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

Abstract

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.