Carotenoid turnover was investigated in mature leaves of Arabidopsis (Arabidopsis thaliana) by 14CO2 pulse-chase labeling under control-light (CL; 130 micromol photons m(-2) s(-1)) and high-light (HL; 1,000 micromol photons m(-2) s(-1)) conditions. Following a 30-min 14CO2 administration, photosynthetically fixed 14C was quickly incorporated in beta-carotene (beta-C) and chlorophyll a (Chl a) in all samples during a chase of up to 10 h. In contrast, 14C was not detected in Chl b and xanthophylls, even when steady-state amounts of the xanthophyll-cycle pigments and lutein increased markedly, presumably by de novo synthesis, in CL-grown plants under HL. Different light conditions during the chase did not affect the 14C fractions incorporated in beta-C and Chl a, whereas long-term HL acclimation significantly enhanced 14C labeling of Chl a but not beta-C. Consequently, the maximal 14C signal ratio between beta-C and Chl a was much lower in HL-grown plants (1:10) than in CL-grown plants (1:4). In lut5 mutants, containing alpha-carotene (alpha-C) together with reduced amounts of beta-C, remarkably high 14C labeling was found for alpha-C while the labeling efficiency of Chl a was similar to that of wild-type plants. The maximum 14C ratios between carotenes and Chl a were 1:2 for alpha-C:Chl a and 1:5 for beta-C:Chl a in CL-grown lut5 plants, suggesting high turnover of alpha-C. The data demonstrate continuous synthesis and degradation of carotenes and Chl a in photosynthesizing leaves and indicate distinct acclimatory responses of their turnover to changing irradiance. In addition, the results are discussed in the context of photosystem II repair cycle and D1 protein turnover.