The petunia, a popular garden annual, is known for its trumpet-shaped flowers and vibrant colors. While some petunias fill the evening air with a sweet perfume, the majority of the showy hybrids sold today are scentless. This difference stems from a complex biological trait connecting the flower’s chemistry and genetics to its ecological role. The presence or absence of fragrance is shaped by chemical production, a biological clock, evolution, and human intervention.
The Chemical Composition of Petunia Fragrance
The scent emitted by petunia flowers is composed of volatile organic compounds (VOCs), small, airborne molecules synthesized within the plant. Fragrance is a unique blend of chemicals, not a single molecule, that creates the flower’s specific aroma profile. Primary contributors include benzenoids and phenylpropanoids.
These aromatic molecules are created through the phenylpropanoid biosynthetic pathway, which converts precursor molecules into volatile products. For example, sweet, almond-like notes come from compounds such as benzaldehyde and methyl benzoate. The relative concentration of these components dictates the perceived smell, which can range from spicy, sweet, or citrusy.
The synthesis of benzenoids occurs mainly in the epidermal cells of the petals, where they are produced and quickly released into the air. Specialized transporter proteins shuttle the molecules out efficiently, preventing them from building up and becoming toxic inside the plant cells. This mechanism ensures the fragrance is available when needed.
Scent Timing and Pollinator Attraction
The presence of a strong scent is an evolutionary adaptation, and its release timing is synchronized with the intended pollinator’s activity window. This temporal regulation is controlled by the plant’s internal circadian rhythm, a 24-hour biological clock. Petunia species fall into two distinct groups based on their pollination syndrome and scent schedule.
One group, including species like Petunia integrifolia, is primarily pollinated by bees and releases fragrance during the day. The other major group, typified by Petunia axillaris, relies on nocturnal insects, specifically night-flying hawk moths. These petunias release their maximum amount of volatile compounds in the evening and around midnight when the moths are foraging.
At the molecular level, this timing is governed by the clock gene LHY (LATE ELONGATED HYPOCOTYL). During the morning, LHY is highly active and suppresses the genes responsible for manufacturing fragrant chemicals. As the day progresses, LHY activity declines, allowing other genes, such as ODORANT 1 (ODO1), to ramp up scent production. This mechanism prevents the petunia from wasting metabolic energy when its target pollinator is inactive.
Why Some Petunias Have Lost Their Smell
The lack of scent in many modern petunia cultivars is a consequence of intensive horticultural breeding practices over the last century. Early hybrids were crosses between fragrant species like Petunia axillaris and the purple-flowered Petunia integrifolia. However, commercial breeders shifted focus, prioritizing other traits over fragrance.
Breeders selectively focused on characteristics appealing to human buyers, such as larger flower size, novel color patterns, and a compact growth habit ideal for hanging baskets. The genes responsible for producing fragrance often compete metabolically with the genes for these desired visual traits. Since VOC production requires significant energy, selecting for massive, colorful blooms inadvertently diverted resources away from scent production.
This process broke the link between the flower and its ancient nocturnal pollinator, resulting in the visually stunning but scentless hybrids dominating garden centers today. Gardeners seeking the classic petunia perfume should look for older, heirloom varieties not subjected to modern breeding pressures. Cultivars such as the ‘Balcony Mix’ or hybrids with Petunia axillaris parentage retain a noticeable fragrance, especially as evening approaches.