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. and 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 filiculoides-T. azollae symbionts exposed to different light intensities. Our results revealed VOC emission mainly comprising isoprene and methanol (82% and 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 m2 s1), along with increased content of both phenolic compounds and carotenoids, was measured in A. filiculoides grown for long-term under high (700 ?mol m2 s1) rather than medium (400 ?mol m2 s1) and low (100 ?mol m2 s1) 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 ?mol m2 s1) to very high light (1000 ?mol m2 s1) 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. 1 | INTRODUCTION Species belonging