Malate as a key carbon source of leaf dark-respired CO2 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 CO2. The carbon isotopic composition of leaf dark-respired CO2 (i.e. delta C-13(R)) shows daily enrichments up to 14.8% under different environmental conditions. However, the reasons for this C-13 enrichment in leaf dark-respired CO2 are not fully understood, since daily changes in delta C-13(R) of putative leaf respiratory carbon sources (d13CRS) are not yet clear. Thus, we exposed potato plants (Solanum tuberosum) to different temperature and soil moisture treatments. We determined delta C-13(R) with an in-tube incubation technique and d13CRS with compound-specific isotope analysis during a daily cycle. The highest delta C-13(R) values were found in the organic acid malate under different environmental conditions, showing less negative values compared to delta C-13(R) (up to 5.2%) and compared to delta C-13(R) of soluble carbohydrates, citrate and starch (up to 8.8%). Moreover, linear relationships between delta C-13(R) and delta C-13(RS) among different putative carbon sources were strongest for malate during daytime (r2= 0.69, P= 0.001) and nighttime (r(2)= 0.36, P= 0.001) under all environmental conditions. A multiple linear regression analysis revealed delta C-13(RS) of malate as the most important carbon source influencing delta C-13(R). Thus, our results strongly indicate malate as a key carbon source of C-13 enriched dark-respired CO2 in potato plants, probably driven by an anapleurotic flux replenishing intermediates of the Krebs cycle.