Exposure to different light intensities affects emission of volatiles and accumulations of both pigments and phenolics in Azolla filiculoides

Many agronomic trials demonstrated the nitrogen-fixing ability of the ferns Azolla spp. arid its obligate cyanobiont Trichormus azollae. In this study, we have screened the emission of volatile organic compounds (VOCs) and analyzed pigments (chlorophylls, carotenoids) as well as phenolic compounds in Azolla azollae symbionts exposed to different light intensities. Our results revealed VOC emission mainly comprising isoprene and methanol (similar to 82% and similar to 13% of the overall blend, respectively). In particular, by dissecting VOC emission from A. filiculoides and T. azollae, we found that the cyanobacterium does not emit isoprene, whereas it relevantly contributes to the methanol flux. Enhanced isoprene emission capacity (15.95 +/- 2.95 nmol m(-2) s(-1)), along with increased content of both phenolic compounds and carotenoids, was measured in A. filiculoides grown for long-term under high (700 mu mol m(-2) s(-1)) rather than medium (400 mu mol m(-2) s(-1)) and low (100 mu mol m(-2) s(-1)) light intensity. Moreover, lightresponses of chlorophyll fluorescence demonstrated that A. filiculoides was able to acclimate to high growth light. However, exposure of A. filiculoides from low (100 mu mol m(-2) s(-1)) to very high light (1000 mu mol m(-2) s(-1)) did not affect, in the short term, photosynthesis, but slightly decreased isoprene emission and leaf pigment content whereas, at the same time, dramatically raised the accumulation of phenolic compounds (i.e. deoxyanthocyanidins and phlobaphenes). Our results highlight a coordinated photoprotection mechanism consisting of isoprene emission and phenolic compounds accumulation employed by A. filiculoides to cope with increasing light intensities.