Our understanding of the regulation of respiration in C
plants, where mitochondria play different roles in the different types
of C photosynthetic pathway, remains limited. We
examined how leaf dark respiration rates (R),
in the presence and absence of added malate, vary in monocots
representing the three classical biochemical types of C
photosynthesis (NADP-ME, NAD-ME and PCK) using intact leaves and
extracted bundle sheath strands. In particular, we explored to what
extent R are associated with mitochondrial
number, volume and ultrastructure. 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 types. However, despite the obvious differences in
mitochondrial traits, no clear correlation was observed between these
traits and R. We suggest that
R 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.