous ly generated ammonium.