Testing the photoprotection hypothesis for foliar anthocyanin accumulation

Background/Question/Methods

Though plants rely on sunlight to excite chlorophyll and drive energy production via photosynthesis, too much sunlight can adversely affect plants. So-called excess visible light can lead to the formation of highly reactive forms of oxygen that can modify and disable constituents of the photosynthetic pathway, in doing so lowering photosynthetic efficiency. Plants have evolved a variety of mechanisms to counteract this threat.  A principal mechanism involves conversions among a suite of carotenoid pigments, the xanthophyll cycle, which allows plants to modulate the harmless dissipation of excess light as heat. A role in photoprotection has also been proposed for red anthocyanins, which may screen certain wavelengths of visible light when they accumulate at or near the adaxial (upper) surface of the leaf. Examining the photoprotective role of anthocyanins is complicated by the fact that leaf anthocyanin content generally co-varies with other protective features of leaf morphology and physiology in leaves of different developmental state or growth environment. Here, we compare the response to experimental light stress of a red- and a green-leafed variety of Coleus (Solenostemon scutellarioides), examining chlorophyll fluorescence emission and chlorophyll and carotenoid composition.

Results/Conclusions

After experimentally imposed high intensity white light, red- and green-leafed Coleus exhibited the same manifestations of light stress (decreased photosystem II efficiency). This, alone, could be interpreted as evidence that anthocyanins do not serve a photoprotective role. However, during excess light exposure, the green-leafed variety employed a greater level of xanthophyll cycle-mediated thermal energy dissipation, compensating for its lack of anthocyanin accumulation. During exposure to red light stress (note: anthocyanins do not absorb red light), levels of thermal energy dissipation did not differ between Coleus varieties. Taken together, our findings suggest that foliar anthocyanins can minimize stress associated with excess sunlight absorption. Thus anthocyanin accumulation should be considered alongside the suite of well-described photoprotective mechanisms employed by leaves, including thermal energy dissipation.