Identification of Volatile Compounds and Key Aroma Components in Ornamental Crabapple Flowers

Floral fragrance is an important trait of ornamental plants. Identifying the chemical basis underlying floral scents among different ornamental crabapples can provide foundations for the fragrance breeding of ornamental crabapple and the development of aromatic products.

The flowers at the full-bloom stage of six ornamental crabapples in landscape applications, namely Malus hupehensis, Malus micromalus, Malus Strawberry Parfait, Malus Kelsey, Malus Radiant and Malus Royalty, were used as plant materials. The composition and content of their volatile compounds were analyzed using headspace-solid-phase microextraction (HS-SPME) coupled with comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOF MS). The key aroma compounds of each ornamental crabapple were identified through orthogonal partial least squares-discriminant analysis (OPLS-DA) coupled with odor activity value (OAV) and relative odor activity value (ROAV), and the similarities and differences in the aroma characteristics among different ornamental crabapples were further analyzed through hierarchical clustering analysis (HCA).

A total of 154 volatile compounds were identified, comprising 36 terpenes and derivatives, 50 benzenoids/phenylpropanoids, 65 fatty acid derivatives, and 3 amino acid derivatives. The aroma composition and content among different tested ornamental crabapples exhibit significant differences. For each single ornamental crabapple, 53-95 kinds of compounds were detected, and the total content ranging from 7.95 to 38.31μg·g^-1. Among the materials, M. Radiant contains the most types of volatile compounds, M. Strawberry Parfait contains the highest total content, and M. micromalus contains the fewest categories and the lowest content. The OPLS - DA model had a good fitting effect (R²X=0.972, R²Y=0.991, Q²=0.974), which could effectively distinguish the volatile compounds of different ornamental crabapples, with 27 volatile compounds making important contributions to distinguishing their aroma characteristics, mainly including linalool, indole, cinnamyl alcohol, hexa-2,4-dienal, etc. Linalool acts as the key aroma compound in M. Strawberry Parfait, M. hupehensis, M. Kelsey, and M. Royalty, while indole serves as the principal aroma material in M. Radiant and M. micromalus, with benzeneacetaldehyde also contributing significantly to the aroma of M. micromalus. These compounds form the unique aroma characteristics of each ornamental crabapple.

A total of 154 volatile compounds were identified in this study, and the detection performance was significantly superior to that of conventional HS-SPME-GC-MS used in previous studies. The significant differences of aroma composition and content, characteristic aroma compounds and the chemical basis underlying floral scents differences among different ornamental crabapples were clarified. Linalool, indole and cinnamyl alcohol were identified as key aroma components, and it was verified that the distinctive floral fragrance of different ornamental crabapples is not governed by a single compound, but arises from the synergistic modulation of key aroma components.