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To explore differences in sucrose metabolism between peach fruit subjected to chilling stress (5 8C) and nonchilling stress (10 8C), sucrose concentration as well as the activities and gene expression levels for enzymes associated with sucrose metabolism were compared. Fruits stored at 5 8C accumulated higher concentrations of H2O2 and developed severe chilling injury (CI) compared with fruit kept at 10 8C. Activities and gene expression levels for enzymes related to sucrose metabolism, such as acid invertase (AI), neutral invertase (NI), sucrose synthase (SS), and sucrose phosphate synthase (SPS) were higher in fruit stored at 5 8C than at 10 8C throughout or late in storage. A sharp increase in net sucrose cleavage activity dramatically decreased sucrose concentration and increased reducing sugars at 5 8C. The sucrose concentration at 10 8C increased over the first 21 days and then declined slightly, and was higher than in fruit at 5 8C throughout storage. The increase in net sucrose cleavage activity at 5 8C is contrary to the expectation that biochemical reactions ordinarily proceed more rapidly with increasing temperature. We conclude that chilling stress stimulates the activities and transcription levels of enzymes involved in sucrosemetabolism, resulting in increased sucrose cleavage. Peaches [Prunus persica (L.) Batsch] are one of the most important fruit crops in the world and are priced because of their rich flavor and nutritional value. Sucrose is the major sugar present, contributing around 75% of total sugar in the fruit after harvest (Aubert et al., 2014), and the concentration of sucrose is directly related to fruit quality and taste (Borsani et al., 2009). Sucrose metabolism is the primary system controlling sugar concentration and composition. Sucrose can be converted to glucose and fructose by AI and NI, or to fructose by SS cleavage in the presence of uridine diphosphate (UDP). In the reverse reaction, sucrose can be synthesized from glucose and fructose by SS-synthesis or SPS in the presence of UDP-glucose (UDPG). Sucrose also modulates the response in field and model plants to cold (Uemura et al., 2003), heat (Zhao et al., 2013), drought (Hoffmann, 2010), and anoxia (Lara et al., 2011). Sucrose may protect the cytomembrane by acting as a cytoprotectant, scavenge reactive oxygen species (ROS), act as a signaling molecule, and regulate gene expression to protect plants against stress (Wang et al., 2013). CI is a physiological disorder accompanying cold stress and is a great concern among peach growers. CI appears within 1 or 2 weeks if fruit is stored at 2–5 C, or 3 or more weeks at 0 C (Lurie and Crisosto, 2005), and is characterized by internal browning, juicelessness, mealiness, internal reddening, and other adverse effects (Crisosto and Labavitch, 2002; Lurie and Crisosto, 2005). CI seriously degrades fruit quality and frequently renders fruit not saleable, resulting in economic loss. Researchers have focused considerable effort on understanding the cause of CI and identifying potential methods for reducing it. One important contributing factor may be the accumulation of ROS, which destroy membrane systems and induce CI development (Lukatkin et al., 2012). Yang et al. (2011) found that g-aminobutyric acid enhanced the activities of antioxidant enzymes such as superoxide dismutase, catalase, and ascorbate peroxidase, and were associated with the increased chilling tolerance in peaches. 1-Methylcyclopropene markedly inhibited CI incidence in peaches by increasing the activities of antioxidant enzymes and alleviating oxidative damage (Jin et al., 2011). Some reports have called attention to a possible relationship between sucrose and CI in harvested fruit. In mandarin fruit, sucrose rather than glucose, fructose, or starch appeared to be responsible for heatinduced chilling tolerance during cold stress temperature storage (Holland et al., 2002). High sucrose and glucose concentration alleviated CI symptoms in peach fruit (Abidi et al., 2015), and higher concentration of sucrose in ‘Yulu’ peach fruit may be positively correlated with chilling tolerance (Wang et al., 2013). In other fruit, such as loquat, evidence suggests that reducing sugars, rather than sucrose, are related to CI tolerance (Cao et al., 2013). Although 5 C is commonly used for the storage of fruits and vegetables, it causes severe CI in peach fruit (Lurie and Crisosto, 2005). 0 C storage is better than 5 C for peaches because CI develops more slowly at this temperature (Lurie and Crisosto, 2005). In contrast, 10 C does not cause chilling stress although it is still cold enough for peach storage. In a previous study, we compared soluble carbohydrates in peach fruits stored at 0 and 5 C (Wang et al., 2013). However, sugar metabolism in fruit stored at chilling (5 C) and nonchilling (10 C) stress temperatures has not yet been compared, and the relationship between sugar and chilling resistance is not understood. In this study, we investigated differences in sucrose metabolism, sucrose concentration, and activities and gene expression levels for related enzymes, in peach fruit subjected to chilling (5 C) and nonchilling (10 C) stress for 28 d. Materials and Methods Plant material Peaches [P. persica (L.) Batsch cv. Yulu] were hand harvested at commercial maturity Received for publication 20 July 2015. Accepted for publication 2 Sept. 2015. This study was supported by the Natural Science Foundation of Zhejiang Province (no. LR15C200002), Key Laboratory of Healthy & Intelligent Kitchen System Integration of Zhejiang Province (no. 2014E10014), the National Science Foundation of China (no. 31000825), the Scientific Research Foundation of Graduate School and the K.C. Wong Magna Fund at Ningbo University. Corresponding author. E-mail: shaoxingfeng@ nbu.edu.cn. 1542 HORTSCIENCE VOL. 50(10) OCTOBER 2015 (135 d after full bloom) from a commercial plantation located in Fenghua, Zhejiang Province, China, and immediately transported to the laboratory. Treatments Fruit were precooled to 10 C and manually selected for uniform color, size, and absence of mechanical damage, and then randomly divided into experimental units for either 10 or 5 C experiments. For 10 C storage, three replicates of 150 fruit were stored at 10 C, 95% relative humidity (RH) for up to 28 d. For 5 C storage, three replicates of 150 fruit were stored at 5 C, RH 95% for 28 d. Ten fruit per treatment replicate were sampled after periods of 0, 7, 14, 21, and 28 d of cold storage. CI indexes were assessed immediately after removal from cold storage, and then slices of mesocarp ( 1 cm thick) were frozen in liquid nitrogen and stored at –40 C for further use. The entire experiment was conducted twice (in the 2011 and 2013 harvest seasons). The date from the 2013 harvest season was used in here because the same pheromone was used in the two seasons. Physical and molecular assays CI index. CI index was determined using the following scale (Meng et al., 2009), 0: no signs of internal browning; 1: browning area less than 25%; 2: browning area between 25% and 50%; 3: browning area more than 50%. CI index was calculated as CI index = P [(CI scale) · (number of corresponding fruit at the CI scale)]/(total number of fruit in the treatment group). H2O2 concentration. H2O2 concentration was determined using the method described by Wang et al. (2014). Two grams of flesh tissue were ground with 5 mL chilled 100% acetone, followed by centrifugation at 10,000 gn for 20 min at 4 C. The supernatant was collected and assayed for H2O2 using kits purchased from Nanjing Jiancheng Institute of Bioengineering (Nanjing, Jiangsu, China). Soluble sugar concentrations. Soluble sugars were measured using the method described by Wang et al. (2013) with modifications. Five grams of fruit flesh were ground with 0.5 mL of 300 mM zinc acetate and 0.5 mL of 63 mM potassium ferrocyanide. The homogenate was diluted to 25 mL and passed through 0.22-mm membrane filters. A 20-mL aliquot was injected into a highperformance liquid chromatography system (model 2695; Waters, New York, NY) fitted with an amino column (Kromasil 100A; Sweden) and a refractive index detector (model 2414; Waters) operating at a flow rate of 1.0 mL/min at 35 C, using acetonitrile/water (75:25, v/v) as the mobile phase. Individual sugars were identified and quantified by comparisonwith the retention times and peak areas of pure sugar standards. The results were expressed as milligrams per gram fresh weight (FW). Extraction and assay of sucrosemetabolismrelated enzyme activities. One gram of frozen mesocarp tissuewas homogenizedwith 5mL of 100 mM sodium phosphate buffer (pH = 7.5) containing 5 mM MgCl2, 1 mM EDTA, 2.5 mM DTT, and 0.1% (v/v) Triton X-100. The homogenates were centrifuged at 10,000 gn for 20 min at 4 C, and the supernatants were diluted 1:10 with distilled water before being used for enzyme activity assays. AI activity was assayed in an incubation mixture of 100 mM sodium citrate buffer (pH = 4.5), 1% (m/v) sucrose, and crude enzyme extract. The mixture was incubated at 34 C for 30 min, and then the reaction was stopped by boiling for 5 min. NI activity was measured using a similar protocol, except that the incubation mixture was 100 mM sodium phosphate buffer (pH = 7.5), 1% (m/v) sucrose, and crude enzyme extract (Shao et al., 2013). Glucose produced by these reactions was measured using the 3,5-dinitrosalicylic acid method (Miller, 1959). AI and NI activities were expressed as micromoles glucose per hour per gram FW. SS activity consisted of SS-cleavage activity and SS-synthesis activity. For the SS-cleavage activity assay, the reaction, containing 80 mMHepes-NaOH (pH = 5.5), 5 mM NaF, 100 mM sucrose, 5 mM UDP and the mixture was incubated at 34 C for 30 min, then stopped by boiling for 5 min. The remainder of the assay for SS-cleavage activity was conducted using the same method as for AI. SS-synthesis and SPS activities w

Differences in Sucrose Metabolism in Peach Fruit Stored at Chilling Stress versus Nonchilling Stress Temperatures