Comparative Analysis and Genetic Study of Aroma Volatiles from Different Citrus Selections and Hybrids

Improvement in seedlessness, flavor, and color ranks high among the prioritized breeding goals for mandarin (Citrus reticulata Blanco). Given long seedling juvenility, large tree size, and associated costs in citrus breeding, trait-associated marker development and marker-assisted selection could potentially lead to a more expeditious and economical approach to these challenges. This study investigated aroma volatiles in citrus selections and hybrids using gas chromatography-mass spectrometry (GC-MS) from 2012 to 2014, compared juice aroma profile among different citrus genotypes, identified QTL regions controlling aroma volatiles and validated molecular markers in relevant citrus germplasm accessions. The objective was to develop markers for aroma-oriented citrus breeding. The citrus selections and hybrids included six mandarins (‘Fortune’, ‘King’, ‘Murcott’, ‘Pimpled’, ‘Ponkan’, and ‘Sunki’), one satsuma (‘Owari’), one Clementine (‘Nules’), one blood orange (‘Moro’), three sour orange types (‘Goutou’, ‘Zhuluan’, and one unknown), one complex mandarin hybrid (‘Rangpur’ lime), and a population of 116 ‘Fortune’ x ‘Murcott’ F₁ hybrids. The aroma volatile composition varied among the citrus selections from 35 volatiles identified in ‘Ponkan’ mandarin to 123 in ‘Guotou’ sour orange. The aroma volatile profile was able to differentiate the 13 citrus selections and revealed complex interactions between them. The phylogenetic analysis results for the citrus selections based on aroma volatile compositions were matched well with that based on the genotypic data of 1536 SNPs. The volatile compounds in ‘Fortune’ and ‘Murcott’ behaved differently during the fruit maturation period. In ‘Murcott’, the total aroma volatiles obtained from samples were very similar at each developmental stage, however, the aroma constituents changed as fruit maturity progressed to a tree ripe stage. The concentration of valencene, the most abundant sesquiterpene accumulated during the last two development stages in ‘Fortune’, but in ‘Murcott’, it was present only in the first development stage and then disappeared. A total of 183 QTLs were identified on a previous generated mandarin genetic map for 84 volatile compounds, with 79 of them being consistent over two or more sampling times. A QTL interval on LG2 controlling monoterpenes and sesquiterpenes corresponded to a genomic region that contains terpenoid biosynthetic pathway genes GPS1, TPS3, TPS4, and TPS14. QTLs were validated in the 13 citrus selections, and some QTL linked markers showed significant association with aroma volatile content.