Physiological and biochemical changes during postharvest ripening in ‘Docteur Jules Guyot' pear and their correlations

Abstract：【Objective】‘Docteur Jules Guyot', a representative European pear (Pyrus communis L.) cultivar was used in this study to investigate the changes in various physiological and biochemical indexes and analyze their correlations during the postharvest ripening of European pear.【Methods】Loss of fruit firmness is an obvious change in the after ripening of‘Docteur Jules Guyot'pear. Fruit were transported to the laboratory and precooled at (0±0.5) ℃ for 24 h. Uniform sized fruit without lesions were selected as the materials. The precooled fruit were packed with PE bags (the thickness: 0.02 mm) and stored at room temperature (20±1 ℃) . The fruit in the bags were sampled every day until the fruit firm-ness reduced to 2.0 kg·cm-2. Three biological replicates were performed each with 10 fruit. Chromatic aberration, chlorophyll fluorescence, soluble solids, titratable acid content, vitamin C content, pulp firmness, fruit respiration intensity, ethylene release, starch staining, starch content and various enzymes were measured for investigating the physiological and biochemical changes.【Results】During the storage of‘Docteur Jules Guyot'fruit, the skin color changed from green to yellow, with L* value increasing gradually. Photochemical efficiency (Fv/Fm ) showed no significant change in the first 6 d but decreased rapidly later, and the contents of chlorophyll a, chlorophyll b and total chlorophylls decreased with the after-ripening and softening of the fruit. h° value of the‘Docteur Jules Guyot'declined with the prolongation of storage time at room temperature. When the firmness reduced to 1.64 kg · cm-2 (less than 2.0 kg · cm-2) , h° value was 95.50 and the peel turned completely yellow. Chlorophyll fluorescence detection indicated that chlorophylls were activated by light. With the ripening of‘Docteur Jules Guyot'at room temperature, Fv/Fm declined continuously, and the lowest value of 0.3 appeared at the 8 th day. The coordinated changes in chlorophyll content and fruit color suggested that a high content of soluble sugar played an important role in the regulation of chloroplast thylakol disintegration and chlorophyll degradation. In this experiment, the chlorophyll content showed continuous decline with storage time. There was a significant negative linear correlation between starch dyeing index and fruit firmness (r = 0.92**) . In this study, we found that the amylum content decreased with storage time from 44.91 mg · g-1 to 24.89 mg · g-1. Amylase activity increased at the beginning and then decreased, and reached its maximum value of 6.60 mg· g-1· min-1 at the 4 thday. The cellulase activity decreased gradually, and the cellulase activity decreased rapidly from harvest to day 5, after which, the cellulase activity decreased to the lowest value of 0.36 μ g· g-1· h-1 and maintained this low value thereafter. Polygalacturonase (PG) activity was 1.31 μg· g-1·h-1 at the 5 thday, and then decreased to its lowest value of 0.67 μg· g-1·h-1. The content of soluble sugars increased continuously, and in contrast, titratable acid content decreased continuously. Vitamin C content decreased slowly to 2.30 mg · 100 g-1. The values of respiratory intensity and ethylene release both reached their highest values on the 6th day, which were 155.76 μL· kg-1·h-1 and 91.14 mg · kg-1·h-1, respectively.【Conclusion】During the after-ripening of European pear fruit, starch content and firmness of fruit decreased gradually and the skin color changed from green to yellow. Fruit with a firmness of 3.0 kg· cm-2 had the best taste. During the after-ripening and softening of the fruit, the skin color and chlorophyll fluorescence parameters could be used as the standards for judging the fruit status with the best taste. In the case of‘Docteur Jules Guyot', the best eating quality was attained when fruit h° value was below 105, L*value above 70 and Fv/Fm below 0.7.