Variation in the activity of some enzymes of photorespiratory metabolism in C₄ grasses

BACKGROUND AND AIMS: Photorespiration occurs in C₄ plants, although rates are small compared with C₃ plants. The amount of glycine decarboxylase in the bundle sheath (BS) varies among C₄ grasses and is positively correlated with the granal index (ratio of the length of appressed thylakoid membranes to the total length of all thylakoid membranes) of the BS chloroplasts: C₄ grasses with high granal index contained more glycine decarboxylase per unit leaf area than those with low granal index, probably reflecting the differences in O₂ production from photosystem II and the potential photorespiratory capacity. Thus, it is hypothesized that the activities of peroxisomal enzymes involved in photorespiration are also correlated with the granal development. METHODS: The granal development in BS chloroplasts was investigated and activities of the photorespiratory enzymes assayed in 28 C₄ grasses and seven C₃ grasses. KEY RESULTS: The NADP-malic enzyme grasses were divided into two groups: one with low granal index and the other with relatively high granal index in the BS chloroplasts. Both the NAD-malic enzyme and phosphoenolpyruvate carboxykinase grasses had high granal index in the BS chloroplasts. No statistically significant differences were found in activity of hydroxypyruvate reductase between the C₃ and C₄ grasses, or between the C₄ subtypes. The activity of glycolate oxidase and catalase were smaller in the C₄ grasses than in the C₃ grasses. Among the C₄ subtypes, glycolate oxidase activities were significantly smaller in the NADP-malic enzyme grasses with low granal index in the BS chloroplasts, compared with in the C₄ grasses with substantial grana in the BS chloroplasts. CONCLUSIONS: There is interspecies variation in glycolate oxidase activity associated with the granal development in the BS chloroplasts and the O₂ production from photosystem II, which suggests different potential photorespiration capacities among C₄ grasses.