Ozone-induced hormonal effects underpin germination dynamics in temperate tree species

Tropospheric ozone is a secondary air pollutant that negatively affects plant growth, but its impact on seed germination remains poorly understood despite the occurrence of ozone episodes in early spring. This study investigated the direct effects of ozone exposure on non-dormant seed germination, together with the impact on the key phytohormones that govern germination. We exposed seeds of five widely distributed temperate tree species (Pinus sylvestris, Betula pendula, Alnus glutinosa, Acer pseudoplatanus, and Picea sitchensis) to elevated (+ 60 ppb) or ambient atmospheric ozone concentrations over 27-day germination periods. Elevated ozone led to a significant delay and reduction in germination (by 17%) in P. sylvestris, with the same but weaker trends in B. pendula and P. sitchensis, and no detectable effects in A. glutinosa or A. pseudoplatanus. At the end of the trial, ozone altered the balance of the key germination-regulating hormone abscisic acid (ABA; 1.5-fold change) in ungerminated P. sylvestris seeds, as well as the levels of hormones that influence ABA transcription (auxin; 9-fold change) and plant responses to abiotic stress (jasmonic acid; 3-fold change; salicylic acid; 2-fold change). These phytohormonal effects provide a suggested mechanism through which ozone can induce changes in seed germination, with species sensitivity likely depending on oxidant uptake, antioxidant capacity and physical seed characteristics. Our results underscore ozone's potential role as a selective pressure by differentially affecting seed germination dynamics.