Aims The primary metabolism and secondary metabolism of plants are influenced by elevated CO₂ concentration. There are many studies on primary metabolism, but few on secondary metabolism; therefore, we studied the effect of CO₂ concentration on plant growth (primary metabolism), contents and components of essential oil (secondary metabolism) in fennel (Foeniculum vulgare).

Methods After adding CO₂ at three concentration levels (287.11, 532.88 and 780.46 μmol·mol ^-1) to a sealed phytotron for 70 days, we investigated plant growth, contents and components of essential oil in fennel.

Important findings Plant height, inflorescence number, fresh and dry weight of inflorescence, whole plant dry weight and dry mass ratio increased, while node number and whole plant fresh weight did not change significantly. Soluble sugar and total carbon increased continuously, while total nitrogen and protein nitrogen exhibited the opposite trend. Pigment content in leaves decreased, but chlorophyll a/b ratio was not significantly different among plants. Contents of essential oil and yields of essential oil per plant were improved under elevated CO₂ concentrations. We identified 22 kinds of components in essential oil, but there were no differences in components of essential oil among different CO₂ concentration treatments. However, α-pinene, β-pinene, myrcene, p-cymene, tran-fenchyl acetate and (Z)-anethole were significantly different in relative contents of components (p<0.01), and sabinene, phellandrene, ocimene, γ-terpinene, 2,4,6-octatriene, 3,4-dimethyl-, estragole, fenchyl acetate, copaene, fanesene and germacrene were significantly different (p<0.05). Contents of (E)-anethole in essential oil increased under elevated CO₂ concentrations, while contents of limonene decreased, but differences between contents of (E)-anethole and those of limonene were not significant.