The exact mechanism is not fully understood, but its believed that the toxins in cigarette smoke may damage the melanocytes, the cells responsible for producing hair pigment
Besingi RN, Wenderska IB, Senadheera DB, Cvitkovitch DG, Long JR, Wen ZT, Brady LJ (2017) Functional amyloids in Streptococcus mutans, their use as targets of biofilm inhibition and initial characterization of SMU_63c
PMID: 24570145 pubmed.ncbi.nlm.nih.gov/24570145/
They're indistinguishable from regular petunias during the day, but thanks to some genes spliced in from naturally bioluminescent mushrooms, their leaves and flowers emit a soft green glow in the dark, increasing in intensity around the edges and at places of new growth