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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.

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Controlled environment experiments were conducted to study the influence of four night/day temperature regimes; 17.8/25.6, 7.2/18.3, 4.4/15.6 and 2.8/13.9°C on the quality of tomato fruits, Lycopersicon esculentum Mill. c.v. Early Red Chief, measured objectively at three harvest dates. Temperature effects on vegetative and reproductive growth and fruit cracking were also examined. In addition, the stability of carotenoid pigments of macerated and cubed fruit stored at -20° for 0, 10, 20 and 40 days was studied. Low air temperatures decreased plant growth, caused chlorosis of vegetative growth, and reduced the frequency of fruit cracking but had little effect on fruit weight. Flower formation continued at all temperatures with the exception of the 2.8/13.9 environment while fruit set occurred only at the two highest thermal regimes. Fruits harvested at 17.8/25.6 were considerably lower in total solids, reducing sugars and titratable acidities and had substantially higher pH values than fruit exposed to 7.2/18.3, 4.4/15.6 and 2.8/13.9. Temperature had little or no effect on fruit refractive indices and total pectic substances. The failure of total pectic substances to reflect the apparent firmness differences between treatments indicates that total pectic substances are not a satisfactory index of this quality parameter. Surface and internal lightness and yellowness declined with increasing temperatures and later harvests, while redness values increased. Fruit harvested at 17.8/25.6 attained a full red coloration in 7 days, while those exposed to 7.2/18.3 required about 14 days to reach a comparable level of colour development. Fruit exposed to 2.8/13.9 were of inferior colour as evidenced by high L and bL values and low aL values. The high degree of association between lightness and yellowness values under all treatment conditions suggests that surface colour and, to a lesser extent, internal colour can be adequately specified in terms of a constant and two, rather than three, variables. The high overall correlation coefficient obtained between surface and internal Lb/a ratios immediately indicated the possibility of utilizing surface Lb/a ratios to predict internal colour. Temperature and harvest dates influenced the relationship between internal and surface colour ratios as evidenced by the decrease in correlation coefficients with higher temperatures and later harvests. The effect of decreasing temperatures on tomato colour was found to be largely a function of temperature effects on lycopene synthesis. Colour values showed marked changes as total carotene concentrations increased up to about 55 μg/g fresh weight. Continued increases above this level were not accompanied by parallel changes in surface or internal colour. Temperatures and harvest dates affected all pigment concentrations with the exception of Ƴ-carotene and, for the most part, β-carotene. The temperature regimes ranked in order of decreasing fruit quality were as follows: 7.2/18.3; 17.8/25.6; 4.4/15.6; 2.8/13.9. Although of satisfactory coloration, fruits harvested at 17.8/25.6 were rated below the 7.2/18.3 fruit for reasons of lower dry matter, sugar and acid contents. Storage duration had little effect on carotenoid concentrations of cubed samples. In macerated samples, phytoene, phytofluene and ζ-carotene concentrations decreased with storage time. When fresh samples were analysed, all pigment concentrations with the exception of lycopene were found to be much lower in macerated than in cubed samples. / Land and Food Systems, Faculty of / Graduate

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