Drought decreases crop productivity, with economic consequences for farmers. For soybean, drought particularly affects the reproductive phase. There is therefore a need for strategies that minimize drought effects, such as agronomic fortification with micronutrients. Here, we evaluated the mitigation of drought stress in soybean using composite formulations of three micronutrient nanoparticles, ZnO, B2O3, and CuO, and their salts: ZnSO4·7H2O, H3BO3, and CuSO4·5H2O, in a greenhouse. The micronutrients were soil or foliar applied 3 weeks after seed germination. Drought was imposed at 50% field moisture capacity. We measured parameters related to growth, yield, and nutrient uptake dynamics during 19 weeks. Results show that drought decreased soybean shoot growth by 27% and grain yield by 54%. Application of salt formulations to soil was more effective than foliar application, in mitigating drought stress. For foliar application, the effects of nanoparticles and salts were similar. On average, the formulations reduced drought effects by increasing shoot growth by 33% and grain yield by 36%. On average, the formulations increased shoot N by 28%, K by 19%, Zn by 1080%, B by 74%, and Cu by 954%. Likewise, the formulations, on average, increased grain N by 35%, K by 32%, Zn by 68%, B by 56%, and Cu by 13%. In contrast, drought did not alter shoot P, but the formulations, on average, reduced shoot P by 33%. Whereas micronutrient salts are known to reduce drought effects in plants, our findings demonstrate for the first time a novel use of micronutrient nanoparticles to boost crop performance and N and P uptake under drought stress.
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