Petunia belongs to the family Solanaceae, subfamily Petunioideae. The popular flower of the same name takes its epithet from the French, which took the word pétun, 'tobacco', from a Tupi-Guarani language. Petunias are generally insect pollinated, with the exception of P. exserta, a rare, red-flowered, hummingbird-pollinated species.
The hybrid Petunia × atkinsiana encompasses all hybrid petunias between P. axillaris and P. integrifolia. P. axillaris, the large white or night-scented petunia, bears night-fragrant, buff-white blossoms with long, thin tubes and somewhat flattened openings. Its flowers, the only white ones found in the genus, are 3 to 7 cm long. It is native to temperate South America, comprising southern Brazil, parts of Argentina and Uruguay. The scent molecules emitted by the hybrids are generally similar to those from P. axillaris.
P. integrifolia, a violet-flowering species with small, broad flowers lacking strong scent, is visited by bees during the day. P. axillaris, a white-flowering species with slender flowers, is visited by hawk moths at night and starts producing a strong scent at dusk. The flowers of the hybrid cultivar Mitchell are only fragrant in the evening and at night. Volatile benzenoids are the main compounds of the floral scent of this cultivar. Methyl benzoate is the major floral volatile of petunia, and daily light and dark transitions are the predominant cues for the timing of its emission. In Petunia hybrida, volatile emissions are dominated by products of the floral volatile benzenoid/phenylpropanoid pathway, and four major ones are methyl benzoate, benzyl benzoate, benzaldehyde and isoeugenol.
In the cultivar Mitchell, the transcription factor ODORANT1 controls the synthesis of volatile benzenoids. Its transcript levels in the nonfragrant cultivar W138 are only 10% of those in Mitchell. Experiments with the clock gene LATE ELONGATED HYPOCOTYL (PhLHY) indicate that it directly sets the timing of floral volatile emission by restricting the expression of ODO1 and other genes of the volatile pathway to the evening. Experiments showed that the hawkmoth Manduca sexta prefers scented plants and that scent determines its choice at a short distance.
Flowers of Petunia integrifolia emit mostly benzaldehyde. Flowers of P. axillaris subspecies axillaris emit a mixture of volatile benzenoid and phenylpropanoid compounds that include isoeugenol and eugenol. The flowers of P. axillaris subspecies parodii emit neither isoeugenol nor eugenol but contain high levels of dihydroconiferyl acetate in the petals. The isoeugenol synthase gene of this subspecies contains a frame-shift mutation that renders it inactive. Benzaldehyde, benzyl alcohol, methyl benzoate, benzyl acetate, methyl salicylate, benzyl benzoate and benzyl salicylate were identified among the volatiles emitted by its petals. Eugenol has a pleasant, spicy, clove-like scent. Methyl salicylate is a colorless, viscous liquid with a sweet, fruity odor reminiscent of root beer.
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