Selenomethionine regulates the sugar-acid ratio of strawberry fruit by modulating the activities of related enzymes

To study the effects of exogenous organic selenium on the sugar-acid ratio (SAR) of strawberry (Fragaria x ananassa) fruit, we investigated the effects of different concentrations of selenomethionine (SeMet) on the activities of key enzymes in sugar and organic acid metabolism in fruit during Large fruit period (LP, 20 d after flowering), Colour changed period (CP, 24 d after flowering) and Mature period (MP, 28 d after flowering) of fruit. The results showed that SeMet increased the contents of total sugars (TS), sucrose (Suc), glucose (Glu) and fructose (Fru) by improving the activities of acid invertase (AI), sucrose synthase (SS) and sucrose phosphate synthase (SPS), and reducing hexokinase (HK) activity. Meanwhile, SeMet markedly decreased the contents of total acids (TA), citric acid (CA) and malic acid (MA) by reducing the activities of NAD-malic dehydrogenase (NAD-MDH), NADP-malic enzyme (NADP-ME) and citrate synthase (CS). By this way, SeMet improved the SAR of fruit. Among different concentrations, 60 mg L−1 SeMet was the best concentration in improving the value of SAR in ripe fruit and could be used in the production of 'Sweet Charlie' strawberry to improve fruit SAR.