Fermentation and malate metabolism in response to elevated CO₂ concentrations in two strawberry cultivars

Concentrations of acetaldehyde, ethanol, ethyl acetate (EA), organic acids and activities and gene expression of alcohol dehydrogenase (ADH; EC 1.1.1.1), pyruvate decarboxylase (PDC; EC 4.1.1.1), alcohol acyltransferase (AAT; EC 1.4.1.14), malate dehydrogenase (MDH; EC 1.1.1.37), malic enzyme (ME; EC 1.1.1.40) and glutamate dehydrogenase (EC 1.4.1.14) were investigated in two strawberry (Fragaria x ananassa Duch) cultivars with different responses to CO₂ during storage. 'Jewel' fruit treated with CO₂ accumulated acetaldehyde and ethanol but little EA, while 'Cavendish' accumulated little acetaldehyde or ethanol but accumulated EA. In CO₂-treated fruit, PDC activity was positively correlated with EA accumulation in 'Jewel' but not in 'Cavendish', while no differential effect of atmosphere was observed on its gene expression. ADH activity and gene expression show a correlation with ethanol accumulation in 'Cavendish'. In 'Jewel', there was a positive correlation between ADH gene expression and enzyme activity; however, this correlation does not explain ethanol accumulation in this cultivar. EA accumulation did not show any correlation with AAT activity and gene expression in any of the cultivars. Succinate concentrations were highest and those of malate lowest in CO₂-treated fruit of both cultivars, but MDH and ME activities were not affected by CO₂. Gene expression of MDH and ME were not affected by atmosphere in 'Cavendish', although in 'Jewel' the MDH expression was slightly lower in CO₂- than air-treated fruit. The results of this study show that differences in fermentation products and malate accumulation in CO₂-treated strawberry fruit are not consistently correlated with enzyme activities and gene expression.