Fluorescence Imaging of Habanero and Bell Pepper Fruits

Abstract

Imaging of visible fluorescence from plants has evolved into an important research discipline with a broad spectrum of practical applications. Capsicum peppers are important commercial spice crops with many culinary and medicinal applications that are related to unique alkaloids, known as capsaicinoids, for the most pungent component, capsaicin [1]. Pepper fruits contain chlorophyll and many autofluorescent secondary metabolites, including carotenoids and other phytochemical, natural products that are differentially located in constituent cells and tissues, including the placental mesocarp, which accumulates capsaicin in swollen vesicles between the epidermis and cuticle [2, 3]. Habanero peppers have numerous intensely fluorescent vesicles and the most capsaicin. Sweet Bell peppers, known for their flavor and aroma, have vestigial vesicles with dim fluorescence and little or no capsaicin [4]. We used a TCS spectral confocal microscope and MZ FL3 stereofluorescence microscope to explore the distribution of fluorescence in placentas of both types of peppers, in an effort to further understand the mechanism of synthesis and secretion of capsaicinoids. Fluorescence of vesicles in habanero peppers was concentrated in an elevated cuticle and in large, cellular vacuoles adjacent to vesicles and not in the subtending droplet of oleoresin which stained with Nile Red, along with intercellular channels extending throughout the epidermis. The dim yellow fluorescence on placentas in Bell peppers was limited to small groups of enlarged cells. Correlative, topographical imaging of habanero placentas using scanning electron microscopy and the Haggis method of freeze-fracture [5] revealed (a) for the first time an intricate fine structure in the oleoresin of the vesicle, (b) patterned features of the cuticle covering the vesicle, and (c) vacuolated epidermal cells under the vesicle. Although only harvested pepper fruits from retail markets were examined, the distribution of Nile Red fluorescence in epidermal channels and vacuolated apical surfaces localized on the vesicular epidermis visualized by freeze-fracture suggest a complicated mechanism with multiple tissue factors involved in secretion and accumulation of oleoresin and capsaicin.