JIA-2019-11

2515 Maratab Ali et al. Journal of Integrative Agriculture 2019, 18(11): 2514–2520 decarboxylase (PDC) and alcohol dehydrogenase (ADH), respectively, and this process has a vital role in maintaining fruit flavor quality by regulating ethanol fermentation metabolites (Tietel et al . 2011). Generally, the shift from aerobic to anaerobic condition boosts up activity of PDC and ADH, which result in high production of acetaldehyde and ethanol (Kader 1986; Kader and Yahia 2011). Endogenous acetaldehyde and ethanol can improve the quality of fruit flavor, enhance disease resistance, inhibit ethylene production, and reduce chilling injury (Pesis 2005). However, high accumulation of these metabolites results in toxicity (Polenta et al . 2005) and physiological disorders in relation to flesh browning (Vidrih et al . 1999) and off- avors occurrence in fruit during postharvest (Shi et al . 2007). Therefore, regulation of ethanol fermentation metabolism to control the accumulation of acetaldehyde and ethanol is beneficial to maintaining fruit quality during postharvest. Oxalic acid (OA) is an organic acid ubiquitously present in plant tissues and generally known as eco-friendly and recommended as safe compound (Razavi and Hajilou 2016). In recent years, OA treatment has accounted for its beneficial effect on postharvest quality of different fruit and vegetables due to delaying ripening and senescence, reducing decay, controlling browning, and alleviating chilling injury (Zheng and Brecht 2018). However, most of previous works have reported postharvest application of OA, while a few studies have investigated the effect of pre-harvest spraying of OA on fruit and vegetables, such as peach (Razavi and Hajilou 2016), sweet cherry (Martínez-Esplá et al . 2014), and artichoke (Martínez-Espl et al . 2017), for evaluating various quality parameters. Our previous work has reported that pre-harvest spraying of OAon kiwifruit plants increases the postharvest quality and disease resistance of the fruit, especially inhibits the blue mold rot and patulin accumulation by P . expansum , which attributes to the apparent induced resistance of kiwifruit against postharvest diseases (Zhu et al . 2016). In this paper, for further clarifying and understanding the pre-harvest application of OA to maintain the postharvest quality of kiwifruit cv . Bruno with aim to prolong fruit storage, the effect of pre-harvest OA spraying on fruit quality, especially associated with regulation of ethanol fermentation metabolism in kiwifruit cv . Bruno was investigated during storage at room temperature. 2. Materials and methods 2.1. Materials and treatments Each six of eleven-year old kiwifruit ( A . deliciosa ) cv . Bruno plants was selected randomly and sprayed with 5 mmol L –1 OA (as a treatment) or water (as a control) until leaves and fruit were fully wetted by hand-held mist sprayer at a commercial orchard in Wenzhou, Zhejiang, China. The plants were treated at 130, 137 and 144 d after the flowering period of kiwifruit, respectively, according to the method described by Zhu et al . (2016). Fruit selected for uniformity of size and maturity and without blemish or apparent infections were harvested at 151 d after full bloom, when the soluble solids content (SSC) had reached to approximate 6.5%, and then transported to an experimental laboratory in Hangzhou City within 5 h in an air-conditioned car. Each fruit group (treated or control) without physical injuries was placed into 9 clean plastic racks (each contained 15 fruit) with fruit touching, and covered with 0.05 mm thick polyethylene bag, and then was stored at room temperature (20±1)°C. Triplicate of flesh samples were collected from the middle part of 27 fruit being removed peels and cores (9 fruit/each replicate) on the 1st d and at 3-d intervals thereafter during storage, and were rapidly frozen in liquid nitrogen and stored at –80°C for future analysis in triplicate. 2.2. Determination of SSC and titratable acid SSC was detected in flesh juice (obtained by squeezing fruit pulp) by using refractometer (Master-α, ATAGO ATC, Japan), and titratable acid (TA) was determined by titration method, 10 mL of juice was taken with addition of phenolphthalein indicator (2–3 drops), titrated against 0.1 mol L –1 NaOH up to pH 8.2 and results were expressed as g malic acid L –1 juice. 2.3. Assays for AsA and total-AsA One gram of sample was homogenized in 10 mL of 5% (w/v) trichloroacetic acid (TCA) and then centrifuged at 12000×g and 4°C for 20 min. The supernatant was used for assaying the content of AsA or total-AsA (T-AsA) by the method of Jiang et al . (2018). DHA content equaled T-AsA minus AsA in terms of as mg 100 g –1 FW. 2.4. Determination of OA and tartaric acid A total of 2 g of flesh samples were grounded with 2 mL 0.2% HPO 3 , and then centrifuged at 12 000×g and 4°C for 30 min. The combined supernatant was filtered through 0.22 μm filter paper, and the filtrate was used to assay OA and tartaric acid according to the method described by Jiang et al . (2018). Chromatographic separation was performed on a PRONTOSIL120-10-C18 Vennusil MP C18 (250 mm×4.6 mm i.d., 10 μm). The mobile phase was 5% (v/v) acetonitrile in 0.01 mol L –1 K 2 HPO 4 (pH 2.08) with a flow rate of 0.5 mL per min at 30°C.