Zeaxanthin, a Molecule for Photoprotection in Many Different Environments

Abstract

Conversion of sunlight into photochemistry depends on photoprotective processes that allow safe use of sunlight over a broad range of environmental conditions. This review focuses on the ubiquity of photoprotection associated with a group of interconvertible leaf carotenoids, the xanthophyll cycle. We survey the striking plasticity of this process observed in nature with respect to (1) xanthophyll cycle pool size, (2) degree and speed of interconversion of its components, and (3) flexibility in the association between xanthophyll cycle conversion state and photoprotective dissipation of excess excitation energy. It is concluded that the components of this system can be independently tuned with a high degree of flexibility to produce a fit for different environments with various combinations of light, temperature, and other factors. In addition, the role of genetic variation is apparent from variation in the response of different species growing side-by-side in the same environment. These findings illustrate how field studies can generate insight into the adjustable levers that allow xanthophyll cycle-associated photoprotection to support plant photosynthetic productivity and survival in environments with unique combinations of environmental factors.

Features of the Duckweed Lemna That Support Rapid Growth under Extremes of Light Intensity

“…This was seen in the present study in the form of a rapid lowering of the rate of thermal energy dissipation upon transfer of high-light-grown L. gibba to low light with little to no sustained depression of photosystem II photochemical efficiency or photoinhibition. Notably, such rapid return to a high PSII photochemical efficiency is also seen in shade-tolerant species subsequent to exposure to rapid sunflecks in natural understory settings [3,41] as well as in sun-grown plants of terrestrial Araceae upon return to low light after extended exposure to high light ( [42]; see also [43]).…”

“…Evergreens and perennials often exhibit relatively lower maximal electron transport rates associated with very high fractions (around 90%) of absorbed light allocated to nonphotochemical routes as well as very high fractions of the xanthophyll cycle pool converted to zeaxanthin at midday in sunny, but otherwise favorable, habitats [3,16,22,45]. In contrast, annuals and biennials often exhibit relatively higher maximal electron transport rates and relatively lower fractions (around 50%) of absorbed light allocated to non-photochemical routes and fractions of the xanthophyll cycle pool converted to zeaxanthin at midday in the same habitats [3,16,22,45].…”

“…Small, floating plant species in the duckweed family (Lemnaceae) possess attractive nutritional features as they accumulate large quantities of high-quality protein (with all essential amino acids for humans) throughout the plant [1]. Furthermore, our group has highlighted the exceptional ability of Lemna gibba to accumulate high levels of the carotenoid zeaxanthin under conditions when the plant is growing rapidly [2,3]. Zeaxanthin (and its close isomer lutein) is an essential human micronutrient required to support brain function and fight systemic inflammation [4].…”

“…We previously reported a notable ability of L. gibba to maintain uniformly high growth rates, paired with profound modulation of photoprotection, over a range of growth photon flux densities (PFDs) from 100 to 700 µmol m −2 s −1 of continuous light [2]. Plants grown under higher PFDs exhibited higher levels of the interconvertible xanthophyll cycle carotenoids (violaxanthin, antheraxanthin, and zeaxanthin) and pronounced conversion to zeaxanthin that dissipates potentially harmful excess absorbed light [3,9,10]. In the present study, we further broadened the range of growth PFDs to test whether duckweed's phenotypic plasticity with respect to photoprotective capacity and maintenance of a high growth rate may extend to even more extreme growth PFDs.…”

“…Furthermore, the present study tested the hypothesis that the combination of exceptionally rapid growth with a remarkable ability to grow under a wide range of light intensities in duckweed may be associated with pigment patterns not seen in other fastgrowing species. In particular, prior studies of leaf pigment composition in slow-growing evergreens or perennials versus fast-growing annual species often reported an inverse relationship between growth rate and accumulation of photoprotective pigments (for a review, see [3]). The present study compared a population of Lemna minor in an open outdoor pond with a nearby population of the fast-growing terrestrial biennial weed Malva neglecta that was previously shown to exhibit a pigment composition and photoprotective capacity similar to that of fast-growing annual crop species [11].…”