Enzymes of ammonia assimilation, photosynthesis, and respiration in alfalfa leaves of different ages

The activities of enzymes involved in ammonia metabolism ferredoxin-depende nt glutamate synthase (Fd-GOGAT), glutamine synthetase (GS) and glutamate d ehydrogenase (GDH), the rates of photosynthetic oxygen evolution, dark resp iration, and the activity of RuBP carboxylase (RuBPC) were determined in al falfa (Medicago sativa L.) leaves taken from the apex (apical leaves), from the second to the fourth internode (mature leaves) and from the bottom of the canopy (basal leaves), Photosynthetic rate and the activities of RuBPC, GS and Fd-GOGAT showed their maximum in the mature leaves. The respiration rate together with amino acid and ammonium contents decreased with leaf ag e, whereas the opposite was true for GDH activity, Basal leaves still maint ained substantial levels of chlorophylls, GS and Fd-GOGAT activities and ox ygen evolution rate, thus suggesting that photosynthesis has some role in t he reassimilation of the nitrogen liberated during protein degradation.