Diversity and interrelations among the constitutive BVOC emission blends and changes during salt and drought stress of four broad-leaved tree species at seedling stage

Biogenic volatile organic compounds (BVOCs) emitted by plants consist of a broad range of gasses which serve purposes such as protecting against herbivores, communicating with insects and neighboring plants, or increasing the tolerance to environmental stresses. Evidence is accumulating that the composition of BVOC blends plays an important role in fulfilling these purposes. Constitutional emissions give insight into species-specific stress tolerance potentials and are an important first step in linking metabolism and function of co-occurring BVOCs. Here, we investigate the blend composition and interrelations among co-emitted BVOCs in unstressed and drought- and salt stressed seedlings of four broad-leaved tree species, Quercus robur, Fagus sylvatica, Betula pendula, and Carpinus betulus. BVOCs of Q. robur and F. sylvatica were mainly isoprene and monoterpenes, respectively. B. pendula had relatively high sesquiterpene emission; however, it made up only 1.7% of its total emissions while the VOC spectrum was dominated by methanol (∼72%). C. betulus was emitting methanol and monoterpenes in similar amounts compared to other species, casting doubt on its frequent classification as a close-to-zero VOC emitter. Under drought and salt stress the main emitted BVOCs of F. sylvatica and B. pendula slightly decreased, whereas an increase was observed for Q. robur and C. betulus. Beside these major BVOCs, a total of 22 BVOCs could be identified, with emission rates and blend compositions varying drastically between species and treatments. Principal component analyses among species and treatments revealed co-release of multiple compounds. In particular, new links between pathways and catabolites were indicated, e.g., correlated emission rates of methanol, sesquiterpenes (MVA pathway), and green leaf volatiles (hexanal, hexenyl acetate, and hexenal; LOX pathway) during unstressed conditions. Drought stress led to a decrease of all BVOC emissions except for a slight increase of isoprene emissions of Q. robur, which might be due to decoupling from the photosynthesis and led to emptying C storages. Hexenyl acetate (LOX) follows the same pattern as isoprene but might have decreased due to a long droughting period. Salt stress led to an increase of LOX-related BVOCs, and acetaldehyde which supports the hypothesis that acetaldehyde emissions are linked to the oxidation of C₁₈ fatty acids of cell membranes. Our results thus indicate that certain BVOC emissions are highly interrelated, pointing toward the importance to improve our understanding of BVOC blends rather than targeting dominant BVOCs only.