Radiation damage to grapefruit in relation to ethylene production and phenylalanine ammonia-lyase activity

Irradiation of grapefruit with 200 krads of gamma rays enhanced ethylene production which attained a peak 12 hr after treatment. Subsequently ethylene production rapidly decreased either to control values (fruit at early maturity), or to about 50 per cent of maximum (fruit fully ripe). Phenylalanine ammonia-lyase (PAL) activity curves followed the same patterns, but with a certain time lag. With early fruit no damage ensued; with fully ripe fruit pitting of peel occurred. Dose effects studies on fully ripe fruits, 22 and 120 hr after irradiation, showed that effects of relatively low doses (25-100 krads) resembled the effects of 200 krads on early fruits (a decrease of ethylene and PAL to control level, and no damage). At 400 krads ethylene production and PAL activity increased, resulting in a severe peel damage.