Photoprotection under low temperatures

Abstract

An unusual auxiliary xanthophyll cycle involving lutein epoxide and lutein operates in parallel with the violaxanthin cycle in leaves of an Australian mistletoe Amyema miquelii. Compared with summer, A. miquelii exhibited a marked increase in the pool size of the two xanthophyll cycles in winter (+60% and +120% increase on a Chl basis for the violaxanthin and lutein epoxide cycle, respectively) along with an extremely high de-epoxidation state (DPS=A+Z/V+A+Z). In sun leaves during winter, the violaxanthin cycle remained highly de-epoxidised overnight (pre-dawn DPS=0.77). The intrinsic efficiency of PSII (Fv/Fm) measured before dawn was remarkably low in winter mistletoe leaves (0.27), but then rapidly recovered in spring (0.68) as the xanthophyll cycle pool size and DPS decreased. The seasonal changes in DPS and Fv/Fm showed that the relationship between zeaxanthin concentration and Fv/Fm was not linear but saturated. Pronounced accumulation of b -carotene, another photoprotective pigment, was also observed in sun leaves in winter and spring (+115% increase on a Chl basis). The variations in the carotenoid concentrations, except for neoxanthin that remained unchanged, coincided with a decrease in Chl contents (-40%). Together, all these changes in the pigment composition determined the balance between light absorption, thermal dissipation and PSII electron transport in mistletoe leaves throughout different seasons. Despite the extensive acclimation in the pigment composition, damage symptoms were observed in some chloroplasts in mistletoe leaves in winter. Dynamic rearrangement of pigments in the parasite will be discussed in terms of cold acclimation and photoprotection.