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Carotenoid turnover was investigated in mature leaves of Arabidopsis (Arabidopsis thaliana) by 14CO2 pulse-chase labeling under control-light (CL; 130 µmol photons m-2 s-1) and high-light (HL; 1,000 µmol photons m-2 s-1) conditions. Following a 30-min 14CO2 administration, photosynthetically fixed 14C was quickly incorporated in β-carotene (β-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 β-C and Chl a, whereas long-term HL acclimation significantly enhanced 14C labeling of Chl a but not β-C. Consequently, the maximal 14C signal ratio between β-C and Chl a was much lower in HL-grown plants (1:10) than in CL-grown plants (1:4). lut5 mutants, containing α-carotene (α-C) together with reduced amounts of β-C, remarkably high 14C labeling was found for α-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 α-C:Chl a and 1:5 for β-C:Chl a in CL-grown lut5 plants, suggesting high turnover of α-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. [ABSTRACT FROM AUTHOR]