Internal Ethylene Levels during Ripening and Climacteric in Anjou Pears

Previous investigations have shown that ripening of Anjou pears (Pyrus communis L.) was initiated by applied ethylene separately and independently of the respiratory climacteric (12). Softening occurred prior to or without the development of the climacteric (14). It has been reported that a critical triggering concentration of ethylene is required to produce a response (3). A small peak of ethylene production was related to the initiation of softening and a larger increase associated with the climacteric (14). However, analysis of the ethylene evolution alone does not provide an adequate test for the physiologically active concentration of ethylene dissolved or adsorbed within the tissue (7). Therefore, the status of internal ethylene in Anjou pears during ripening and climacteric needs to be determined. This study was undertaken to determine if a lower level of endogenous ethylene is required for softening than for the respiration climacteric. Unlike cantaloupe (7) or honeydew melons (9), Anjou pears do not have a central cavity from which the direct measurement of the ethylene concentration in the internal atmosphere can be made. Although a 1-ml sample of air has been obtained from the core of a pear for the measurement of internal CO2 and 02 (15), a larger sample is required to detect minute amounts of ethylene present. While several methods (1, 2, 11) have been described for extracting internal ethylene from plant tissues where gas samples cannot be readily obtained with a syringe, the procedure described by Blanpied (2) was followed in this experiment using a vacuum of 36 mm. Beyer and Morgan (1) reported that using a vacuum below 100 mm Hg may induce the release of bound or dissolved ethylene from plant tissues. Therefore, the absolute values of internal ethylene may vary as different amounts of vacuum are applied in the extraction procedure. The main objective of this experiment was to compare the relative ethylene levels during the initiation of softening and onset of climacteric. In order to avoid the wounding effect (8) on ethylene synthesis, whole fruits rather than slices were used. Since the opening of a 4-liter suction flask described by Blanpied was too small for a mature pear, a 250-mm desiccator with tubulated cover was used. Sampling techniques for ethylene analysis, respiration, and fruit firmness have been described previously (14). Anjou pears do not ripen evenly without exposure to either ethylene or cold storage treatment (6). Fruits in this experiment were picked at desired maturities (85% and 100% based on a postbloom period of 147 days) and stored at -1.1 C for 60