Effect of Fertilization and Harvesting Time on Antioxidant Activity of Three Leafy Vegetables Commonly Used in Benin

Abstract

Leafy vegetables are an excellent source of bioactive factors, traditionally used as important medical ingredients. Recently, some leafy vegetables are domesticated without the use of fertilizer, as well as the assessment of the effect of fertilizer on their nutritional value. This study aims at testing the effect of three mineral and organic fertilizers (Cowpat, NPK and NPK + cowpat) on three traditional leafy vegetables: Ceratotheca sesamoïdes, Sesamum radiatum and Justicia tenella. Their antioxidant activities were assessed at different harvesting times varying from six (6) to fourteen (14) Weeks After Transplantation (WAT) using three in vitro methods: Ferric reducing antioxidant power (FRAP), 2,2’-Azinobis3-ethylbenzothiazoline-6- sulfonic acid (ABTS) and 2,2-diphenyl-1-picryl hydrazil (DPPH). The FRAP, DPPH, ABTS assays were consistent and positively correlated (p < 0.001). Total antioxidant activities of leafy vegetables depended on species, harvesting time and methods. They ranged from 32.0 to 45.7 μmol Fe/g DW (for FRAP), with non fertilized J. tenella cut at 12 WAT giving the highest antioxidant activity. Percentage of inhibition using DPPH assay ranged from 11.4 to 87.2 % and showed that J. tenella fertilized with NPK and cowpat, and harvested at 9 WAT had the highest antioxidant activity. Regarding ABTS, the range of 17.6 to 28.9 μmol TE/g DW was recorded, and the leaves of C. sesamoïdes harvested at 10 WAT and fertilized with cowpat showed the highest level. Compared to other species, those studied here may best contribute to improve human health related to degenerative diseases. Moreover, significant and positive correlations were observed between the total phenolic compounds content and antioxidant activities of leaves regardless the methods used. The positive and significant correlations between the three assays (FRAP, DPPH, ABTS) allow to suggest the use of only anyone of them to check factors in the study. This paper highlights the potential of antioxidant capability of the leafy vegetables even fertilized.

Key words: Fertilizer, antioxidant, Sesamum, Ceratotheca, Justicia