Nanoparticulate materials have aroused growing commercial and scientific interests in several fields, including their use as fertilizer. Here, we evaluated the effects of a nanoparticulate zinc source (ZnO-NP), compared to bulk ZnO on biomass yield, mineral composition, chlorophyll fluorescence, and Zn concentration in cowpea (Vigna unguiculata) plants and grains. The two sources of Zn were applied to soil in five rates (0, 100, 300, 500, and 800 mg dm −3 ) to cover the micronutrient's sufficiency and toxicity ranges. Plants were grown for 110 days and concentrations of N, P, K, Fe, Cu, and Mn were determined in roots and shoots, while Zn and Fe were also measured in grains. The chlorophyll fluorescence and concentrations of the chlorophyll a, chlorophyll b, carotenoids, and xanthophyll pigments in leaves were measured. The results showed that at the rate of 100 mg dm −3 ZnO, the bulk and nanoparticulate Zn oxides adequately supplied the Zn concentrations required by cowpea, promoting increases in shoot and grain biomass. At doses higher than 300 mg dm −3 of ZnO, however, bulk ZnO application caused more significant toxicity in plants, promoting decreased biomass and damage to the photosynthetic apparatus. The chlorophyll fluorescence parameters were efficient in predicting the damage to cowpea plants' photosynthesis before the appearance of visual symptoms. The sources were similar in the increase of Zn in the grains and the reduction of the Fe concentrations; therefore, Zn biofortification programs would not benefit from nanoparticulate sources of the element via soil in the conditions of the present study.
