I am sure most of us know the flowers don’t have scents just to please us gardeners but why? I started pondering this question recently when I noticed that the petunias that I have started from seed (and currently reside in every southern window of my house) have a much more notable scent in the evenings. I have noticed this before; a friend has an orchid that smells delightful in the mornings and like pepper in the evenings, to my nose anyway. Being the science geek and master gardener that I am, I decided to do a bit of research.
Petunias originated in South America and were primarily pollinated by the hawk moth (also known as hummingbird moth) which is more active in the evening. The blossoms actually have an enzyme that regulates this diurnal difference in scent. (Floral Scent Diversity is Differently Expressed in Emitted and Endogenous Components in Petunia axillaris Lines, M. KONDO,1 N. OYAMA-OKUBO,2,* T. ANDO,3 E. MARCHESI,4 and M. NAKAYAMA2) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3292273/ I recommend this reference for anyone that is totally into the chemistry of floral scents, it is on the technical side.