Inheritance of some quality traits of the fruits in triploid hybrids derived from two citrus 2x × 4x interploidy crosses

Abstract: 【Objective】The study aimed to investigate the inheritance of some fruit quality traits and to provide a basis for selecting parents in the interploidy cross breeding of citrus.【Methods】Two previously produced triploid hybrid populations derived from the cross of Fallglo mandarin × NS (Nova tangor + Succari sweet orange) and another cross of Kiyomi tangor × SD (Succari sweet orange + Dancy red tangerine) were used as materials. Fully matured fruits were collected in two successive years (2018 and 2019) for fruit quality analysis. For each triploid plant, twelve fruits were collected and pooled for three technical replicates. The transverse diameter, longitudinal diameter and peel thickness were measured using a digital vernier caliper. The fruit weight was determined by a precise 1/100 electronic balance. Furthermore, the soluble solids content and titratable acid content of the fruit pulp were deter-mined using an ATAGO handheld digital refractometer. The genetic variation of the fruit traits was assessed by the coefficient of variation (CV), transmitting ability (Ta) and transgression rate. The normal distribution map was employed to depict the genetic tendencies of the triploid progenies. All data were processed using Microsoft excel.【Results】The distribution of fruit transverse and longitudinal diameter, fruit weight, peel thickness, and soluble solids content frequency in two citrus sexual offspring populations showed a continuous normal distribution, while the distribution of the titratable acid content showed a partial normal distribution. The fruit transverse diameter and fruit quality of the two triploid sexual offsprings were mostly distributed between those of the parents, indicating a trend of intermediate variation. The fruit longitudinal diameter exhibited diverse patterns in different combinations. For instance, 60.47% and 42.00% of the fruit longitudinal diameters of individuals from the Fallglo mandarin× NS cross was significantly higher than that of the higher value of the parents in 2018 and, respectively, indicating a genetic trend of transgressive variation. In contrast, 94.12% and 97.0% of the fruit longitudinal diameter of the individuals from the Kiyomi tangor × SD combination in 2018 and 2019 fell between the values of the two parents, respectively, showing an intermediate variation trend. The average peel thickness of the fruits was greater than the median value of the parents and most of them were higher than the high-value parents, showing a phenomenon of transgressive inheritance. The ratios of the average value of the soluble solids content of the fruits from the two triploid crosses lower than that of the lower value of the parent were 74.4% and 66.88%, respectively. The average value of the titratable acid content in the fruits of the both two triploid progeny populations was lower than that of the median value of the parents, which was different from the results reported previously, implying that the occurrence rate of the low-acid offspring might be related to the acid content of the female parents. The coefficient of variation for the fruit weight and titratable acid content in the fruit traits exceeded 30%, indicating extensive separation of these traits in the triploid sexual offsprings. In contrast, the coefficient of variation for the fruit transverse and longitudinal diameter, peel thickness, and soluble solids content remained below 30%, implying a limited separation in these traits in the triploid sexual offsprings. The genetic transmission ability of the fruit transverse and longitudinal diameter, peel thickness, soluble solids and titratable acids of the triploid sexual offspring exceeded 70%, suggesting that genetic factors were the primary drivers of variation in these traits. The genetic transmission ability of the fruit weight varied significantly with the different combinations. For instance, the genetic transmitting ability of the fruit weight in the Fallglo mandarin × NS cross for two consecutive years were both high (99.79% and 91.84% ). However, the genetic transmitting ability in the Kiyomi tangor × SD combination was relatively low (55.93% and 63.30%), indicating that the inheritance of the fruit weight is greatly influenced by the parents.【Conclusion】The fruit weight, fruit transverse diameter, longitudinal diameter, peel thickness and soluble solids content of the sexual offspring in citrus triploids were polygenic quantitative traits, whereas the titratable acid content might be primarily influenced by a major gene, potentially of paternal origin. Notably, the significant genetic variation was observed in single fruit quality and titratable acid content, facilitating the trait segregation, while the variation was comparatively low in the other traits. The order of the genetic heritability values for several traits was as follows: fruit peel thickness＞fruit longitudinal diameter＞fruit transverse diameter＞titratable acid content＞soluble solids content＞fruit weight. The enhanced genetic transmission ability were correlated with the prominent heterosis, providing a foundational framework for the parent selection in ploidy hybridization aiming to generate elite triploid progeny in the future.