Can anthocyanins photoprotect purple genotypes of Ocimum basilicum L against boron toxicity and high light?

In many areas worldwide boron (B) is frequently added as fertilizer, while in other regions the concentration and/or the availability of B in soils or water sources exceed the requirements for crops production. Although the degree of tolerance can significantly differ among different species, and even among cultivars of the same species, once B becomes toxic it leads to visible symptoms of damages on leaf lamina. As a consequence, both the reduction of the photosynthetic leaf area caused by necrosis on and chlorosis as well as the impairment of chloroplast efficiency in leaf areas close to the necrosis strongly reduces the photosynthetic rate. Anthocyanin-rich plant morphs have been demonstrated many times more tolerant to a wide range of stresses (salinity, drought, heavy metal, etc.) compared to acyanic genotypes belonging to the same species. The reason of the higher tolerance has not always completely understood, albeit in most of the cases it is likely attributable to the photoprotection and/or the antioxidant ability of anthocyanins. Due to these features, anthocyanins can protect the photosynthetic machinery especially when light intensity exceeds the requirements of chloroplasts that are already partially compromised by other stressors. In view of above, in this work we explored the possibility that anthocyanins may exert a benefit for a largely used Mediterranean crop (Ocimum basilicum L.) and we utilized a purple (‘Red Rubin’) and a green-leafed (‘Tigullio’) cultivar of sweet basil to evaluate the effects of B toxicity. Our results demonstrate the negative effects of B on photosynthesis occurred either due to stomatal or mesophyll limitations independently to basil plants leaf pigmentation. Furthermore, anthocyanins effectively represent a benefit for plants subjected to high B (20 mg L-1) as leaves of ‘Red Rubin’ were less damaged than those of ‘Tigullio’. Being anthocyanins localized in the abaxial and adaxial epidermises of basil leaves, in both intact plants and protoplasts we demonstrated that the main advantage of anthocyanin-rich leaves consists in their photoprotective role. Indeed, while the purple cultivar has the possibility to screen a proportion of light otherwise absorbed by chloroplasts, the green cultivar ‘Tigullio’ cannot avoid surplus of excitation energy burden to chloroplasts already compromised by B toxicity. However, when we attempted to simulate the removal of the anthocyanic layer isolating protoplasts from leaf mesophyll of both ‘Tigullio’ and ‘Red Rubin’, the photosynthetic process was still less impaired and the degree of oxidative stress lower in acyanic protoplasts of ‘Red Rubin’ than in ‘Tigullio’ protoplasts treated with B. Thus, we postulate that in addition to the photoprotective role of anthocyanins in ‘Red Rubin’, other mechanisms, including a higher antioxidant pool, contribute to higher performance of that cultivar when exposed to concomitant conditions of excess B and light. Levels of both ascorbate and glutathione, key antioxidants in plant cells, are indeed 2-fold higher in ‘Red Rubin’ than in ‘Tigullio’. Noteworthy, ‘Red Rubin’ plants grown under high sunlight, exhibited typical morphological and biochemical features of plants grown under shade suggesting that the ‘shade syndrome’ described for anthocyanin-rich species plays a key role in plant adaptations to sunny environments. In conclusion, due to the sunscreen effect of anthocyanins, and, probably, for the high amount of antioxidants, ‘Red Rubin’ plants appear naturally equipped to survive in an environment, as the Mediterranean area, where light excess may represent a stressor, especially in concomitance to others such as B excess. Nevertheless, other possible functions of anthocyanins in B tolerance can be hypothesised, such as sequestration of boric acid and/or borate in the vacuole of epidermal cells where anthocyanins are localized.