The role of light reactions in the regulation of anthocyanin synthesis in Petunia corollas

Abstract

The role of light reactions in anthocyanin synthesis was studied in both attached and detached corollas of Petunia hybrida (cv. Hit Parade Rosa), the latter grown in vitro in media containing 150 mM sucrose and 50 μM gibberellic acid (GA). Light was essential for the synthesis of anthocyanin in detached corollas, whereas in intact corollas its effect was only to enhance anthocyanin synthesis. Continuous white light at a fluence rate of at least 20 μmol m−2 s−1 was needed for anthocyanin synthesis in detached corollas. Blue light was more effective than red or green, and far‐red was ineffective. Pigmentation of detached corollas exposed to light was inhibited by the photosynthetic inhibitor 3‐(4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU). The chloroplast uncoupler NH4Cl did not affect anthocyanin synthesis, which was, however, inhibited by the blocking of ATP synthesis in both the chloroplast and the mitochondria by dicyclohexylcarbodiimide (DCCD). Sucrose uptake in vitro was inhibited by DCMU and by darkness, and was promoted equally by blue and red light. The activity of phenylalanine ammonialyase (EC 4.3.1.5) was inhibited in detached corollas grown in the dark or in the light in the presence of DCMU. The activity of chalcone isomerase (EC 5.5.1.6) was not affected by light. These findings suggest that at least two different light reactions are involved in the regulation of anthocyanin synthesis in petunia corollas, namely the high irradiance reaction (HIR) and photosynthesis.