Optimizing foliar nutrient timing: Soluble monoammonium phosphate applications improve maize physiological parameters and productivity under water stress

Abstract In Brazil, most ( Zea mays L.) production is in the off‐season, following the summer soybean [ Glycine max (L.) Merrill] harvest, a period often affected by water scarcity during critical growth stages, which can substantially reduce yields. To address this challenge, foliar nutrient applications can mitigate drought damage if timed correctly. Thus, this study aimed to evaluate the effect of monoammonium phosphate (MAP: 12‐61‐00) via foliar application at various development stages on maize plant physiology and productivity under water‐limited conditions. The research employed a randomized block design with eight treatments and four replicates: a control; MAP applications at V 4 , V 6 , V 8 , R 1 , and MAP applied at all stages (MAP‐All) (all four stages: V 4 + V 6 + V 8 + R 1 ); and two additional treatments supplying nitrogen (N) via urea equivalent to that from MAP: N‐V 6 and nitrogen applied at all stages (N‐All) (V 4 + V 6 + V 8 + R 1 ). Assessments included leaf nutrient content, photosynthetic pigment content, gas exchange parameters, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) activity, lipid peroxidation, antioxidant enzyme activity, carbohydrate content, and grain yield (GY). The MAP‐All treatment demonstrated the most significant positive effects, enhancing the contents of chlorophyll a and b, total chlorophyll, and carotenoids and improving net photosynthesis, stomatal conductance, carboxylation efficiency, and Rubisco activity. This increase in photosynthetic efficiency led to reduced lipid peroxidation and the necessary actions of antioxidant enzymes, leading to greater sugar production in leaves. This resulted in an average increase in GY of 16% compared to the control, equivalent to 884 kg ha −1 . The N‐All treatment, despite providing a lower dose of N, increased GY by 10% compared to the control, or 408.6 kg ha −1 . These findings indicate that foliar MAP applications effectively enhance maize physiological conditions by supplying phosphorus and nitrogen to boost photosynthetic and productive metabolism in off‐season maize. Core Ideas Monoammonium phosphate (MAP) boosts chlorophyll, photosynthesis, and water use efficiency, reducing oxidative stress in maize under drought. N from MAP has an important physiological influence on maize plants. Regular MAP applications at multiple stages maximize maize resilience and yield under water‐limited conditions. Plain Language Summary This study evaluated the foliar application of monoammonium phosphate (MAP) at different growth stages of off‐season maize under water‐stress conditions. MAP improved chlorophyll content, photosynthetic rate, stomatal conductance, carboxylation efficiency, and ribulose‐1,5‐bisphosphate carboxylase/oxygenase activity. It also reduced oxidative stress by lowering reactive oxygen species and antioxidant enzyme activity while increasing sugar accumulation in leaves. The treatment with applications at all four stages (V4, V6, V8, and R1) had the strongest effect, resulting in a 16% increase in grain yield. These findings show that foliar MAP application is a practical and efficient strategy to improve maize physiological performance and yield under limited water availability during the off‐season.