PHOTORESPIRATION IN C-3-C-4, INTERMEDIATE SPECIES OF ALTERNANTHERA AND PARTHENIUM - REDUCED AMMONIA PRODUCTION AND INCREASED CAPACITY CO2 REFIXATION IN THE LIGHT

The pattern of photorespiratory ammonia (PR-NH3) formation and its mod ulation by exogenous bicarbonate or glycine were investigated in C-3-C -4 intermediates of Alternanthera (A. ficoides and A. tenella) and Par thenium hysterophorus in comparison to those of C-3 or C-4 species. Th e average rates of PR-NH3 accumulation in leaves of the intermediates were slightly less than (about 25% reduced) those in C-3 species, and were further low in C-4 plants (40% of that in C-3). The levels of PR- NH3 in leaf discs decreased markedly when exogenous bicarbonate was pr esent in the incubation medium. The inhibitory effect of bicarbonate o n PR-NH3 accumulation was pronounced in C-3 plants, very low in C-4 sp ecies and was moderate in the C-3-C-4 intermediates. Glycine, an inter mediate of photorespiratory metabolism, raised the levels of PR-NH3 in leaves of not only C-4 but also C-3-C-4 intermediates, bringing the r ates close to those of C-3 species. The rate of mitochondrial glycine decarboxylation in darkness in C-3-C-4 intermediates was partially red uced (about 80% of that in C-3 species), corresponding to the activity -levels of glycine decarboxylase and serine hydroxymethyltransferase i n leaves. The intermediates had a remarkable capacity of reassimilatin g photorespiratory CO2 in vivo, as indicated by the apparent refixatio n of about 85% of the CO2 released from exogenous glycine in the light . We suggest that the reduced photorespiration in the C-3-C-4 intermed iate species of Alternanthera and Parthenium is due to both a limitati on in the extent of glycine production/decarboxylation and an efficien t refixation/ recycling of internal CO2.