FACTORS AFFECTING UV-INDUCED RESISTANCE IN GRAPEFRUIT AGAINST THE GREEN MOLD DECAY CAUSED BY PENICILLIUM-DIGITATUM

Exposure of harvested grapefruit to ultraviolet (UV) light induced resistance against the green mould decay caused by Penicillium digitatum. Grapefruit picked at various times during the harvest season responded differently to UV treatments. The UV dose required for development of maximum resistance increased as the season progressed. The initial UV dose required in November-picked fruit for maximum response was 4.8 kJ/m2. It declined to 1.6 and 3.2 kJ/m2 in December- and January-picked fruit, respectively, and increased to 8 kJ/m2 in February-picked fruit. Correspondingly, the minimum percentage infection developing after UV treatment increased throughout the season from 0 to 35%. Resistance in UV-treated fruit developed to its maximum extent at 24-48 h following exposure to UV light and then decreased. Development of induced resistance in grapefruit peel was affected by the temperature at which the fruit was stored 24 h after UV treatment and before infection with P. digitatum. In the UV-treated fruit, the fungus developed a sporadic mycelium with marked inhibition of sporulation. The activity of phenylalanine ammonia-lyase and peroxidase markedly increased in the peel following exposure of the fruit to UV light.