Application of metabolomics in assessment of functional diversity and quality traits of selected African solanaceae accessions

Abstract

African eggplants and tomatoes are among the most economically important and valuable vegetable fruit crops which have played an important historical role in the traditional diets in Africa. Their nutritional quality and diversified metabolites which contribute to their usefulness as food and the effect of drought stress on these metabolites has not been well defined. The study aimed at application of metabolomics in identification and characterization of metabolites in African eggplant and tomatoes under drought stress and assessment of antidiabetic activities of fruits of five selected African eggplant accessions. Seeds of the two accessions were obtained from World Vegetable Centre (AVRDC), Arusha, Tanzania. They were grown in the greenhouse and subjected to drought stress. Fresh African eggplant leaves were sampled at three different growth stages, namely before stress, 2 weeks and 4 weeks after stress. African eggplant and tomato fruits were harvested at three stages of ripening; mature green, breaker and mature red stages. Ascorbic acid and carotenoid contents were analyzed using HPLC. Ascorbic acid was seen to increase considerably during growth and fruit ripening, and it was significantly high in mature stages (week 4) and mature red fruits (p < 0.05). The most pronounced difference was also observed between control and stressed leaves where high ascorbic acid levels was observed in drought stressed GBK50591 leaves (28.49±1.20mg/100g), RV100265 fruits (23.57±2.81 mg/100g) and V1035028 fruits (44.42±0.58 mg/100g). Major carotenoids such as Xanthophylls (neoxanthin, violaxanthin, zeaxanthin and lutein) and carotenes (β–carotene and α–carotene), phytofluene, lycopene, phytoene as well as chlorophylls (chlorophyll-b and Chlorophyll-a) increased with growth and ripening stage of the crops. Although the leaves of the stressed crops reported decreased amount of carotenes, chlorophylls, neoxanthin and violaxanthin, the concentration of zeaxanthin increased with stress whereas lutein had no significant change (p > 0.05). There was an increase in lycopene during fruit ripening and a decrease in chlorophylls (a and b). Beta carotene increased, neoxanthin slightly decreased, while lutein was virtually constant during development. The total carotenoid contents varied from 1.07±0.16–23.21±4.61 mg/100g and 1.89±0.88–108.14±1.90 mg/100g for the African eggplant and tomato fruit tissues, respectively. Metabolite analyses were carried using a GC–MS and LC–MS and metabolite identification carried out with the Golm, Germany metabolomics library software. Proline, glutamate, sucrose, fructose and tricarboxylic acid cycle metabolites were shown to significantly increase with stress (p < 0.05). Principal component analysis (PCA) showed a clear discrimination between the different accessions, growth stages, and stress/control conditions. The results for antidiabetic studies showed that immediately after diabetes induction, the blood glucose was elevated in the mice and administration of the extracts significantly (p < 0.05) reduced the blood glucose levels. The effect was dose dependent with 300 mg/kg dosage showing good activity and maintaining the levels within the normal level. Metformin treated mice recorded the biggest fall in blood glucose levels (48.08 %) followed by RV10333 (44.86 %), RV10511 (41.66 %), RV10265 (40.01 %), RV101201 (35.23 %) and RV10445 (17.23 %). In addition, the treatment with the extracts did not induce any signs of toxicity. The results of this study illustrates the common aspects associated with drought stress effects on vegetable quality and indicate that drought stress affects the concentration of ascorbic acid and carotenoids as well as metabolites such as amino acids, sugars and organic acids. The findings also indicate that harvesting the leaves and fruits at mature stages has improved nutritional benefit. The study also suggests that the African eggplant fruits possess antidiabetic properties and their consumption may be important in management of diabetes.