Analysis of floral volatile components and aroma features during flower development of Annona atemoya

Abstract: 【Objective】Annonaceae is the largest family in Magnoliales. Several genera produce edible fruit. Among them, Annona atemoya is the most popular commercial cultivated species. A. atemoya is the hybrid of A. squamosa and A. cherimola, both native to the American tropics. The flower of A. atemoya is with three large thick petals and a cone of pistils surrounded by stamens. Though the flower is hermaphrodite with both female and male structures, the pistils mature earlier than stamens. The so called dichogamy combined with the rare visitation of insect pollinators, with the crop depends on artificial pollination for fruit set. The floral volatiles of A. atemoya seem not attractive enough for insect pollinators, however,the opening flowers emit aroma pleasant to human beings. This study aimed to identify the volatile compounds in the flower of A. atemoya during different flowering stages and to analyze the scents of aromatic components.【Methods】Flowers from four development stages were harvested for the analysis of the volatile components. The four development stages were young flower bud stage, closed flower bud stage, halfopen stage and fully open stage. Half open stage was characterized by pistil maturation and full opening accompanied by stamen maturation, corresponding to the female stage and male stage, respectively. Flowers at the four stages were separately collected, quickly frozen in liquid nitrogen and preserved under-80 ℃. The volatile compounds were identified by Headspace solid phase micro-extraction combined with gas chromatography-mass spectrometry(HS-SPME-GC-MS).【Results】A total of 37 major volatile compounds with relative content over 1% were detected in different stages, and the volatile compounds in young flower bud were obviously different from those in the other three stages. They included 3 alcohols, 1aldehyde, 4 esters, 2 ketones and 27 terpenes. And the terpenes were the most abundant chemicals in both content and quantity in all the stages. Moreover, the relative content of terpenes increased with flower development. In young flower buds, there were 19 main volatile compounds with relative content over than1%, including β-pinene(15.48%), α-pinene(9.37%), isocaryophillene(7.28%),(+)-dipentene(6.34%),9-methyl-5-methylene-8-decen-2-one(5.38 %), etc. In closed flower buds, there were 21 major volatile compounds including ocimene(11.25%), β-pinene(10.65%), α-pinene(9.03%), β-caryophyllene(7.15%),(+)-dipentene(5.64%), etc. In half open flowers, there were 17 main volatile compounds such as ocimene(14.08%), β- pinene(10.09%), α-pinene(9.37%),(+)-dipentene(6.93%), γ-elemene(6.18%),β- caryophyllene(5.80%), etc. And 23 volatile compounds including ocimene(14.49%), γ- elemene(8.27%), β-pinene(6.88%),(+)-dipentene(6.71%), β-caryophyllene(6.33%), etc., were detected in fully open flowers. In young flower buds, compared with the other three stages, the exclusive volatile compounds were cis-verbenol, cis-trans-farnesol, cubebene, 2,4,6-trimethyl-3-cyclohexen-1-carboxaldehyde and 9-methyl-5-methylene-8-decen-2-one. And the volatile compounds unique in close flower buds, half open flowers and fully open flowers were ethyl caprylate, 2-nonanone, cosmene,neo-allocimene, elixene,(-)-α-cubebene, β-elemene, γ-elemene, γ-cadinene, β-cadinene,(+)-epi-bicyclosesquiphellandrene and(1E,6E,8S)-1-methyl-5-methylene-8-isopropyl-1,6- cyclodecadiene. The relative contents of myrcene, ocimene, linalool and ethyl acetate increased during flower opening. 4-methyl-5-vinylthiazole, which had been identified in A. montana, A. crassiflora, A. coriacea and A. dioica as the key attractant of pollinator beetles was not found in the flowers of A. atemoya. This partially explained why A.atemoya lacks pollinator insects. At half open stage and fully open stage, the flower emitted pleasant smells. The differences in these two stages were mainly in the relative content of volatile compounds. To evaluate the general fragrance of the flowers, scents of 15 aromatic compounds including linalool, ethyl acetate, isoamyl tiglate, ethyl caprylate, L-bornyl-acetate, 2-nonanone, α-pinene, β-pinene, myrcene,(+)-dipentene,(E)-β-ocimene, ocimene,(-)-α-cubebene, β-caryophyllene and α-caryophyllene were analyzed integratedly which. All these 15 aromatic compounds contributed to 18 aroma scents, among which conifer scent, herb scent, rose scent, muguet scent, orchid scent, wood scent, fruit scent, green scent had higher loads, and they could be considered as the typical floral scent of A. atemoya.【Conclusion】The volatile compounds in A. atemoya are made up by alcohols, aldehyde, esters, ketones and terpenes, and terpenes are most diverse and abundant. The volatile constitutions in closed buds, half open flowers and fully open flowers are similar. The rare visitations of pollinator insects might be due to the lack of attractive volatile compound. Moreover, the typical scent of the flower is the mixture of conifer, herb, rose, muguet, orchid, wood, fruit and green scents. These results provide reference for the extraction of essential oils and development new perfumes from A. atemoya flower and also shed light on the development of lures to attract pollinators for increased fruit set.