Foliar Essential Oil Glands of Eucalyptus Subgenus Eucalyptus (Myrtaceae) Are a Rich Source of Flavonoids and Related Non-Volatile Constituents

Abstract

The sub-dermal secretory cavities (glands) embedded within the leaves of Eucalyptus (Myrtaceae) were once thought to be the exclusive repositories of monoterpene and sesquiterpene oils. Recent research has debunked this theory and shown that abundant non-volatile compounds also occur within foliar glands. In particular, glands of four species in subgenus Eucalyptus contain the biologically active flavanone pinocembrin. Pinocembrin shows great promise as a pharmaceutical and is predominantly plant-sourced, so Eucalyptus could be a potential commercial source of such compounds. To explore this we quantified and assessed the purity of pinocembrin in glands of 11 species of E. subg. Eucalyptus using Electro-Spray Ionisation Liquid Chromatography Mass Spectrometry of acetonitrile extracts and Gas Chromatography Mass Spectrometry analyses of hexane extracts of isolated glands which were free from other leaf tissues. Our results showed that the glands of subgenus Eucalyptus contain numerous flavanones that are structurally related to pinocembrin and often present in much greater abundance. The maximum concentration of pinocembrin was 2 mg g-1 dry leaf found in E. stellulata, whereas that of dimethylpinocembrin (5,7-dimethoxyflavanone) was 10 mg g-1 in E. oreades and that of pinostrobin (5-hydroxy-7-methoxyflavanone) was 12 mg g-1 in E. nitida. We also found that the flavanones are exclusively located within the foliar glands rather than distributed throughout leaf tissues. The flavanones differ from the non-methylated pinocembrin in the degree and positions of methylation. This finding is particularly important given the attractiveness of methylated flavonoids as pharmaceuticals and therapeutics. Another important finding was that glands of some members of the subgenus also contain flavanone O-glucosides and flavanone-β-triketone conjugates. In addition, glands contain free β-triketones, β-triketone heterodimers and chromone C-glucosides. Therefore, the foliar glands of this taxonomically distinct group of plants are a rich source of a range of flavonoids and other biologically active compounds with great commercial potential.

Emission of constitutive isoprene, induced monoterpenes, and other volatiles under high temperatures in Eucalyptus camaldulensis: A 13C labelling study

“…Indeed, we observed a threefold increase in dark respiration at temperatures >50°C, and the majority of CO 2 emitted was not labelled (data not shown). Previous studies have shown that respiration rates increase at temperature higher than 40°C in Eucalyptus spp., ( (Goodger, Seneratne, Nicolle, & Woodrow, 2016), and the increase in temperature, shifting isoprenoid equilibrium from liquid to gas phase, becomes the principal environmental driver of their emission (Grote, Monson, & Niinemets, 2013;Harley, 2013). Although isoprene is expected to be synthesized in the mesophyll and not stored in permanent pools (Niinemets, Loreto, & Reichstein, 2004), we cannot discard the idea that an isoprene pool that is stored in glands and is not quickly labelled by 13 C contributes to isoprene emission after volatilization at very high temperatures.…”

“…Previous studies have shown that respiration rates increase at temperature higher than 40°C in Eucalyptus spp., (Crous, Wallin, Atkin, Uddling, & Ekenstam, ; Way, Holly, Bruhn, Ball, & Atkin, ), with an optimum at about 55°C in Eucalytpus pauciflora (O'Sullivan et al, ). However, Eucalyptus leaves have numerous subdermal secretory cavities (glands) containing terpenes (Goodger, Seneratne, Nicolle, & Woodrow, ), and the increase in temperature, shifting isoprenoid equilibrium from liquid to gas phase, becomes the principal environmental driver of their emission (Grote, Monson, & Niinemets, ; Harley, ). Although isoprene is expected to be synthesized in the mesophyll and not stored in permanent pools (Niinemets, Loreto, & Reichstein, ), we cannot discard the idea that an isoprene pool that is stored in glands and is not quickly labelled by 13 C contributes to isoprene emission after volatilization at very high temperatures.…”

“…However, emitted monoterpenes were not labelled by 13 C, and their source of carbon must be older than that producing isoprene. Pools of monoterpenes are expected to be larger due to their lower volatility compared with isoprene (Loreto et al, 1996;Niinemets et al, 2004), and they are stored in specialized glands in eucalypts (Goodger et al, 2016). Nevertheless, there may be other factors that are limiting the process and for which isoprene and monoterpenes are competing (e.g., chemical energy or reducing power; Loreto & Sharkey, 1990;Morfopoulos et al, 2014).…”

High marker density GWAS provides novel insights into the genomic architecture of terpene oil yield in Eucalyptus

“…In the case of tea tree and Eucalyptus, terpenes are stored in secretory cavities within the leaf, and their capacity may set a limit to the accumulation of oil in a leaf (King et al, 2004). Goodger & Woodrow (2012) showed that foliar terpene…”

Quantitative Analysis of Various B-ring Unsubstituted and Substituted Flavonoids in Ten Australian Species of Eucalyptus

“…Feeding rates of animals were significantly depressed by B ring unsubstituted flavanones but not by flavanones with substitutions in the B ring or unsubstituted flavones [9]. Critically, it appears that B ring unsubstituted flavanones only occur in the subgenus Monocalyptus (= Eucalyptus) of the Eucalyptus genus [8,10].…”