Phenolic profile of a Parma violet unveiled by chemical and fluorescence imaging.

Moustafa Khatib,Alain Jauneau,Cécile Pouzet,Claude Lafitte,Marie-Thérèse Esquerré-Tugayé,Valérie Bonzon-Ponnet,Justine Chervin,Colleen Doherty

doi:10.1093/aobpla/plab041

Abstract

The ability of phenolic compounds to autofluoresce upon illumination by UV or blue light was exploited to explore the nature and distribution of these metabolites within the flower petals, leaves and roots of the violet, Viola alba subsp. dehnhardtii. This was achieved through a dual complementary approach that combined fluorescence microscopy imaging of living intact tissues and chemical extraction of pulverized material. The blue to red fluorescence displayed by living tissues upon illumination was indicative of their richness in phenolic compounds. Phenolic acids were found in all tissues, while flavonoids characterized the aerial part of the plant, anthocyanidins being restricted to the petals. The chemical quantification of phenolics in plant extracts confirmed their tissue-specific distribution and abundance. A key finding was that the spectral signatures obtained through confocal microscopy of endogenous fluorophores in living tissues and their counterpart extracts share the same fluorescence patterns, pointing out the potential of fluorescence imaging of intact organs for a proper estimation of their phenolic content. In addition, this study highlighted a few distinct morphology cell types, in particular foliar-glandular-like structures, and jagged petal cell walls. Altogether, these data provide a comprehensive histochemical localization of phenolics in living tissues of a violet. Converting fluorescence imaging into a chemical imprint indicated that one can rely on fluorescence microscopy of intact living tissues as a rapid, non-destructive means to follow their phenolic imprint under various environmental conditions.

Highlights

The Viola genus comprises some 500–600 species of violets and pansies whose fragrant and medicinal properties have long attracted botanists and herbalists
Converting fluorescence imaging into a chemical imprint indicated that one can rely on fluorescence microscopy of intact living tissues as a rapid, non-destructive means to follow their phenolic imprint under various environmental conditions
The blue fluorescence was distributed in patches throughout the blade which holds numerous stomata, each being surrounded by highly fluorescent epidermal cells arranged in a typical jigsaw puzzle-type pattern (Fig. 2F); an intense fluorescence was observed at the distal part of the midrib (Fig. 2G)

Summary

Introduction

The Viola genus comprises some 500–600 species of violets and pansies whose fragrant and medicinal properties have long attracted botanists and herbalists. In the mid-19th century, they were brought to Toulouse (Southern France) where they became, and still are, the emblematic flower of the city, known ‘Toulouse violet’ (Morard et al 2000). The uniqueness of this violet resides in its double, Received: 12 April 2021; Editorial decision: 1 June 2021; Accepted: 5 July 2021. Sweet-smelling chasmogamous flowers, which harbour 20–40 petals and petal-like stamens (Fig. 1).

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Discussion

Conclusion