4. DiscussionCichoric acid and rosmaric acid exist naturally as majorphenolic compounds in lettuce (Rajashekar et al., 2012; Beckeret al., 2013) and basil (Lee and Scagel, 2009; Taulavuori et al., 2013),respectively. In accordance with previous studies, cichoric acid wasthe most abundant phenolic compound found in red leaf lettuce(Table 2) and similarly rosmaric acid in basil (Table 3). Inaccordance with our previous study (Taulavuori et al., 2013), theenhanced blue light increased only few of theflavonoidcompounds in basil. This may indicate that the phenolic pool isnot the most responsive to changes in light quality in this plantspecies. This is supported by the fact that terpenoid biosynthesismay be highly increased under environmental stress in basil (Misraet al., 2014). And indeed, the blue light has induced essential oilproduction in basil, especially under blue light exposure (Amakiet al., 2011). However, the supplemental blue light has increasedrosmaric acid concentration up to 4 fold compared to plants grownunder high sodium pressure lamps (Taulavuori et al., 2013). In thepresent work, the rosmaric acid in basil was relatively high in alltreatments, but marginally decreased during the experiment inresponse to the extension of the blue light period.It has been shown that specifically red light increases therosmaric acid concentration in basil (Shiga et al., 2009). Thisindicates that rosmaric acid biosynthesis is reactive to both blueand red wavelengths. However, cichoric acid production wasboosted clearly by supplemental blue light in both species(Tables 2 and 3). When compared to our previous study (Taulavuoriet al., 2013) analogousfindings may be observed in concentrationsof chlorogenic acid, p-OH cinnamic acid derivative and feruloyl-tartaric acid in basil. There were no differences in concentrations ofthese compounds between blue and red light treatments in thisstudy (Table 3), while supplemental blue increased thesecompounds significantly compared to basil plants grown underhigh sodium pressure lamps (Taulavuori et al., 2013). This indicatesthat biosynthetic accumulation of these phenolic acids may beincreased both blue- and red- weighted spectra, similarly rosmaricacid in basil.The red leaf lettuce is known to be rich in quercetins andchlorogenic acid (Crozier et al., 2000; Arabbi et al., 2004; Beckeret al., 2013). Total phenols, chlorogenic acid and anthocyaninconcentrations in the seedlings of this cultivar were found to besignificantly higher under blue-containing LEDs (Johkan et al.,2010). There may exist a direct link between blue light biosynthesisof certain phenylpropanoids through photosynthesis. Becker et al.(2013) showed a close correlation between quercetin-glycosidesconcentrations with the reducing sugar concentration indicatingthat glucose might directly increase glucosylation of certainflavonoid aglycones to their glucosides. Accordingly, concentra-tions of two quercetin-glucosides increased in red leaf lettuceunder supplemental blue light (Table 2).The two species investigated responded differentially in termsof phytochemical accumulation, in accordance with thefirsthypothesis and previous literature (e.g., Julkunen-Tiitto, 1989;Crozier et al., 1997; Pichersky et al., 2006; Cheynier et al., 2013).Taken together, supplemental blue light increased many bioactivecompounds in red leaf lettuce, while in basil only two compoundsshow this response. In addition, although cichoric acid increasedunder supplemental blue light in basil, its concentration was alsorelatively high under the red-weighted light. Finally, lettuce andbasil yielded a different mixture of substances. Thus, thesefindingssupport the hypothesis that possible responses are speciesspecific.Consequently, the hypothesis that blue light rather thanred is behind the phenolic acid andflavonoid biosynthesis, is onlypartially supported. The species-specific difference was evident inrelation to hypothesis 3: certainflavonoids accumulated over timein red leaf lettuce while not in basil. However, hypothesis 4 –according to which the developmental stage may affect theresponse – can be neither supported nor rejected by the data.In addition to the light spectrum, the species-specific responsesmay also be modified by other aspects of light. For example, lightintensity may be a factor behind the regulation of these responses.Especially dihydroxy-B ring-substitutedflavonoids (e.g., quercetinglycosides) have been found to be up-regulated by high irradiance(Agati et al., 2013). As the light saturation level of basil is muchhigher, exceeding >1000 mmol m2 s1, compared to the respectivelevel (<300–400 mmol m2 s1) of lettuce (e.g., Kitaya et al., 1998;Chang et al., 2008), it is understandable that basil was not asresponsive to the experimental treatments as lettuce under thelight intensity supplied (300 mmol m2 s1). It should be rememberedthat red leaf lettuce is bred for production of highanthocyanins, the pigments causing the red color, and givingfurther protection against high light intensity.There were practically no differences in shoot elongation ofbasil grown under either red-weighted or blue dominating light(Fig. 4). This result is in accordance with the growth suppressingeffect of red light found by (Ballare, 2014, and references therein),and the conclusion that removal of blue light increased elongationof many species (Sarala et al., 2011). In Scots pine seedlings theobserved stem elongation is thought to be a photomorphogenicresponse, which is not dependent on gas exchange or accumulationof growth resources (Sarala et al., 2009). These reactions maybe explained byflavonoids through altered p-coumaroyl CoAmetabolism mediating photomorphogenic responses (e.g., Briggsand Huala, 1999; Folta and Spalding, 2001; Parks et al., 2001).Flavonoids and monolignols are synthesized by two differentroutes: if synthesis of quercetin and otherflavonoids is enhancedunder blue light it may lead to a decrease in the monolignolsneeded in lignin biosynthesis. This may cause decreased growththrough affecting chalcone synthase activity and auxin transport.In fact, Besseau et al. (2007) showed that repression of ligninsynthesis in Arabidopsis thaliana leads to the light-controlledredirection of the metabolicflux intoflavonoids and theyconcluded that the reduced size phenotype of the plants is dueto the presence offlavonoids. Moreover, inhibition of thecondensed tannin pathway leads to the accumulation of thecompounds preceding the phenylpropanoid pathways (Kosonenet al., 2015). The increase is especially strong inflavonolderivatives, for example quercetins which are known to be amongthe most active phenolic compounds in perturbing auxintransport. Moreover,flavonoids have been shown to be locatedin the nucleus where they may control the transcription of genesinvolved in growth and development (e.g., Saslowsky et al., 2005).For future prospects, it is interesting to identify in more details,which range or peak of the blue light is inducible for thebiosynthesis of a given specificflavonoid or phenolic acid. Indeed,the question concerns the UV radiation as well, since both UV-A(e.g., Wilson et al., 2001) and UV-B radiation (e.g., Gerhardt et al.,
2008) modify
flavonoid composition. The enzyme behind
flavo-
noid biosynthesis is chalcone synthase (CHS), and it is shown that
UV-A radiation and blue light inductions in CHS expression are
mediated by cryptochrome (cry1) (Wade et al., 2001). Phenylala-
nine, a precursor in
flavonoid biosynthesis, is required for specific
developmental responses mediated by UV radiation (Sullivan et al.,
2014). It is also well-known, that UV-B radiation limits growth and
alters plant morphology (e.g., Teramura and Sullivan,1994), i.e., the
changes also responsive to blue light (e.g., Sarala et al., 2011).
In conclusion, this study supports earlier work, according to
which the
flavonoid and phenolic acid biosynthesis in plants are
species (genetic) dependent. However, manipulation of the
environmental light spectrum may boost the synthesis of certain
products. Of the species studied, red leaf lettuce was much more
responsive to supplemental blue light. Supplemental blue light
increases production of these compounds especially in red leaf
lettuce, while both blue and red light may regulate their
production in basil. Some of the compounds detected accumulated
continuously as a function of the time spent under supplemental
blue light in red leaf lettuce, but not in basil.
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