Identification of extremely low-terpene-emitting variants from an urban greening tree species Quercus myrsinifolia

Terpene-emitting plants may risk air quality and agricultural yields by increasing tropospheric ozone with nitrogen oxides. However, the number of non- or low-terpene-emitting species is limited, leading to a potential trade-off in species diversity when air quality considerations are prioritized for afforestation and urban greening. To expand the available choices, developing non- or extremely low-terpene-emitting cultivars from species with high-terpene emissions could be a feasible solution. In this study, we observed terpene emissions from multiple individuals of a common afforestation tree in Asia with high monoterpene emission rates, Quercus myrsinifolia, to identify natural low-monoterpene-emitting variants. A screening result showed that the total monoterpene emission rate at temperature of 30 °C and photosynthetic photon flux density of 1000 μmol m^-2 s^-1 among the 47 individuals ranged from no detection to 8.8 nmol m^-2 s^-1 in summer, where two out of them showed no detectable monoterpene emissions. With additional validation over months using the leaf cuvette method, extremely low monoterpene emissions which were solely composed of β-ocimene were detected for the two candidates during a warm month, and no monoterpenes were detected in the later months. Concurrently, net photosynthetic rates of the two candidate individuals were comparable to those of other individuals. Subsequent observations of the clones propagated from these variants detected no monoterpene emissions. These findings indicated that their extremely low-monoterpene-emitting trait is reproducible and attributed to their genotype. These variants are expected to be the foundation for establishing a low-air-quality-impacting cultivar for afforestation and urban greening.