Foliar nickel fertilization enhances photosynthesis and defines critical leaf levels and optimal rate in maize

Nickel (Ni) use in agriculture has expanded, especially the foliar application of this micronutrient, which has had positive effects on the health and grain yield of several crops. Thus, critical levels of the element in the leaves and the optimal dose for the maize crop need to be identified. The aim of this study was to evaluate the effect of foliar application of Ni doses on gas exchange, nutrition, biochemistry, physiology and grain yield of maize plants. Field experiments were carried out in randomized block design with four replicates and five doses of foliar Ni fertilizing: 0; 20; 40; 80 and 160 g ha^{- 1}, divided into two applications, totaling 20 plots. A single application of Ni increased the total chlorophyll content by 38% with 70 g ha^{- 1} Ni in year I and 44% in year II when 80 g ha^{- 1} Ni was applied. Photosynthesis also improved, increasing by 46% in year I and 45% in year II. Ni content in the leaves varied from 1.02 to 3.35 mg kg^{- 1} in year I and from 0.33 to 6.32 mg kg^-¹ in year II, while the content in the grains increased from 0.68 to 1.03 mg kg^-¹ Ni in year I and from 0.13 to 1.05 mg kg^{- 1} in year II. Yield peaked at 46.50 g ha^{- 1} Ni in year I and 92.50 g ha^{- 1} in year II, increasing grain yield by 15.62% and 5%, respectively, compared to the control (no Ni). The appropriate range of Ni in the leaves was 1.29 to 1.86 mg kg^-¹ in year I and 1.40 to 3.60 mg kg^-¹ in year II, with toxic levels above 2.43 mg kg^-¹ and 5.80 mg kg^-¹, respectively, reducing grain yield by approximately 32% in year I and 6% in year II. Overall, foliar Ni fertilization improved photosynthesis and grain yield when applied within moderate ranges, although the magnitude of the response varied substantially between years. These results indicate that Ni use in maize should not rely on a single fixed dose but should instead be calibrated according to environmental conditions and crop responses. Grain Ni levels remained within acceptable limits, reinforcing that moderate foliar Ni application can be used safely when properly managed.