Critical soil and leaf zinc toxicity thresholds in field-grown maize cultivated on a clayey oxisol

Zinc (Zn) is one of the most important micronutrients for maize cultivation and, when applied in excess, may cause physiological damage to plants. However, studies on the critical toxicity levels of Zn in clayey Oxisol under tropical conditions are still scarce, limiting the establishment of safe corrective fertilization practices. We hypothesized that: (i) Zn rates and critical soil toxicity levels would be relatively high due to the strong adsorption in clayey soils, reflected in the low Zn recovery rate by plants, and that the critical foliar toxicity level would also be relatively high, indicating a relative tolerance of maize to Zn toxicity under field conditions; and (ii) the damage caused by Zn toxicity would be more associated with oxidative stress and photosynthetic limitation than with nutritional imbalances. The objective of this study was to determine the critical toxicity levels of Zn in soil and leaves, as well as to evaluate the physiological, nutritional, and productive responses of maize. The experiment was conducted under field conditions in Jaboticabal, Brazil, using a randomized complete block design with five Zn rates (0, 12, 24, 48, and 96 kg ha⁻¹), applied as zinc sulfate in a clayey Oxisol. Soil and leaf Zn concentrations, nutrient accumulation, gas exchange, malondialdehyde content, and grain yield were evaluated. The application of 80 kg ha⁻¹ Zn reduced maize productivity by inducing toxicity and was associated with critical Zn toxicity levels of 9.9 mg dm⁻³ in soil and 74 mg kg⁻¹ in leaves. The results indicated a relative tolerance of maize to Zn toxicity in clayey soil, evidenced by the low Zn recovery rate by plants (< 1.3%) and by the high critical toxicity levels observed in both soil and leaves. In addition, the damage caused by excess Zn was mainly associated with oxidative stress and reduced photosynthetic rate, whereas the nutritional changes were not sufficient to impair the nutritional status of the crop. This study contributes to the establishment of critical Zn toxicity levels for maize and provides a scientific basis for safer corrective Zn fertilization practices through soil incorporation in tropical agricultural systems.