Dark respiration rates are not determined by differences in mitochondrial capacity, abundance and ultrastructure in C 4 leaves.

Our understanding of the regulation of respiration in C 4 plants, where mitochondria play different roles in the different types of C 4 photosynthetic pathway, remains limited. We examined how leaf dark respiration rates (R dark ), in the presence and absence of added malate, vary in monocots representing the three classical biochemical types of C 4 photosynthesis (NADP-ME, NAD-ME and PCK) using intact leaves and extracted bundle sheath strands. In particular, we explored to what extent rates of R dark are associated with mitochondrial number, volume and ultrastructure. Based on examination of a single species per C 4 type, we found that the respiratory response of NAD-ME and PCK type bundle sheath strands to added malate was associated with differences in mitochondrial number, volume, and/or ultrastructure, while NADP-ME type bundle sheath strands did not respond to malate addition. In general, mitochondrial traits reflected the contributions mitochondria make to photosynthesis in the three C 4 types. However, despite the obvious differences in mitochondrial traits, no clear correlation was observed between these traits and R dark . We suggest that R dark is primarily driven by cellular maintenance demands and not mitochondrial composition per se, in a manner that is somewhat independent of mitochondrial organic acid cycling in the light.