The present invention relates to a method of modulating the density of lignin in a test plant compared to the corresponding lignin density in a wild-type plant cultured under the same conditions, the method comprising modulating the concentration and/or activity of a polypeptide encoded by a core circadian clock gene, wherein modulation of the concentration and/or activity of the polypeptide modulates lignin density in the test plant. The present invention further relates to the use of a genetic construct which is capable of modulating the concentration and/or activity of a polypeptide encoded by a core circadian clock gene, for modulating the density of lignin in a plant, wherein the genetic construct encodes a polypeptide comprising the amino acid sequence substantially as set out in SEQ ID No's: 2 or 4, or a functional variant or fragment thereof. In addition the present invention relates to a plant host cell comprising a genetic construct which is capable of modulating the concentration and/or activity of a polypeptide encoded by a core circadian clock gene is also taught.