Storage Temperature and Fruit Calcium Alter the Sequence of Ripening Events of ‘d’Anjou’ Pears

Pear trees (Pyrus communis L.), cv. d’Anjou, received foliar applications of X-77 surfactant and 32.3 mM CaCl2 at 55, 85, 125, and 137 days after full bloom (DAFB) and fruit were harvested at 147 DAFB. Samples of fruit were stored in air either at 20 °C continuously or at 5 or 10 °C for several periods, then transferred to 20 °C, to determine the effects of storage temperature and CaCl2 treatments on the development of the ethylene climacteric and flesh firmness loss. Control fruits held continuously at 20 °C required 70 days for the onset of climacteric ethylene production, which commenced when firmness had decreased to ≈20 N. Calcium-sprayed fruit required 80 days at 20 °C before the rise in ethylene and resisted softening for ≈50 days. Regardless of calcium treatment, pears stored at 5 or 10 °C required only 40 days to produce climacteric ethylene; fruit softening and internal ethylene concentration after storage at 10 °C were intermediate between those of fruits stored at 5 and 20 °C. Calcium application did not alter the sequence of ripening events. ethylene concentration (IEC) was measured by extracting a 1-mL sample from waterimmersed fruit. Ethylene was determined with a flame ionization gas chromatograph (Carle Model 311; Carle instruments, Fullerton, Calif.) equipped with a 2-m-long × 3-mm-diameter column packed with 80/100 mesh activated alumina. While threshold ethylene may be in the range of 0.05 to 0.5 μL·L (Wang et al., 1972), for comparative purposes an internal ethylene concentration of 1 μL·L was chosen as being above the concentration having threshold physiological activity (Gerasopoulos and Richardson, 1996). Flesh firmness was measured on the same five fruits with a force gauge (Hunter model LKG-1; Western Industrial Supply, San Jose, Calif.) equipped with an 8-mm tip in a Univ. of California– Davis penetrometer, using two pared surfaces on opposite sides of the fruit at the equator. Calcium determinations. At the termination of each ripening experiment, all five fruit groups were washed and calcium was extracted by the method of Perring (1968), as modified by Richardson and Al-Ani (1982). Ten grams of a 1 fruit : 1 water homogenate were transferred into a serum-test tube; 10 mL of concentrated HCl (37.8% A.R.) was added and the tube was capped. The suspension was boiled for 20 min in a water bath, cooled, then filtered through Whatman 41 low-ash paper. Strontium chloride was added to all extracts to a final concentration of 3% to reduce atomic absorption mineral interference. The filtrate was diluted to 25 mL with distilled water, and calcium was determined with an atomic absorption spectrophotometer (Perkin Elmer Model 303; Perkin Elmer, San Jose, Calif.) calibrated against CaCl2 standards in acid. Statistical analysis. For comparisons between treatments during poststorage ripening, LSDs were calculated with the SAS (SAS Institute, Cary, N.C.) statistical computer software. For comparing firmness and internal ethylene, LSDs were obtained by application of factorial analysis. Results and Discussion The average fruit calcium concentrations (fresh fruit basis) were 56 and 66 μg·g in the controls and the calcium-treated pears, respectively (LSD0.05 = 9.2). At harvest IEC was 0.16 μL·L for the control fruit and 0.08 μL·L in calcium-sprayed pears (LSD0.05 = 0.12). Flesh firmness was 67.3 and 71.7 N (LSD0.05 = 6.2) for control and calcium treated-pears, respectively. Preharvest CaCl2 sprays have been reported to increase ‘d’Anjou’ pear Ca by about 10 μg·g –1 and to improve firmness retention at harvest by about 5 N (Gerasopoulos and Richardson, 1996; Richardson and Lombard, 1979). Storage of ‘d’Anjou’ pears at –1 °C maintains firmness at about harvest levels (maximum loss of 5%) while stimulating ACC oxidase activity, leading to accumulation of ACC (Blankenship and Richardson, 1985; Gerasopoulos and Richardson, 1997d) and increasing mRNA expression of genes complementary to clones associated with ethylene biosynthetic enzymes (Cregoe et al., 1993). Thereafter, Fruit of ‘d’Anjou’, a winter pear variety, require ≈60 d at –1 °C to gain the ability to produce ethylene and ripen upon transfer to room temperature (Chen et al., 1982). The duration of chilling temperatures required to induce ripening and the sequence of ripening events vary, however, depending on storage temperature and fruit calcium concentration, and the pears lose firmness more rapidly as storage temperature increases. At high temperatures (5 to 20 °C), firmness loss can take place before the climacteric rise in CO2 (Porritt, 1964) or ethylene (Gerasopoulos and Richardson, 1997d). Thus, ‘d’Anjou’ pears stored at 20 °C reach complete ripening capacity within 50 to 60 d (Gerasopoulos and Richardson, 1997d). Calcium is applied to prevent physiological disorders and delay ripening in avocado (Tingwa and Young, 1974), apples, pears (Richardson and Lombard, 1979), small fruit (Ali et al., 1994), and kiwifruit (Gerasopoulos et al., 1994). When ‘d’Anjou’ pears were treated with Ca, they required longer storage at –1 °C in order to initiate autocatalytic ethylene production and ripening than did control fruit (Richardson and Lombard, 1979; Vaz, 1984). Although winter pears require chilling to ripen normally, treating immature or mature ‘d’Anjou’ pears with ethylene can induce softening and ripening (Wang et al., 1972). Received for publication 28 Oct. 1987. Accepted for publication 13 Aug. 1998. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. The purpose of this study was to determine the effects of storage temperature and preharvest calcium application on the ripening capability of ‘d’Anjou’ pear, as measured by climacteric ethylene production and firmness loss. Materials and Methods General procedures. Two groups of 10 mature ‘d’Anjou’ pear trees at the MidColumbia Experiment Station, Hood River, Ore., were sprayed with 32.3 mM CaCl2 plus X-77 surfactant or surfactant only (control) 55, 85, 125, and 137 DAFB. The fruit were harvested at 147 DAFB and placed directly into perforated polyethylene bag liners (folded over) in cartons (30 × 50 × 30 cm) holding ≈20 kg of fruit each, and stored in air at 5 or 10 °C. Additional fruit were placed in 20-L jars and stored at 20 °C for the measurement of internal fruit ethylene and flesh firmness. Fruits were not treated with fungicide after harvest to allow evaluation of possible effects on storage rots; however, they were screened for cork spot incidence throughout the experiment. Only sound fruit were used. Fruits were stored at 5 or 10 °C for 25, 40, and 55 d and then placed in 20-L jars at 20 °C to ripen for 11 d for the measurement of internal fruit ethylene and flesh firmness. All jars were ventilated with humidified air at 1 L·min; CO2 accumulation (as monitored by infrared analysis) did not exceed 0.5%. Ripening characteristic determinations. Five fruit per treatment were taken from the 20-L jars every other day and the internal