Risk of Poisoning from Garden Plants: Misidentification between Laurel and Cherry Laurel

Abstract

The misidentification between edible and poisonous plants is an increasing problem because of the new trend to collect wild plants, especially by amateur collectors who do not have the botanical skills to distinguish between edible and toxic species. Moreover, morphologically similar species are sometimes responsible for accidental contamination or used in the intentional adulteration of products for human and animal consumption. Laurus nobilis L. (laurel) and Prunus laurocerasus L. (cherry laurel) are typical ornamental shrubs of the Mediterranean region. Laurel is considered a non-toxic plant, widely used as flavorings. Conversely, cherry laurel leaves, morphologically similar to those of laurel, contain toxic cyanogenic glycosides. Considering this, the aim of this study was to carry out an in-depth evaluation of laurel and cherry laurel leaves by using light and scanning electron microscopy coupled with three step phytochemical analyses (qualitative and quantitative colorimetric assays and liquid chromatography). This allowed to highlight the distinguishing features of plant species investigated features such as the venation pattern, presence/absence of nectaries, calcium oxalate crystals, secretory idioblasts, and cyanogenic glycosides. Concluding, this multidisciplinary approach can be useful for the identification of plants but also fragments or pruning residues containing cyanogenic glycosides, in quality control tests, intoxications, and criminal cases.

Keywords: Laurus nobilis; Prunus laurocerasus; edible plants; microscopy; phytochemistry; toxic plants.

Figure 1

Macro-morphology of the leaf: (A,B)—L. nobilis; (C,D)—P. laurocerasus; (A,C)—upper surface; (B,D)—lower surface.

Figure 2

Macro-morphological characteristics of the leaf lower surface: (A,B)—L. nobilis; (C,D)—P. laurocerasus. LM: leaf base near the petiole (A), leaf base near the petiole with extrafloral nectaries on both sides of the midrib (C), detail of the different venation pattern (B,D). En: Extrafloral nectaries (arrows). Bar = 2 mm.

Figure 3

Stomata in the lower epidermis of the leaf: (A–C)—L. nobilis; (D–F)—P. laurocerasus. LM: paracytic stomata enclosed in areoles (A,B), solitary stomata (D), anomocytic stomata (E). SEM: focus on a sunken paracytic stomata (C), focus on an anomocytic stomata (F). Ss: solitary stomata (arrows). Bar = 100 micron (A,D). Bar = 500 micron (B,E).

Figure 4

Micro-morphological characteristics of the leaf of L. nobilis (A–C) and P. laurocerasus (D–F). (A,D)—Phloroglucinol-HCl; (B)—Fluorol Yellow 088; (E)—Polarized light; (C,F)—SEM. L. nobilis: leaf cross section with slight collenchymatous protrusion (A); secretory idioblasts containing droplets of essential oil stained by Fluorol Yellow (B); cross-section showing secretory idioblasts without oil content, scattered in the mesophyll (C). P. laurocerasus: leaf cross section with evident collenchymatous protrusion (D); epidermal peel showing many calcium oxalate crystals along the veins (E); a single calcium oxalate crystal, at high magnification, within a collenchyma cell (F). Xy: xylem, Ph: phloem, Sc: sclerenchyma, Pr: collenchymatous protrusion, Oc: oil cell (arrows). Bar = 100 micron (A,B,E). Bar = 200 micron (D).

Figure 5

SEM–EDS structural characterization of a prismatic crystal of calcium oxalate in P. laurocerasus leaf. The peaks denoted by “Au” correspond to the gold sputter coating of the sample.

Figure 6

Semi-quantitative determination of cyanogenic glycosides by the picric acid method: (A) prunasin calibration curve, built by plotting the densitometric readings of the test strips against the standard amount (µg) tested. (B) Representative figure of the test strips, after treatment with different volumes (100, 50, 25, 12.50 µL) of the reference standard prunasin (STD, 1 mg/mL), blank (B) and with two different volumes (50 and 100 µL) of P. laurocerasus and L. nobilis leaf extracts (PL and LN, 1 mg/mL).

Figure 7

Spectrophotometric determination of cyanogenic glycosides with the pyridine–barbituric acid method: (A) calibration curve of prunasin built by plotting the absorbance against the standard amount (µg) tested; (B) representative figure of the colorimetric test after treatment with different volumes (25, 12.50, 6.25, 3.125, 1.5625, 0 µL) of the reference standard prunasin (1 mg/mL); (C) representative figure of the colorimetric test after treatment with different volumes (25 and 12.50 µL) of L. nobilis and P. laurocerasus leaf extracts (1 mg/mL).

Figure 8

Representative HPLC–DAD chromatogram acquired at 220 nm of the reference standard prunasin 20 µg/mL (A), leaf extract (1 mg/mL) of P. laurocerasus (B) and leaf extract (5 mg/mL) of L. nobilis (C).

Figure 9

L. nobilis (A) and P. laurocerasus (B).