Malate as a key carbon source of leaf dark-respired CO2across different environmental conditions in potato plants

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Abstract
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. δ13CR) shows daily enrichments up to 14.8‰ under different environmental conditions. However, the reasons for this 13C enrichment in leaf dark-respired CO2 are not fully understood, since daily changes in δ13C of putative leaf respiratory carbon sources (δ13CRS) are not yet clear. Thus, we exposed potato plants (Solanum tuberosum) to different temperature and soil moisture treatments. We determined δ13CR with an in-tube incubation technique and δ13CRS with compound-specific isotope analysis during a daily cycle. The highest δ13CRS values were found in the organic acid malate under different environmental conditions, showing less negative values compared to δ13CR (up to 5.2‰) and compared to δ13CRS of soluble carbohydrates, citrate and starch (up to 8.8‰). Moreover, linear relationships between δ13CR and δ13CRS among different putative carbon sources were strongest for malate during daytime (r2=0.69, P≤0.001) and nighttime (r2=0.36, P≤0.001) under all environmental conditions. A multiple linear regression analysis revealed δ13CRS of malate as the most important carbon source influencing δ13CR. Thus, our results strongly indicate malate as a key carbon source of 13C enriched dark-respired CO2 in potato plants, probably driven by an anapleurotic flux replenishing intermediates of the Krebs cycle.

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