This study was aimed at determining the expression and activity of polyphenoloxidase (PPO) during apricot fruit development together with the biochemical characteristics of the enzyme extract at different development stages. Biochemical factors considered include substrate, pH, NaCl level, inhibitor type, high temperature inactivation and sulphur dioxide level. Changes in apricot (Prunus armeniaca L., cv. 'Moorpark') polyphenoloxidase (PPO) were measured during fruit development from a few days after full bloom until over-ripe at 92 days after full bloom. Cold ground amples in McIllvaine's buffer were analysed for PO activity over a range of pH (5.0, 6.0, 6.8 and 7.2); for response to intact fruit sample pre-heating (25, 35, 45, 55 and 65 °C); for sulphite and NaCl inhibition (0.2, 0.5, 2 and 5mM) and other inhibitors (SHAM 0.2mM, cinnamic acid 2.5mM and tropolone 0.5mM). PPO activity was measured at 25°C using a Clark-type oxygen electrode with 4-methyl catechol (20mM) as substrate. As fruit ripened PPO activity increased under all conditions tested. The increase in activity was not even with fruit development. Three common peaks of PPO activity occurred at ages 22-29 days, 57 days and for fully-ripe fruit at 85-92 days. Optimum pH was found to be 6.8 with a wide range for all ages of fruit. PPO activity tended to be higher for more mature fruit at a higher pH of 7.2 to 8.0, whereas activity tended to be higher in less developed fruit at the lower pH of 6.0. Catechol and chlorogenic acid showed reduced PPO activity compared with 4-methyl catechol over all development ages, however, there was a different pattern of response. Both catechol and chlorogenic acid showed greater PPO activity in the fully mature, day 92 fruit and less in the very young day 8 fruit, relative to the control 4-methyl catechol substrate. L-DOPA, as a substrate, showed a reaction lag as previously reported, and quite depressed PPO activity with no particular variation with development age compared to the control. Pre-heating of fruit samples in air for 30 minutes resulted in increased inactivation with holding temperature (35°C - 31%, 45°C - 82%, 55°C - 97%, 65°C - 99%). Sulphite and NaCl acted as inhibitors with increasing effect as concentration increased. Added sulphite depressed PPO activity by about 30% at the level (2mM) used. This was less than the literature would suggest and it appeared that fully-ripe fruit were less inhibited than mature, non-ripe fruit. NaCl has a greater inhibitory effect on apricot PPO activity at the lower pH 5.0 tested. As NaCl added increases PPO activity decreases after an initial small rise. Again, less sensitivity to NaCl inhibition is shown by fruit of greater development age. Sensitivity to inhibition by SHAM, cinnamic acid and tropolone decreased with development age. Tropolone was the most effective inhibitor of apricot PPO. The pattern of change in PPO activity, was consistent with physiological and biochemical changes reported by other workers as fruit develop from hard, green to soft, ripe. Regarding the existence of different PPO isozymes during development, no evidence of a isozyme based differential response with age was found within the constraints of the parameters tested. / Thesis (M.App.Sc.)--School of Agriculture & Wine, 2002.