ETHANOL PRODUCTION AND CHLOROPHYLL FLUORESCENCE IN HEAT-STRESSED APPLE FRUIT DURING STORAGE

Abstract

Ethanol concentration and chlorophyll fluorescence were measured as signs of heat stress in apple fruit [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.]. 'Mclntosh', 'Cortland', 'Jonagold', and 'Northern Spy' apples were held at 46°C for 0, 4, 8, or 12 h. Following treatments, fruit were stored at 0°C in air and evaluated after 0, 4, 8, or 12 weeks. Peel and flesh browning, headspace ethanol concentration, and chlorophyll fluorescence (Fv/Fm) were measured. Peel browning became apparent in all 4 cultivars 4 weeks after treatment and increased during storage. Exposure to 46°C for 12 h caused severe flesh browning in all cultivars. Severity of flesh browning increased with increasing heat treatment time and storage time. Headspace ethanol concentrations immediately following 12 h heat treatments increased and were 48-, 70-, 9-, and 170- fold higher in 'Mclntosh', 'Cortland', 'Jonagold', and 'Northern Spy' apples than in the controls, respectively. Ethanol concentrations also increased in fruit treated for 8 h but not in those treated for 4 h. Ethanol concentrations were maintained or increased during the 12 weeks of storage. Heat treatments reduced chlorophyll fluorescence (Fv/Fm). One day following treatment, fruit exposed to 46°C for 12 h had Fv/Fm values that were 36%, 59%, 63%, and 36% of the control values in 'Mclntosh', 'Cortland', 'Jonagold', and 'Northern Spy', respectively. The 8 h heat treatment also decreased Fv/Fm in treated fruit, but the 4 h treatment had no effect. Fv/Fm decreased in all treatments during storage. Correlations between flesh browning and ethanol concentration ranged from 0.87 to 0.78, and flesh browning and Fv/Fm ranged from -0.91 to -0.67 depending on cultivar. The increase in ethanol production and decrease in chlorophyll fluorescence were apparent before browning was visually apparent and have the potential to predict injury that develops during storage.