Magnolia (Figlar and Nooteboom, 2004)

Porter, Elaine A., Kite, Geoffrey C., Veitch, Nigel C., Geoghegan, Ivey A., Larsson, Sonny & Simmonds, Monique S. J., 2015, Phenylethanoid glycosides in tepals of Magnolia salicifolia and their occurrence in flowers of Magnoliaceae, Phytochemistry 117, pp. 185-193 : 189-191

2.4. Systematic occurrence of phenylethanoid glycosides 1–6 in tepals of Magnolia and Liriodendron

Phenylethanoid glycosides were detected in the tepals of all 21 species (48 accessions) of Magnolia and Liriodendron that were studied ( Table 3 ). The species of Magnolia studied included examples from the subgenera Magnolia and Yulania , two of the three subgenera of Magnolia recognised by Figlar and Nooteboom (2004), and represented five of the eight sections in subgenus Magnolia and both sections of subgenus Yulania . Both species in Liriodendron were analysed. In the majority of species examined, one or more of the phenylethanoid glycosides 1–6 were among the major UV absorbing components in the tepal extract, along with one or more flavonoid glycosides. Exceptions were Magnolia acuminata , Magnolia delavayi , Magnolia liliiflora , Magnolia macrophylla and Magnolia obovata in which phenylethanoid glycosides were at lower levels both compared to the major flavonoid glycosides present and to other species ( Fig. 2 ).

Phenylethanoid glycosides were abundant in tepals of all members of subgenus Yulania section Yulania studied except for M. acuminata and M. liliiflora . Only 2 and 5 were detectable at trace levels in M. acuminata while all of 1–6 were detectable among the three accessions of M. liliiflora studied, although only one contained all six. M. acuminata is the sole member of subsection Tulipastrum and the only North American species of subgenus Yulania ; it has green to yellow tepals as opposed to the white, pink or purple tepals of all other species in subgenus Yulania , all of which originate from Asia ( Figlar and Nooteboom, 2004). It is of interest to note that M. liliiflora has been classified with M. acuminata as they share sepaloid tepals ( Chen and Nooteboom, 1993), although DNA sequence data shows the two to be distantly related ( Kim et al., 2001).

Three hybrids between species in subsection Yulania that were examined, Magnolia x loebneri ( Magnolia kobus x Magnolia stellata ), Magnolia x soulangeana ( Magnolia denudata x Magnolia liliiflora ) and Magnolia x veitchii ( Magnolia heptapetala x Magnolia campbellii ) ( Spongberg, 1976), also contained 1–6 and some of these were among the major components in the extract. In the widely planted ornamental hybrid M. x soulangeana (which to many people is synonymous with Magnolia ), the traits promoting the production of phenylethanoid glycosides gained from its maternal parent ( M. denudata ) clearly predominated over any responsible for the lower levels found in the paternal parent ( M. liliifora ). In Magnolia ‘Yellow lantern’, however, a hybrid between, M. acuminata and M. x soulangeana, the trait of lower phenylethanoid levels seemed to predominate, with only 2 being confirmed in the tepal extract that was otherwise dominated by rutin, as was the tepal extract of M. acuminata ( Fig. 2 ).

In section Michelia of subgenus Yulania , only one species, Magnolia figo , was available for analysis. The tepals of this species contained all of 1–6, with 2 and 5 being the dominant UV absorbing compounds in the extract. A similar result was obtained for Magnolia x fogii ‘Jack Fog’, a hybrid between two species in section Michelia ( M. figo x M. doltsopa ). This also contained all of 1–6 with 2 and 5 being the dominant UV absorbing compounds.

In subgenus Magnolia , phenylethenoid glycosides 2 and 5 (and sometimes 4 and 6) were major components in tepal extracts of species examined from section Magnolia and section Rytidospermum subsection Rytidospermum . These two groups, however, are not considered to be closely related ( Kim et al., 2001; Kim and Shu, 2013). Levels were low in species from other (sub)sections examined: M. delavayi (section Gwillimia ), M. macrophylla (- section Macrophylla ) and M. obovata (section Rytidospermum subsection Oyama ). In M. obovata an isomer of 5 was more abundant; verbascoside (5) isomers, namely magnolosides A and D, have been reported from the stem bark of M. obovata and the relat-ed species Magnolia officinalis ( Hasegawa et al., 1988; Yu et al., 2012) and are possible candidates for the observed compound in the tepals.

Levels were low in species from other (sub)sections examined: M. delavayi (section Gwillimia ), M. macrophylla (section Macrophylla ) and M. obovata and M. hypoleuca (section Rytidospermum subsection Oyama ).

A notable difference in the phenylethanoid glycoside chemistry of subgenera Magnolia and Yulania was in the occurrence of 1 and 3, the two phenylethanoid glycosides bearing a disaccharide at C-6 0 of the core glucose. Compounds 1 and 3 were detected in all studied taxa in subgenus Yulania (except M. acuminata as noted previously). In contrast, 3 was not detected in any studied members of subgenus Magnolia , except for M. macrophylla , and 1 was only dubiously detected in trace amounts in some species (the close elution of compounds giving isobaric ions hindered the scoring of very low levels of 1). In subgenus Magnolia , the absence of 3 was most notable in section Magnolia and section Rytidospermum subsection Rytidospermum in which some phenylethanoid glycosides were the major components in the tepal extracts. The presence of 2 and 4 in subgenus Magnolia , which are possible precursors of 1 and 3 respectively, suggests that members of this subgenus lack the ability to add an additional Glc to the glucose at C-6 0. A further difference between subgenera was noted in the major flavonoid glycoside present in the tepals: in subgenus Yulania , rutin was usually the predominant flavonoid, whereas in subgenus Magnolia kaempferol 3- O -rutinoside was the major flavonoid glycoside in those species that had high phenylethanoid levels and isorhamnetin 3- O -rutinoside was the main UV-absorbing component in M. macrophylla and M. delavayi ( Fig. 2 ), only in Rytidospermum subsection Oyama did rutin tend to predominate. Species in the two subsections of Rytidospermum therefore show differences in both their flavonol glycoside and phenylethanoid glycoside chemistry, although phylogenetic analysis of DNA sequence data suggests the two are related ( Kim et al., 2001; Kim and Shu, 2013).

The broader circumscription of Magnolia ( Figlar and Nooteboom, 2004) leaves Liriodendron as the only other genus in family Magnoliaceae . The two species of Liriodendron , L. chinense and L. tulipifera , contained 2 and 4–6, with 4 and 6 being particularly abundant in L. tulipifera while 5 was abundant in L. chinense . Neither contained 3 (or 1), although in L. tulipifera an isomer of 1 was present with a similar retention time to 1, but collision cell MS/MS suggested it was acylated with ferulic acid rather than caffeic acid. Rutin was the most abundant flavonol glycoside in L. chinense , whereas flavonol glycosides were not major components in the tepal extract of L. chinense .

Phylogenetic analyses of Magnolia based on DNA sequence data support the various sections of Magnolia but have largely not resolved the relationships between sections except for those in subgenus Yulania , which generally form a well supported clade ( Kim et al., 2001; Kim and Shu, 2013). Therefore all species examined in the present study that were found to produce 3 are placed in this clade, except for M. macrophylla , and conversely all species lacking 3 occurred outside this clade except for M. acuminata . The position of M. macrophylla is unstable in published phlyogenetic analyses. In the analysis of the ndhF locus ( Kim et al., 2001), M. macrophylla (together with M. dealbata ) was sister to all other species of Magnolia , whereas in the more recent analysis of ten chloroplast DNA sequences ( Kim and Shu, 2013), members of subgenus Magnolia sections Gwillimia and Talauma were unresolved sister groups to the remainder of Magnolia . In neither analysis is M. macrophylla sister to subgenus Yulania but in both analyses Magnolia species that constitute the early divergent branches with respect to Liriodendron contain relatively low levels of phenylethanoid glycosides (compared to the high levels in Liriodendron ).

The general agreement of phenylethanoid glycoside levels and the occurrence of 3 with certain subgeneric taxa in Magnolia suggests that these traits would provide additional phenotypic characters in future combined ‘morphological-molecular’ phylogenetic analyses of Magnolia aimed at understanding better the relationships between the sections. The presence of 1–6 in leaves of M. salicifolia also provides the possibility of obtaining a complementary dataset on phenylethanoid glycoside occurrence from leaf material.

Figures

Fig. 2. Maximum absorbance (300–500 nm) chromatograms from LC–UV analyses of 80% CH3OH extracts of Magnolia tepals (top to bottom): M. salicifolia (BI-19354), M. kobus (BI-19363), M. virginiana (BI-19682), M. sieboldii (BI-19731), M. macrophylla (BI-19560), M. delavayi (BI-19737) and M. acuminata (BI-20613). Peaks labelled the phenylethanoid glycosides 1–6 and the 3-O-rutinosides of quercetin (a), kaempferol (b) and isorhamnetin (c). The peak next to 2 in M. virginiana is an isomer of 2 that was not investigated further.