Agronomic biofortification of lettuce with zinc under tropical conditions: Zinc content, biomass production and oxidative stress

•Foliar Zn content increased as much as 18-fold as a function of soil Zn concentration.•50 g of biofortified lettuce with Zn contributed up 18% to Zn daily intake.•Oxidative damage in lettuce increased at concentrations of 20 mg Zn dm−3 and higher.•SOD activity increased at concentrations 30 and 40 mg Zn dm−3 in both growing seasons.•Milder temperatures were more favorable for biofortifying lettuce with Zn.