Contributions of photosynthetic and non‐photosynthetic cell types to leaf respiration in V icia faba   L . and their responses to growth temperature

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non‐photosynthetic (epidermal) cells from fully expanded leaves of warm‐developed ( WD ) and cold‐developed ( CD ) broad bean ( V icia faba L .). Rates of respiration were significantly higher in mesophyll cell protoplasts ( MCPs ) than epidermal cell protoplasts ( ECPs ), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs , MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs , suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non‐photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature. The respiratory contribution of different cell types to overall leaf respiration is largely unknown. We measured the respiratory capabilities of both mesophyll and epidermal cell protoplasts from V icia faba L . under warm and cold growth conditions, revealing a capacity for both cytochrome and alternative oxidase activity in both protoplast types. We found that while the contribution of mesophyll cells to overall leaf respiration is dominant, epidermal cells make a small but significant (4–7%) contribution which increases in the cold. Our results confirm the heterogeneous nature of mitochondrial populations within the leaf.