The protective functions of carotenoid and flavonoid pigments against excess visible radiation at chilling temperature investigated in Arabidopsis npq and tt mutants

The npq1 mutant of Arabidopsis thaliana (L.) Heynh. has no xanthophyll cycl e due to a lack of functional violaxanthin de-epoxidase. Short-term exposur e ( < 2 days) of detached leaves or whole plants to the combination of high photon flux density (1,000 mu mol m(-2) s(-1)) and low temperature (10 deg reesC) resulted in PSII photoinhibition which was more acute in npq1 than i n the wild type. This increased photosensitivity of npq1 at chilling temper ature was attributable to the inhibition of nonphotochemical energy quenchi ng (NPQ) and not to the absence of zeaxanthin itself. In contrast to PSII, PSI was found to be phototolerant to chilling stress in the light in both g enotypes. In the long term (10-12 days), PSII activity recovered in both np q1 and wild type, indicating that A. thaliana is able to acclimate to chill ing stress in the light independently of the xanthophyll cycle. In npq1, ph otoacclimation involved a substantial reduction of the light-harvesting pig ment antenna of PSII and an improvement of photosynthetic electron transpor t. Chilling stress also induced synthesis of early light-inducedproteins (E LIPs) which, in the long term, disappeared in npq1 and remained stable in t he wild type. In both genotypes, photoacclimation at low temperature induce d the accumulation of various antioxidants including carotenoids (except be ta -carotene), vitamin E (alpha- and gamma -tocopherol) and non-photosynthe tic pigments (anthocyanins and other flavonoids). Analysis of flavonoid-def icient tt mutants revealed that UV/blue-light-absorbing flavonols have a st rong protective function against excess visible radiations. In contrast to the defect in npq1, the absence of flavonoids could not be overcome in the long term by compensatory mechanisms, leading to extensive photooxidative a nd photoinhibitory damage to the chloroplasts. Depth profiling of the leaf pigments by phase-resolved photoacoustic spectroscopy showed that the flavo noid-related photoprotection was due to light trapping, which decreased chl orophyll excitation by blue light. In contrast to flavonoids, the xanthophy ll cycle and the associated NPQ seem to be mainly relevant to the protectio n of photosynthesis against sudden increases in light intensity.