Magnesium Fertilization Improves Grain Yield, Net Photosynthetic Rate, and Thousand Grain Weight of Spring Hybrid Maize (Zea mays L.) under Alkaline-Calcareous Soils of Southern Punjab

Abstract

Maize (Zea mays L.) productivity in southern Punjab is increasingly limited by latent secondary-nutrient deficiencies on the region's alkaline-calcareous soils, where free CaCO₃, low organic matter, and Ca²⁺-dominated cation exchange sites jointly constrain the plant availability of magnesium. Despite magnesium's central role as the chlorophyll metal, an essential Rubisco cofactor, and the principal cation for sucrose phloem loading, calibrated soil-applied Mg dose–response data for spring-planted hybrid maize on Pakistani alkaline soils are limited. A two-location field trial was therefore conducted during the Rabi 2024–25 season at Hasilpur and Chishtian (District Bahawalnagar) to evaluate the response of spring-planted DK6317 hybrid maize to five graded levels of magnesium sulphate heptahydrate (MgSO₄·7H₂O), at 0, 20, 40, 60, and 80 kg MgSO₄·7H₂O ha⁻¹ and applied as a broadcast basal alongside a uniform NPK (225–150–125 kg ha⁻¹) plus Zn–B supply. The experiment was laid out as a randomized complete block design with three replications at each site and analyzed as a two-factor combined ANOVA. Treatment effects were highly significant (p < 0.01) for grain yield, net photosynthetic rate (Pn), and thousand-grain weight (TGW). Pooled mean grain yield rose progressively from 7,624.1 kg ha⁻¹ at the control to 7,964.8 kg ha⁻¹ at the highest Mg rate (+4.5%), while Pn increased significantly from 24.67 to 29.90 mmol CO₂ m⁻² s⁻¹ (+21.2%) and TGW increased from 281.1 to 328.1 g (+16.7%) over the same Mg gradient. The disproportionately large gain in TGW, set alongside the parallel rise in Pn, established a coherent mechanistic chain in which magnesium supply enhanced photosynthetic capacity through the late grain-filling window and translated into greater photo-assimilate partitioning to the developing kernel. All three response curves were still rising at the highest tested rate. On this evidence, the inclusion of 80 kg MgSO₄·7H₂O ha⁻¹ as a basal soil broadcast input is recommended as a regional Mg fertilization practice for spring hybrid maize across the canal-irrigated districts of southern Punjab.