Malate as a key carbon source of leaf dark-respired CO(2) across different environmental conditions in potato plants

Dissimilation of carbon sources during plant respiration in support of metabolic processes results in the continuous release of CO(2). The carbon isotopic composition of leaf dark-respired CO(2) (i.e. δ (13) C (R)) shows daily enrichments up to 14.8‰ under different environmental conditions. However, the reasons for this (13)C enrichment in leaf dark-respired CO(2) are not fully understood, since daily changes in δ(13)C of putative leaf respiratory carbon sources (δ (13) C (RS)) are not yet clear. Thus, we exposed potato plants (Solanum tuberosum) to different temperature and soil moisture treatments. We determined δ (13) C (R) with an in-tube incubation technique and δ (13) C (RS) with compound-specific isotope analysis during a daily cycle. The highest δ (13) C (RS) values were found in the organic acid malate under different environmental conditions, showing less negative values compared to δ (13) C (R) (up to 5.2‰) and compared to δ (13) C (RS) of soluble carbohydrates, citrate and starch (up to 8.8‰). Moreover, linear relationships between δ (13) C (R) and δ (13) C (RS) among different putative carbon sources were strongest for malate during daytime (r(2)=0.69, P≤0.001) and nighttime (r(2)=0.36, P≤0.001) under all environmental conditions. A multiple linear regression analysis revealed δ (13) C (RS) of malate as the most important carbon source influencing δ (13) C (R). Thus, our results strongly indicate malate as a key carbon source of (13)C enriched dark-respired CO(2) in potato plants, probably driven by an anapleurotic flux replenishing intermediates of the Krebs cycle.