γ-Aminobutyric acid (GABA) metabolism in CO2 treated tomatoes

γ-Aminobutyric acid (GABA) metabolism has been investigated in breaker and light red maturation stages of ‘Tricia’ tomatoes exposed to air, or 10% CO2 in air during storage, at 13 °C for 12 d, to examine whether GABA responses to elevated CO2 differ in fruit at different maturity stages. At harvest, the GABA concentration in red fruit was about a third of that in breaker fruit. The ripening of breaker stage fruit was inhibited by CO2 to a greater extent than the red fruit as judged by hue and chroma values, but the red fruit were more sensitive to development of water soaking, pitting and decay than the breaker fruit. During storage, GABA concentrations in breaker fruit decreased during storage but were usually higher in CO2 treated than air treated fruit. GABA concentrations in red fruit increased in CO2 treated fruit, but they increased and then decreased in air treated fruit over time. When CO2 treated fruit were transferred to air, GABA concentrations declined to similar levels as those found in fruit stored in air. Although glutamate decarboxylase (GAD) activity was higher in breaker than red fruit at harvest, activities in air and CO2 treated fruit were similar. GABA transaminase (GABA-T) activity was higher in red than breaker fruit, but activity declined more in CO2 than air treated fruit. Although, CO2 treatment did not affect GAD1 expression, it resulted in higher GAD2 and GAD3 expression, and to a greater extent in breaker than red fruit. No consistent effects of high CO2 conditions on the expression of GABA-T or succinic semialdehyde dehydrogenase (SSADH) genes were found. A decline in succinic semialdehyde reductase 1 (SSR1) gene expression was found only in CO2 treated red fruit, while CO2 treatment resulted in a more rapid decrease of SSR2 expression in both breaker and red fruit. The results reveal differences in GABA metabolism in breaker and red fruit in response to CO2 treatment.