Shading inhibits ear and tassel development of summer maize by altering source-sink balance and regulating carbohydrate metabolism

Shading is a major environmental stressor that strongly affects maize growth, development, and yield formation. It is particularly problematic in high-density planting and intercropping systems, where inadequate light interception around the ear can cause substantial yield losses. To examine how shading influences young ear development, field experiments were conducted during the ear development period (jointing stage to tasseling stage) in the summers of 2023 and 2024, with a shading treatment (S) and a natural-light control (CK). Ear morphology, carbohydrate contents in ears, hormone levels, and leaf photosynthetic performance were measured. Shading significantly reduced leaf photosynthetic capacity in summer maize. Relative to CK, leaf sucrose content under shading declined by 65.0%, 24.2%, and 23.0% at V12 (12th leaf stage), V15 (15th leaf stage), and VT (tasseling stage), respectively, accompanied by reductions in plant biomass of 39.5%, 32.3%, and 41.9%. The weakened photosynthetic performance and reduced assimilate production under shading further constrained carbohydrate supply to, and utilization within, developing ears, resulting in significantly lower fructose, glucose, sucrose, and starch contents in both female and male ears. Shading also markedly altered abscisic acid (ABA), auxin (IAA), and cytokinin (CTK) levels in developing ears, leading to delayed ear development, smaller ear length and diameter, and a pronounced reduction in fertile floret number. Overall, shading during the ear stage inhibited normal ear development by limiting assimilate accumulation and disrupting carbohydrate metabolism in young ears, which contributed to a 32.2% reduction in kernels per ear and a 32.8% decrease in yield per plant. These findings clarify the physiological basis of shading-induced yield loss in maize and provide a foundation for breeding shade-tolerant cultivars and optimizing cultivation practices.