Date Published: April 16, 2019
Publisher: Public Library of Science
Author(s): Jakaria Chowdhury Onik, Yajing Xie, Yuquan Duan, Xiaojia Hu, Zhidong Wang, Qiong Lin, Xiaoming Pang.
Early ripening apples are usually used for fresh marketing because of short storage life, although they are with high acid and low sugar contents. Understanding the malate metabolism in fleshy fruit and underpinning process during ripening is crucial for particular crop improvement where acidity is a concern for direct consumption or further processing. In this research, a traditional Chinese apple cultivar ‘Hongyu’, which belongs to early ripening apple cultivar, were freshly harvested at commercial maturity stage (120 Days after full bloom) and used for different storage temperature (4°C, 20°C) and UV-C treatment (following storage at 20°C after treatment). Simple sugars (glucose, sucrose, and fructose) and organic acids (malic, and oxalic) were assessed after 14 d of storage. Compared to fruits stored at 20°C, the malate content in fruits stored at 4°C significantly higher, while it was decreased significantly in UV-C treated fruits stored at 20°C after 14 d of storage. The sugar content was almost similar throughout the UV-C-treated fruits and fruits stored at different temperature. The higher ratios of total sugars to total organic acids in UV-C treated fruits after 14 d suggest that UV-C treatment has the potential to improve the taste of early ripening apple cultivars. Considering the significant difference in malate the samples at 14 d of storage were subjected for RNA-seq analysis. Transcriptome analysis revealed that the phenomena underlying this change were governed by metabolism of malate by the regulation of NADP-malic enzyme (NADP-ME) and phosphoenolpyruvate carboxylase kinase (PEPCK) in apple during postharvest storage. This transcriptome profiling results have specified the transcript regulation of malate metabolism and lead to possible taste improvement without affecting the other fruit quality attributes.
Apple (Malus domestica Borkh.) is an economically important and popularly grown fruit crop all over the world due to high nutritive value, good taste and availability throughout the year. The postharvest storage period varied among different cultivars and different harvesting time . In general, early harvested apple cultivars such as ‘Gala’, ‘Royal Gala’ tended to be relatively less suitable for the long-term cold storage, comparing the late harvested apple cultivars such as ‘Fuji’, ‘Red Delicious’, ‘Golden Delicious’ [2,3,4]. Therefore, summer apple cultivars or early harvested apple cultivars have been in sale for fresh market but not for long-term storage . ‘Granny Smith’ apples, which were belonging to the late ripening cultivar, can be stored for up to seven months under the cold storage condition, while most of the early ripening apples can be stored for about one and a half months under cold storage .
Considering the health benefits associated with apple progressively understood through consumers, demand is increasing for cultivars with preferred quality features, such as size, texture, flavor and nutritional value . For an apple cultivar, fruit quality characteristics are underlain by metabolite composition and contents at maturity and during postharvest storage. The content of predominant organic acid and metabolism also plays an important role in food processing  and quality . The early ripening apples are mostly used for fresh marketing because of short storage life, but they are usually with high acidity and low sugar contents [5,39].
Low temperature maintained significantly higher malate contents by up-regulating the expression of genes (NAD-MDH and PEPC) related to the TCA cycle and citrate synthesis and down-regulating the expression of NADP-ME and PEPCK. UV-C treatment significantly increased the ratio of total sugars to total acids compared with room temperature stored fruits, which was carried out by the up-regulation expressions of NADP-ME and PEPCK. This finding suggested UV-C as an effective measure to maintain and promote the fruit taste during post-harvest storage of early ripening apple cultivar at room temperature.