Inheritance of sugar and acid contents in the fruits of triploid hybrids originated from two 2x × 4x crosses with Nadorcott tangor as a female parent

Abstract：【Objective】To uncover the inheritance of sugar and acid contents in the fruits of citrus triploid hybrids, we determined the components and contents of soluble sugars and organic acids in the fruit pulp of triploid hybrids and their parents from two citrus triploid hybrid populations by Gas Chromatography (GC). Citrus is one of the most important fruit crops worldwide. Seedlessness is an increasingly important trait in relation to citrus fruit quality. And the triploid breeding strategy has been proved to be an efficient way to create seedless citrus. In the past 20 years, Huazhong Agricultural University has produced a large number of citrus triploid hybrid populations based on the hybridization between diploid and tetraploid. However, less attention has been focused on the genetic trend of sugar and acid components in triploid citrus. Therefore, exploring the inheritance of sugar and acid contents in the fruit of citrus triploid hybrids is necessary to provide a basis for selecting parents in citrus triploid breeding.【Methods】Two triploid hybrid populations, previously obtained from interploidy crosses with diploid Nadorcott tangor as female parent and two allotetraploid somatic hybrids Nova tangor + Succari sweet orange (abbreviated as NS) and Succari sweet orange + Dancy red tangerine (abbreviated as SD) as male parents, were used as materials. And the two triploid hybrid populations were named as MNS and MSD, respectively. Fully ripening fruits were collected in 2018 and 2019 for fruit quality analysis. Twelve fruits from each tree were sampled and pooled for three technical replicates. One part of the fruit pulp was used for the determination of total soluble solid (TSS) and titratable acid (TA) contents and the other part was immediately frozen in liquid nitrogen and stored at -80 ℃. The contents of TSS and TA in the fruit pulp of citrus triploid hybrids and their parents were measured using a refractometer. Additionally, the contents of sucrose, glucose, fructose, malic acid and citric acid of the fruits of two parents and their progenies were further determined by Gas Chromatography (GC). The genetic variation of fruit traits was evaluated by coefficient variation (CV), genetic transmitting ability (Ta) and transgression rate, and the normal distribution map was used to reflect the genetic tendency of the progeny. The data were processed by using Microsoft excel.【Results】Sucrose, fructose and glucose were the major components of sugar, while citric acid and malic acid were the major components of organic acid in the fruit pulp of citrus triploid hybrids. The triploid hybrid populations exhibited a wide phenotypic variation in sucrose, fructose, glucose, total sugar, citric acid, malic acid and total acid contents, where the average coefficient of variation (CV) of sugar and acid contents was more than 15% and 30%, respectively, and the range of acid contents was higher than that of sugar contents, indicating that there was a bigger selecting potential for acid contents. Additionally, the genetic transmitting ability (Ta) of sucrose, fructose, glucose, total sugar, citric acid, malic acid and total acid contents was ranged from 65.50% to 184.85% , among which the highest was citric acid (with an average of 177.18% and 176.46% in the MNS and MSD triploid hybrid populations, respectively), indicating that the variations of these traits mainly resulted from inheritance. The total sugar contents of triploid hybrid populations inclined to low-sugar parent value, where the average ratio lower than low parent (LL) was more than 50% ，showing a tendency of depression. However, the total organic acid contents were higher than those of the mid-parental value, where the average ratio higher than high parent (HH) was more than 70%, displaying obvious heterosis performance. The total sugar content was close to the normal distribution, indicating that it was a quantitative trait controlled by multiple genes. Nevertheless, the content of citric acid displayed a skewed distribution, implying that acid contents might be controlled by the major gene.【Conclusion】Our observations indicated that interploidy hybridization between diploid and tetraploid did not change the contents of sugar and acid components in the fruit pulp of citrus, where sucrose, fructose and glucose were the main components of soluble sugars, while citric acid and malic acid were the major components of organic acids. The sugar content was normally distributed, and therefore likely to be controlled by multiple genes, whereas the organic acid content may be under the control of major gene for displaying a skewed distribution. The contents of sugar and acid in the fruit pulp of citrus triploid hybrids were closer to the parent with high-acid and low-sugar content, and were prone to be affected by the male parent.