## 3 Discussion  
The early and late fruit harvesting period is closely related to fruit yield, quality, and storability, and high maturity usually couples with poor storage resistance. When maturity is low, fruit quality, size, and flavor are not up to the requirements, resulting in poor quality. Grasping the dynamic changes in the physiology and biochemistry indexes of kiwifruit from its mature stage to its physiological maturity is the key to determine the best harvest time for kiwifruit. In our study, at 78 DAF, the hardy kiwifruit had basically the same size and volume, indicating that the morphological maturity had been completed at this time. Although there were morphological differences in the two varieties of hardy kiwifruit, there was no difference in weight for a single fruit. Soluble solids contain many components that can be solubilized in water, such as sugar, acids, vitamins, and some minerals. It is an important to evaluate the quality of fruits and vegetables as an integrated index [14]. Currently, soluble solids are used as harvest indicators for kiwifruit. The harvest maturity of _A. deliciousa_ 'Hayward' revealed a soluble solids content of 6.2% at the minimum harvest time [15]. From our results, the minimum harvest time can be met after three days of morphological maturity. The soluble sugar and reducing sugar content of hardy kiwifruit showed an increasing trend during the harvest period. During physiological maturity, the sugar was mainly converted to sucrose. AsA is not only an essential nutrient for humans but also an antioxidant for scavenging reactive-oxygen species (ROS) in plants. Kiwifruit is rich in AsA--one of the important measures of the fresh quality. Several studies have proposed that there are certain differences in ASA content among different varieties of Kiwifruit, with Issai an AsA-rich cultivar [16]. In the present study, our results showed that the AsA content in Issai was 3.34 folds higher than in Ana, and a higher ratio of AsA/DHA was found in Issai, indicating that Issai has a higher AsA turnover and regeneration capability [17]. To explore in depth the molecular reason for this difference, the RNAseq-based expression profiles of AsA biosynthesis transcripts were estimated. As is generally known, the L-galactose and D-galacturonate pathway was proposed as the predominant and alternative pathway for AsA biosynthesis [18], and changes in the expression of genes in both pathways could manipulate AsA accumulation. GalDH and GalLDH were considered to be the key enzymes for the synthesis of AsA via the L-galactose pathway [19]. GalDH uses NAD+ as an electron acceptor to oxidize L-galactose-1,4-lactone and catalyze the oxidation of the final precursor L-galactose-1,4-lactone to AsA [20]. In Arabidopsis thaliana, inhibition of the GalDH and GalLDH gene resulted in an approximately 50% reduction in AsA content [21]. Based on our present transcriptome data, we found that the expression levels of transcripts encoding enzymes involved in the L-galactose biosynthesis pathway were significantly up-regulated in Issai, including GME, GGP, GalDH, and GalLDH. These results suggested that AsA was synthesized mainly through the L-galactose pathway in hardy kiwifruit, which is consistent with the previous reports in Arabidopsis and Tomato [22; 23]. Overexpression of the kiwifruit GGP gene, a homolog in tobacco, results in a threefold increase in AsA content, and thereby GGP has been suggested as a rate-limiting route for AsA biosynthesis [24; 25]. In our results, the transcript level of GGP (Cluster-4820.44393) in Issai is about 5 folds higher than in Ana, indicating that GGP is the key gene for the AsA accumulation in hardy kiwifruit. Moreover, the increase in PME and PG expression in the alternative D-galacturonate pathway could lead to an increased AsA production in Solanaceae [26].