Seasonal variation of exudate ofCistus ladanifer.

The production of labdanum exudate byCistus ladanifer L. is highly seasonal, reaching a maximum concentration during summer and a minimum concentration in winter. Because this exudate strongly absorbs in the wavelength range of 260-400 nm (the near-UV-visible range), it may be important biologically as an UV-visible filter. Separation of exudate components has been achieved by reverse-phase high-performance liquid chromatography (HPLC).The retention times of HPLC chromatograms and the spectral characteristics (absorption and fluorescence) of the exudate identify flavonoids as the most relevant chromophores regarding the potency of the exudate as a UV-visible filter. HPLC studies show that kaempferol-3-(O)methyl, kaempferol-3,7-di(O)methyl, and apigenin-4'-(O)methyl are the most enriched flavonoids in the exudate.Other flavonoids [apigenin, apigenin-7-(O)methyl, apigenin-7,4'-di(O)methyl, kaempferol-3,4'-di(O)methyl and kaempferol-3,7,4'-tri(O)methyl] are present in the exudate as minor components, e.g., each contributes by less than 10% to total flavonoids.The ratio of kaempferols to apigenins of the exudate also shows seasonal variation (maximum value in summer and minimum in spring). However, due to the similar absorption spectra of both groups of flavonoids, this has a minor influence on the exudate's ability to filter near-UV-visible radiation.