Sex-specific ozone stress responses of poplar: Mechanisms of enhanced tolerance of males

Uncovering whether the ozone (O₃)-sensitivity differs between sexes in Populus deltoides and if so, what are the mechanisms underlying the different sensitivities is vital for understanding plant-adaptation-strategy in O₃ polluted areas. We exposed female and male saplings to 80 nmol mol^-1 O₃ for 14 days, measured the growth, structural and physiological characteristics, metabolite accumulations, and gene transcription levels, to test the hypothesis that the enhanced resistance in males is associated with their traits detoxifying and reducing O₃ entry into the cells. In general, females showed more severe visible injury, larger reductions in leaf biomass, chlorophyll content, and photosynthetic characteristics than males. The emission of isoprene and its synthase gene expression were inhibited by O₃ in both sexes with less reductions in males than females. The up-regulated differentially expressed genes in males under O₃ stress were mainly enriched in phenylpropanoid biosynthesis and glutathione metabolism pathways, while in females they were primarily enriched in the flavonoid biosynthesis pathway. Accordingly, males accumulated more lignin, lignans, and coumarins, while females accumulated more flavonoids. Overall, the stronger tolerance to O₃ in males than females was possibly related to their combined up-regulation of multiple defense pathways that reduce both the oxidative stress and O₃ permeability into cytosol.