Turnover of Anthocyanins and Tissue Compartmentation of Anthocyanin Biosynthesis in Flowers of Petunia hybrida.

The synthesis and degradation of anthocyanins in developing flower buds and in mature flowers of Petunia hybrida were investigated by incubating petals in the presence or absence of an inhibitor of anthocyanin synthesis (α-aminooxyacetic acid). In flower buds only synthesis and no degradation of anthocyanins was observed, whereas in mature flowers no synthesis and a very slow degradation was found (about 10 percent was degraded after 48 hours). During flower bud development the increase in anthocyanin content coincided with an increase in activity of three biosynthetic enzymes: chalcone isomerase, uridine diphosphoglucose: flavonoid 3-O-glucosyltransferase and S-adenosyl-L-methionine: anthocyariidin 3-(p-coumaroyl)-rutinoside-5-glucoside 3',5'-O-methyltransferase. Anthocyanins accumulate in the epidermal layers of the flower. In mature flowers, glucosyltransferase and methyltransferase, both involved in later steps of biosynthesis were mainly found (93-100 %) in the epidermal layers, whereas chalcone isomerase was evenly distributed between epidermal and mesophyll tissue. We suggest that biosynthesis of anthocyanins occurs in the epidermis and that a possible involvement of the mesophyll is restricted to providing C9-precursors of flavonoids.