La scopolétine et le cyanure d’hydrogène de manioc (Manihot esculenta Crantz) : propriétés biologiques et application dans la conservation de maϊs (Zea mays Linné)

Abstract

Scopoletin is a phytoalexin of the coumarin class biosynthetized by several plants including cassava (Manihot esculenta Crantz). This study sought to contribute to the reduction of post-harvest losses through the use of cassava cortex powder containing scopoletin. The study focused on 20 cassava varieties (10 locals and 10 improved), the differents parts of the cassava tuber (cortex, parenchyma, whole), as well as some processed products (gari, fufu, chips). Scopoletin and hydrogen cyanide (HCN) were quantified by Liquid Chromatography/Mass Spectrometry (LC/MS) and picrate paper methods. Agar incorporation and microdilution methods were used to evaluate the antifungal activity of cortex and parenchyma crude extracts of cassava variety "0110" on five mold isolates from stored maize (Aspergillus flavus (A. flavus), Aspergillus niger (A. niger), Fusarium sp, Penicillium sp and an unidentified isolate). The maize kernels preservation test was done by coating with cassava cortex powder "0110" at proportions of 2 mg, 4 mg and 8 mg of powder per 100 g of maize containing respectively 0.07 μg, 0.14 μg and 0.28 μg of scopoletin and 2.8 μg, 5.6 μg and 11.2 μg of HCN. The limit test method of OECD at 2000 mg/kg was used to determine the LD50 of cassava cortex extract "0110" in female Wistar rats and subacute toxicity was assessed in broilers by incorporating this powder into their food at doses of 2 mg, 4 mg and 8 mg/100 g of feed. The results showed that Cameroonian cassava varieties contain scopoletin, and the latter accumulates differentially depending on the variety. The levels in the parenchyma vary from 33.48 ± 0.02 to 155.32 ± 0.42 mg/kg (P > 0.05). The improved variety "0110" is the one that stands out with 155.32; 46.37 mg/kg of scopoletin and 116.43; 142.33 mg/kg of HCN in parenchyma and cortex respectively. Chips are processed product of cassava tuber that promote better accumulation of scopoletin. In addition to HCN content, varietal selection and technological treatments influence its accumulation. The crude extracts from the cortex and parenchyma showed fungicidal activity on the five mold isolates tested: MICs ranged from 0.30 to 2.50 μg/mL with the parenchyma extract and from 0.07 to 0.30 μg/mL with cortex extract. The dose of 8 mg per 100 g of maize (0.28 μg of scopoletin and 11.2 μg of HCN) was more effective in preserving maize. The toxicological study made it possible to classify this extract as a weakly toxic substance because the LD50 is greater than 2000 mg/kg. The effective antifungal dose obtained is non-toxic. Thus, the use of cassava cortex powder "0110" as a biopesticide is possible in order to limit post-harvest grain losses.