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Rhythms of floral fragrance

Plants that emit scent at particular times of day
Day-time scent Night-time scent

Ribes nigrum (blackcurrant)
Malus domestica (apple)
Odontoglossum constrictum (orchid)
Citrus medica (orange)
Trifalium repens (white clover)
Rosa spp. (rose)

Lonicera japonica (honeysuckle)
Nicotiana sylvestris (wild tobacco)
Cestrum nocturnum (lady of the night)
Clarkia breweri (fairy fans)
Oenothera spp. (evening primrose)
Hedychium spp. (wild ginger)
Boronia megastigma (brown boronia)
Jasminum spp. (jasmine)
Many flowers emit scent in the day-time to attract flies, beetles, bees and butterflies. Flowers that emit a heavy, sweet scent at night-time to attract moths are often pale or white and without ultra-violet reflecting surfaces.
A typical floral fragrance may contain between 5 and 150 different types of volatile odour molecules and may be emitted by petal, sepals, pollen or nectar. While the nature of these structures cannot change, the scent composition and intensity can vary rhythmically.
The autumn damask rose Rosa damascena Quatre Saisons, a rose grown for rose oil production, emits its scent in a daily rhythm associated with light and dark periods. The biochemical nature of these rhythms is being studied in the rose and other plants, such as sweet pea (Lathyrus odoratus), night-scented stock (Matthiola spp.), tulips (Tulipa spp.), freesias (Freesia spp.) and Mahonia (Mahonia bealei). Mahonia has a lily of the valley scent, known in the fragrance industry as muguet. Lily of the valley itself has an extraordinary scent that cannot be extracted by conventional means.

Most rhythmic behaviour in plants is controlled by several genes that change the rates of biosynthesis of scent compounds. Alternatively, rhythms in fragrance emission may occur by release of volatiles from storage pools in which they are bonded with sugar groups (glycosides). It is a complex issue! Knowledge of these rhythms helps the fragrance industry to optimise scent harvesting, and develop ingredients for perfumes inspired by the odour of a flower at a particular time of day.