IS THERE A CORRELATION BETWEEN LIGHT-INDUCED ZEAXANTHIN ACCUMULATION AND QUENCHING OF VARIABLE CHLOROPHYLL-A FLUORESCENCE

Studies with young leaves of a green and an aurea mutant of tobacco (Nicotiana tabacum L. su/su and Su/su) demonstrated that the aurea mutant Su/su, which efficiently accumulated zeaxanthin by photoconversion of violaxanthin and by de novo biosynthesis was better protected against photoinhibition (smaller decrease of Fv/Fm and Fv/Fo) in a 5 h high-light treatment than the green tobacco variety su/su, which exhibited only a low capacity for zeaxanthin formation. These results would underline a possible photoprotective function of zeaxanthin. Kinetic studies on the light-induced zeaxanthin accumulation and the decrease of the variable chlorophyll a fluorescence (Fv as well as:the ratios Fv/Fo and Fv/Fm) in green attached maple leaves (Acer platanoides L.) demonstrated, however, that though both processes are irradiance-dependent, they proceeded fairly independent of each other with different kinetics. Zeaxanthin formation via phototransformation of violaxanthin started at an irradiance above 300 mu mol m(-2) s(-1). Depending on the irradiance (range: 350-2000 mu mol m(-2) s(-1)) zeaxanthin formation was completed before a significant decrease in variable chlorophyll a fluorescence was detected. A light-induced decline in the variable fluorescence Fv and its ratios Fv/Fo and Fv/Fm continued even then when at medium irradiances (350 and 450 mu mol m(-2) s(-1)) the originally formed light-induced zeaxanthin was reepoxipided to violaxanthin. The results suggest that zeaxanthin does not play a major or specific role in a direct energy dissipation of absorbed light energy, but may possess a different mode of action. The mechanism of a possible photoprotective function of zeaxanthin is still open and requires new investigations.