HEAT‐ AND LIGHT‐INDUCED CHLOROPHYLL <i>a</i> FLUORESCENCE CHANGES IN POTATO LEAVES CONTAINING HIGH OR LOW LEVELS OF THE CAROTENOID ZEAXANTHIN: INDICATIONS OF A REGULATORY EFFECT OF ZEAXANTHIN ON THYLAKOID MEMBRANE FLUIDITY

Abstract

Abstract Potato leaf discs were infiltrated in darkness with a buffer of pH 5 containing 100 M ascorbate, resulting in a massive conversion of the carotenoid violaxanthin to zeaxanthin. In vivo measurements of modulated chlorophyll a fluorescence indicated that this treatment (1) caused a marked upward shift of the threshold temperature at which photosystem II denatures and (2) noticeably inhibited the rate of dark reoxidation of the reduced plastoquinone (at low temperature). These changes were not induced in leaves infiltrated with a buffer of pH 5 containing no ascorbate or with 100 mM ascorbate at pH &gt;7.2. The above‐mentioned effects were also observed during heat acclimation (34°C for several days) of potato plants and suggested that zeaxanthin interacts with the lipid phase of the thylakoid membranes. Based on those results and the previous data obtained with model systems, it is suggested that the xanthophyll cycle could be a regulatory mechanism adjusting thylakoid membrane fluidity, the significance of which for the photoprotection of the photosynthetic apparatus is discussed.