Partial reduction in activities of photorespiratory enzymes in C₃-C₄ intermediates of alternanthera and parthenium

Abstract

The maximum catalytic activities of several photorespiratory and photosynthetic enzymes were determined in leaf extracts of three C₃-C₄ intermediates (Alternanthera ficoides, A. tenella and Parthenium hysterophorus) and were compared to those of C₃ (A. sessiles, Pisum sativum) and C₄ (A. pungens, Zea mays and Amaranthus hypochondriacus) species. The activity levels of key photorespiratory enzymes, glycolate oxidase, catalase, NADH-hydroxypyruvate reductase and glycerate kinase were less (28 to 35% reduced) in intermediates than those of typical C₃ species. Similarly, the activities of photorespiratory aminotransferases in the C₃-C₄ intermediates were also partially reduced (23 to 37% reduction). The activities of phosphoenolpyruvate carboxylase (PEPC), pyruvate, orthophosphate dikinase and NAD-malic enzyme were higher (2 to 7 times) in leaf extracts of the intermediates than those of C₃ species. But the ratios of PEPC/rubisco in the C₃-C₄ intermediates were more like C₃ than C₄ species. We draw attention to the partial reduction in enzyme activity of photorespiratory metabolism, which could be an important factor for restriction of photorespiration in the C₃-C₄ intermediate species, in addition to enzyme compartmentation and/or operation of a 'C₄-like' cycle