Foliar zinc fertilization: Enhancing sorghum productivity and starch quality under zinc deficiency

Sorghum (Sorghum bicolor (L.) Moench), the fifth-largest cereal crop globally, faces productivity challenges due to soil zinc deficiency, which negatively impacts both yield and nutritional quality. This study aimed to evaluate the impact of foliar zinc fertilizer application on sorghum yield, grain filling dynamics, starch synthesis, and nutrient accumulation across two growing seasons. A comprehensive randomized field experiment was conducted with four treatments: a control (A1), foliar spray at the jointing stage (A2), at the heading stage (A3), and at both stages (A4). The zinc fertilizer was applied as a 0.2 % zinc sulfate solution at a rate of 800 L ha⁻¹, equating to 1.6 kg of zinc ha⁻¹. The results demonstrated that foliar application of zinc, especially at the jointing stage, significantly increased sorghum yield by 8.71 % and 9.22 % in 2021 and 2022, and 1,000-grain weight by 20.5 % and 16.5 %, respectively. This enhancement was attributed to the increased net photosynthetic rate and dry matter accumulation, which in turn boosted grain filling efficiency and starch synthesis. The upper panicle grains exhibited superior filling conditions, with the A2 treatment enhancing the upper grain weight by 12.67 % in 2021 and 6.30 % in 2022, respectively. The foliar zinc application notably elevated the activity of key enzymes involved in starch synthesis, such as ADP-glucose pyrophosphorylase (AGPase), starch branching enzyme (SBE), starch debranching enzyme (DBE), and soluble starch synthase (SS). Furthermore, the foliar application of zinc fertilizer significantly increased the zinc content in sorghum leaves and other organs, with the A2 and A4 treatments showing the highest enhancement. This increase in zinc content was accompanied by a positive effect on the absorption and accumulation of other essential nutrients, including nitrogen, phosphorus, and potassium, suggesting a synergistic role of zinc in improving overall plant nutrition. In summary, the foliar application of 0.2 % zinc sulfate fertilizer is an effective agricultural practice to improve sorghum yield and starch quality by enhancing photosynthesis, dry matter accumulation, and the activity of starch synthesis enzymes. These findings provide valuable insights for optimizing sorghum cultivation practices in regions prone to zinc deficiency.