PARTIAL REDUCTION IN ACTIVITIES OF PHOTORESPIRATORY ENZYMES IN C3-C4 INTERMEDIATES OF ALTERNANTHERA AND PARTHENIUM

The maximum catalytic activities of several photorespiratory and photo synthetic enzymes were determined in leaf extracts of three C3-C4 inte rmediates (Alternanthera ficoides, A. tenella and Parthenium hysteroph orus) and were compared to those Of C3 (A. sessiles, Pisum sativum) an d C4 (A. pungens, Zea mays and Amaranthus hypochondriacus) species. Th e activity levels of key photorespiratory enzymes, glycolate oxidase, catalase, NADH-hydroxypyruvate reductase and glycerate kinase were les s (28 to 35% reduced) in intermediates than those of typical C3 specie s. Similarly, the activities of photorespiratory aminotransferases in the C3-Cl intermediates were also partially reduced (23 to 37% reducti on). The activities of phosphoenolpyruvate carboxylase (PEPC), pyruvat e, orthophosphate dikinase and NAD-malic enzyme were higher (2 to 7 ti mes) in leaf extracts of the intermediates than those of C3 species. B ut the ratios of PEPC/rubisco in the C3-C4 intermediates were more lik e C3 than C4 species. We draw attention to the partial reduction in en zyme activity of photorespiratory metabolism, which could be an import ant factor for restriction of photorespiration in the C3-C4 intermedia te species, in addition to enzyme compartmentation and/or operation of a 'C4-like' cycle.