Effects of Different Maturity Groups on Photosynthetic Performance, Grain-Filling Dynamics, and Yield of Maize Under Film Mulching in a Cool Region

Spring maize production in cool regions of Northeast China is constrained by low temperatures, variable soil moisture, and limited thermal resources. A two-year field experiment was conducted in Dunhua, Jilin Province, China, in 2024 and 2025 to evaluate the growth, physiological performance, grain-filling characteristics, starch accumulation, and yield of six maize hybrids representing early-, medium–early-, and medium-maturing groups under biodegradable film mulching. Soil temperature and moisture content, photosynthetic traits, shoot dry matter accumulation, grain-filling parameters, starch synthase activity, starch content, and grain yield were measured. The medium–early-maturing group maintained higher net photosynthetic rate, stomatal conductance, and transpiration rate at the twelve-leaf stage (V12) and milk stage (R3) and accumulated more shoot dry matter at R3. Compared with the early-maturing group, the maximum grain-filling rate (Rmax), kernel weight at the maximum grain-filling rate (Wmax), and mean grain-filling rate (Vmean) increased by 10.0%, 12.1%, and 7.2%, respectively; compared with the medium-maturing group, the corresponding increases were 5.4%, 2.8%, and 9.0%, respectively. The medium–early-maturing group also maintained relatively high starch synthase activity and starch accumulation during the middle and late grain-filling stages. Averaged across the two years, grain yield was 20.0% and 9.1% higher than that of the early- and medium-maturing groups, respectively. Overall, the medium–early-maturing group achieved better coordination among canopy photosynthesis, dry matter accumulation, grain filling, and starch synthesis and showed better adaptation to the thermal conditions of this agroecological region, where the annual effective accumulated temperature is approximately 2300–2400 °C·d. Therefore, the medium–early-maturing group may represent a suitable option for achieving high yield under biodegradable film mulching in this region.