Correlation Between the Development of Photorespiration and the Change in Activities of NH₃ Assimilation Enzymes in Greening Oat Leaves

Abstract

The development of photosynthetic capacity and photorespiration during chloroplast development in 7-day-old etiolated oat (Avena sativa L.) primary leaves was investigated together with changes in the activity of possible NH3-assimilating enzymes. The development of photosynthetic CO₂ fixation and photorespiration capacity, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and glutamine synthetase (GS) activities comparable to green leaves were completed within 48 h of continuous illumination. Chlorophyll synthesis and glutamate synthase (GOGAT) activity continued to increase beyond this time. Within this 48-h period, the activities of Rubisco, GS and GOGAT increased 2.3, 2 and 3 times repectively. Throughout the greening treatment, the GS and GOGAT activities were always high enough to sustain the expected rate of photorespiratory NH₃ production. In contrast, glutamate dehydrogenase activity decreased during greening, and its measured rate was not high enough for photorespirtory NH₃ assimilation. These results support the idea that the GS/GOGAT pathway is the major, if not the only, route for photorespiratory NH3 assimilation in the light in leaves of higher plants.