Plant carbon allocation in tropical forests under drought stress - Shifting the balance between primary and secondary metabolism such as CO2 and VOC emissions

Climate change exerts increasing pressure on tropical rainforests enhancing their susceptibility to environmental stress. Plants' abilities to rapidly adjust their metabolism are critical for reducing the stress effects caused by external extreme conditions. Plants produce a wide spectrum of volatile organic compounds (VOCs) to cope with oxidative and thermal stresses. Within the framework of our large-scale ecosystem manipulation experiment, Biosphere 2 Water, Atmosphere, and Life Dynamics (B2-WALD), we aim to produce deeper insights into carbon partitioning between primary and secondary metabolism under drought stress, notably into CO₂ and VOCs. In particular, we investigate how drought stress influences carbon allocation between processes of primary and secondary metabolisms and to what extent allocation into secondary metabolism protects plants from drought.

The rainforest mesocosm in Biosphere 2, University of Arizona, provides a unique system for ecosystem manipulation studies, where we will implement a drought stress experiment, excluding rainfall for two months. To investigate changes in carbon allocation, we will perform labelling experiments with position-specific ¹³C-labelled pyruvate with several tropical tree and shrub species before and during the drought period. We will use δ¹³CO₂ laser spectroscopy and high-sensitivity proton-transfer-reaction time-of-flight mass spectrometry with position-specific ¹³C-metabolite labelling to enable real-time analysis to trace metabolic pathways and carbon turnover, using leaf-chambers to quantify fluxes.

Considering our preliminary experiments, we expect net CO₂ assimilation to strongly decline under rain exclusion, due to stomatal closure. The response of VOC emissions, however, is more complex and fluxes might increase or decrease and are expected to change VOC emission patterns. We will present detailed data about [1-13C]- and [2-13C]-pyruvate allocation within primary and secondary metabolism, such as decarboxylation processes and VOC-production. Using this innovative approach, we aim to strengthen our understanding of the regulation of plant carbon metabolism and VOC biosynthesis under ongoing climate change.