Same, same, but different: Drought and salinity affect BVOC emission rate and alter blend composition of urban trees

Drought and salt are key abiotic stressors in temperate cities. Biogenic volatile organic compound (BVOC) emissions, often dominated by trees, influence the urban troposphere and mediate plant-plant and plant-insect interactions. While knowledge on constitutive BVOC emissions is increasing, modulation of blends by stress has yet received little attention. We thus investigated BVOC blends and herbivore-related subsets ('bouquets') of Quercus robur, Fagus sylvatica, Betula pendula and Carpinus betulus seedlings under control, and after 2-weeks of drought and salt stress using PTR-Tof-MS. 22 BVOCs were related to metabolic pathways, and changes among blends and bouquets were assessed. Drought led to a slight increase of isoprene and monoterpene emissions from Q. robur and F. sylvatica, respectively, while total non-isoprene emissions generally declined under water stress. Both drought and salinity led to distinct, species-specific changes in the emission rates of single BVOCs, and resulted in markedly different herbivore-related bouquets in F. sylvatica and C. betulus. Oxygenated VOCs and green leaf volatiles in particular increased in these two salt-sensitive species, while pathway-specific effects were less clear under drought. Findings indicate significant consequences of common urban stressors on BVOC emission spectra, including tropospheric ozone formation and severely hampered plant communication cues under stress.