Leaf-level metabolic changes in response to drought affect daytime CO2 emission and isoprenoid synthesis

Daytime respiration in leaves is an integral component of plant metabolism, with CO2 produced as a byproduct during the synthesis of many secondary metabolites, including many common volatile organic compounds (VOCs). It remains unclear how the carbon fluxes associated with daytime respiration and VOC emission vary among mature trees, and how they are influenced by drought. We used position-specific 13C-pyruvate labeling to investigate CO2 and VOC fluxes from the leaves of four tropical species before and during an artificially imposed drought in an enclosed rainforest. Although daytime respiration was always dominated by non-mitochondrial processes, the relative contribution of CO2 from the tricarboxylic acid cycle increased under drought, likely due to osmolyte production. In the legume tree Clitoria fairchildiana , our data indicate that intermediates from the mevalonic acid pathway are used to produce the volatile monoterpene trans-β-ocimene, but not isoprene. This suggests that geranyl pyrophosphate is the more important intermediate for crosstalk between cytosolic and plastidic isoprenoid synthesis, not isopentenyl pyrophosphate. Crosstalk declined with drought, and although trans-β-ocimene emissions increased, they were primarily sourced from storage and not de novo synthesis. We found highly species-specific dynamics in carbon allocation into different metabolic processes including daytime respiration and VOC synthesis. Highlight Intracellular carbon allocation into daytime respiration and VOCs is species-specific and changes with drought. Isoprenoid synthesis in the plastid can be fed with glyceraldehyde-3-phosphate and geranyl pyrophosphate from the cytosol. ### Competing Interest Statement The authors have declared no competing interest. * acetyl-CoA : acetyl coenzyme A B2WALD : Biosphere 2 Water Atmosphere and Life Dynamics DMAPP : dimethylallyl pyrophosphate FEP : fluorinated ethylene propylene GA-3-P : glyceraldehyde-3-phosphate GABA : γ-aminobutyric acid GC-MS-C : Gas Chromatography – Mass Spectrometry – Combustion interface GPP : geranyl pyrophosphate ID : inner diameter IPP : isopentenyl pyrophosphate IRIS : isotope ratio infrared spectrometer IRMS : isotope ratio mass spectrometer MVA : mevalonic acid MEP : methylerythritol 4-phosphate MSD : mass-selective detector PAR : photosynthetic active radiation PFA : perfluoroalkoxy alkane PTR-TOF-MS : proton transfer reaction – time of flight mass spectrometer TCA : tricarboxylic acid VPDB : Vienna Pee Dee Belemnite VOCs : volatile organic compounds