Understanding drought induced responses in leaf and root CO2 and VOC fluxes through position specific isotope labelling

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 extreme external conditions. Plants produce a wide spectrum of volatile organic compounds (VOCs) to cope with oxidative and thermal stress. The distribution and amount of VOC production thereby vary greatly not only among species but also organs, such as leaves and roots. Within the framework of our large-scale ecosystem manipulation experiment, Biosphere 2 Water, Atmosphere, and Life Dynamics (B2-WALD), we aimed to produce deeper insights into carbon partitioning between primary and secondary metabolism under drought stress, notably into CO₂ and VOCs.

In particular, we investigated how drought stress influences organ-specific carbon allocation between processes of primary and secondary metabolisms and to what extent allocation into secondary metabolism protects plants from drought. The tropical rainforest mesocosm in Biosphere 2, University of Arizona, provides a unique system for ecosystem manipulation studies. We implemented a drought stress experiment, excluding rainfall for two months. To investigate changes in carbon allocation, we performed labelling experiments with position-specific ¹³C-labelled pyruvate on leaves and roots of several tropical tree and shrub species before and during the drought period. We used ¹³CO₂ laser spectroscopy and high-sensitivity proton-transfer-reaction time-of-flight mass spectrometry to enable real-time analysis of metabolic pathways and carbon turnover, using leaf- and root-chambers to quantify fluxes.

Considering our preliminary results, net CO₂ assimilation strongly declined under rain exclusion, due to stomatal closure. Consequently, respiration rates declined strongly in leaves as well as in roots. The response of VOC emissions, however, varied among organs. In leaves, we found that the emission of some VOCs declined under drought stress (acetone, monoterpenes), while other fluxes increased or stayed the same (isoprene). We will present detailed data on [1-13C]- and [2-13C]-pyruvate allocation within primary and secondary metabolism, such as decarboxylation processes and VOC-production. To our knowledge, this is the first time that real-time measurements of ¹³C-labelled root VOC-emissions were conducted, enabling this comparative analysis of drought induced stress effects on leaf- and root-emissions.