Foliar spraying of selenium in inorganic and organic forms stimulates plant growth and secondary metabolism of sage (Salvia officinalis L.) through alterations in photosynthesis and primary metabolism

•Selenium (Se) treatment stimulates plant growth and secondary metabolism of sage.•Organic forms of Se boost photosynthesis, amino acid and carbohydrate metabolism.•Inorganic forms of Se intensify carbon flux from primary to secondary metabolism. The selenium application to plants resulted in increased resistance to biotic and abiotic stress as well as in improvement of nutritional quality of edible parts. The current study was aimed to compare the effects of different selenium forms (selenite, selenate, selenocystine, and selenomethionine) on primary and secondary metabolism in sage (Salvia officinalis L.). In Se-treated plants, the enhancement of growth traits, photosynthetic activity, carbohydrate and protein content were recorded, especially under selenocystine treatment, and of selenium concentration in sage leaves, particularly upon selenate application. Selenate and selenocystine had a greater stimulating effect on both ascorbic acid and phenolic compounds compared to selenite and selenomethionine. Through high performance liquid chromatography (HPLC), it arose that the organic forms of selenium showed a higher stimulating effect on the content of caffeic acid and apigenin derivatives, while the inorganic forms had a greater impact on the content of rosmarinic and ferulic acids. Compared to organic forms, selenite and selenate exerted a stronger phenylalanine ammonia-lyase stimulation (2.3–2.8 times vs. 1.2–1.7 times). In sage treated with selenium, the content of ascorbic acid and phenolic compounds positively correlated with the antioxidant activity. The organic forms of selenium stimulated plant growth and the accumulation of secondary metabolites through the intensification of primary metabolism (photosynthesis, amino acid and carbohydrate metabolism). The inorganic forms of selenium elicited the activation of phenylalanine ammonia-lyase, intensifying carbon flux from primary to secondary metabolism.