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Stress physiology of pedicel browning in sweet cherries Schick, Joanne Louise

Abstract

The overall objective of this research was to investigate the influence of some physiological stress factors on postharvest pedicel browning in sweet cherry (Prunus avium L.). While the research focused on examining the effects of stresses on stem quality; decay, pitting and soluble solids content (SSC) in the fruit were also monitored. Because PPO is a major enzyme involved in browning, PPO activity under the influence of some stress factors was also examined. Recommendations aimed at improving commercial sweet cherry quality were made as a result of this work. The use of a reflective tarp was assessed for its potential to improve quality retention of sweet cherries. The tarp was applied as cover to bins of harvested cherries in the orchard and also during open-truck transport to the packing house. Controls were bins that were left uncovered in the orchard and during transport. The results show that reflective tarps, applied with a white painted side facing the sun and the shiny metallic surface next to the cherries, reduced pedicel browning, fruit decay and pitting, when cherries were stored in modified atmosphere packages (MAP) for several weeks. Reflective tarps helped to retain cherry pedicel quality through modification of temperature and relative humidity (RH). Fruit that was covered remained up to 5 °C cooler than fruit that was uncovered and the RH in the immediate air space surrounding the fruit was increased by 15-20%. Under these modified conditions the pedicels of covered fruit retained more water than those that were uncovered. UV-absorbing substances leaking from pedicel tissue was measured. From this inferences were made regarding damage to pedicel cellular membranes. RLR always negatively correlated with pedicel browning (R = -0.92, P ≤ 0.01). This indicated that browning was related to some structural damage to the plant tissue. The effects of temperature, RH and light on pedicel browning were examined separately in several laboratory experiments. RH had the single largest effect on pedicel browning. As RH decreased, browning increased. Higher temperatures also resulted in more pedicel browning. Radiation from metal halide lamps and fluorescent bulbs had no effect on pedicel browning. The effect of postharvest handling at 0 °C, 10 °C, 20 °C and 30 °C on 'Van', 'Bing' and 'Lapins' cherry pedicels was investigated at two stages of maturity. 'Van'pedicels were more resistant to browning than 'Bing' or 'Lapins' pedicels regardless of temperature or maturity. 'Lapins' pedicels showed the highest amount of bruising. Less browning occurred in more mature 'Lapins' and 'Bing'pedicels than in less mature 'Lapins' and 'Bing' pedicels. Cherries that are harvested too early may be more susceptible to handling injury. Cherry pedicels were most susceptible to bruising at 30 °C and least susceptible at 10 °C. These results suggest that optimum harvest time for 'Lapins' and 'Bing' for pedicel quality was at the mahogany maturity stage and the best handling temperature was 10 °C. Preharvest water stress had no effect on pedicel browning. Cherry trees were stressed by withholding water for four weeks and two weeks before harvest. Controls were cherries from trees that were watered weekly. Overall soil moisture content and pressure bomb readings indicated that the water stressed trees were without sufficient irrigation. Withholding water for up to four weeks before harvest did not result in higher rates of pedicel browning. Some properties of cherry pedicel polyphenol oxidase (PPO) were examined by kinetic analysis and electrophoresis. PPO activity did not correlate with pedicel browning. The optimum pH for PPO activity was 5.5. Two PPO isozymes were isolated from 'Lambert' and 'Lapins' pedicels by SDS-PAGE on 4-15% gradient gels under partially denaturing conditions. A smaller, more intense band with an apparent molecular weight of 43.5 kD and a less intense larger band with apparent molecular weight of 37.5 kD, were isolated from both brown and green 'Lambert' and 'Lapins' pedicels. More PPO was expressed in the pedicels of 'Lapins' than those of 'Lambert'.

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