Variations in Essential Oil Compositions and Changes in Oil Cells during Leaf Development of Citral Chemotype of Camphora officinarum Nees ex Wall.

Abstract

The citral chemotype of Camphora officinarum Nees ex Wall. is a promising industrial plant that contains an abundance of citral, which is widely used in medical, chemical, food, and other fields. For a more in-depth exploration, the dynamic characteristics of its essential oil (EO), oil compositions (OCs), and oil cells during leaf development were determined in the present study. The leaf phenotype changed rapidly from the 1st to the 4th week after leaf bud germination. The oil yield (OY), obtained via supercritical carbon dioxide extraction, reached the highest value of 2.82% ± 0.20% in the 12th week. Leaf development is a prerequisite for the production of EO, and the difference in the OY was not significant after leaf maturation. The OCs, analyzed using gas chromatography–mass spectrometry (GC-MS), mainly included aldehydes, alcohols, and hydrocarbons. Different types of compounds accumulated differently during leaf development: the highest relative content of alcohol in the OCs was 30.18% in the 2nd week, while that of aldehyde was 76.11% in the 6th week. In total, 130 OCs were detected, and two isomers of citral, namely, geranial and neral, had the highest relative levels of 51.12% (12th week) and 28.63% (6th week), respectively. The OY was closely related to the developmental stage of the oil cells. In the 1st–2nd weeks, the oil cells were mostly in the non-essential oil stage and essential oil-forming stage, with a lower OY; oil cells reached saturation in the 12–24th weeks, with a higher OY. Transmission electron microscopy showed that osmium droplets were present in large quantities during leaf development and gradually integrated into the vacuoles, finally making the vacuoles become oil bladders for oil storage. In conclusion, EO may have new uses due to the different OCs in leaf development; additionally, the microscopic changes in C. officinarum provide a reference for the cellular mechanism of EO accumulation.