Canopy position and heat treatments influence gamma-irradiation-induced changes in phenylpropanoid metabolism in grapefruit

Irradiation is being evaluated as a quarantine treatment of grapefruit (Cit rus paradisi Macf. 'Marsh'), but it can cause damage to the fruit. Research was conducted to determine if preirradiation heat treatments would improve fruit; tolerance to irradiation as they improve tolerance to low temperatu re injury and to determine if canopy position influenced fruit tolerance to irradiation. Initially, grapefruit were irradiated at 0 or 2.0 kGy at a do se rate of 0.14 kGy . min(-1) and selected biochemical changes were monitor ed over time. There was a marked increase in phenylalanine ammonia-lyase (P AL) activity following irradiation. Maximum activity (approximate to 18-fol d increase) was attained 24 hours after irradiation. Subsequently grapefrui t mere harvested from interior and exterior canopy positions and irradiated at 0 or 1.0 kGy at a dose rate of 0.15 kGy . min(-1) before storage for 4 weeks at 10 degrees C, Following storage, pitting of flavedo was the most e vident condition defect noted as a result of irradiation. Fitting was obser ved on 27% and 15% of irradiated exterior and interior canopy fruit, respec tively, whereas there was no pitting on nonirradiated fruit, Heat treatment before irradiation decreased susceptibility. of fruit to damage. Fitting w as 26%, 19%,and 17% when fruit were held 2 hours at 20 (ambient), 38 or 42 degrees C respectively. Irradiation-induced PAL activity was reduced by tem perature conditioning at 38 or 42 degrees C, Exterior canopy fruit flavedo contained higher levels of total phenols, including flavanols and coumarins compared with interior canopy fruit. Deposition of lignin was not related to canopy position. Neither irradiation nor heat treatment had an effect on total phenols or lignin deposition, Generally, cholesterol, campesterol, s tigmasterol, beta-sitosterol, and isofucosterol mere found to be higher in four steryl lipid fractions in exterior canopy fruit compared with interior canopy fruit,Irradiation increased campesterol in the free sterol and ster yl glycoside fractions and decreased isofucosterol in the free sterol fract ion. Heat treatments had no effect on individual sterol levels. It seems th at irradiation causes a stress condition in the fruit, which leads to pitti ng of peel tissue. Heat treatment before irradiation reduced damaging effec ts of irradiation.