Cadmium (Cd) contamination harms plant growth and human health. The application of hemin (ferroprotoporphyrin IX) can effectively relieve abiotic stresses in plants. This work investigates the effects of hemin on alleviating Cd toxicity and enhancing Cd tolerance in maize seedlings. In this study, maize seedlings were cultivated in nutrient solutions, with a combination of CdCl₂ (464 μmol L⁻¹) and hemin (100 μmol L⁻¹). We measured plant growth status, water status, Cd concentration, and Cd distribution in maize seedlings. The results indicated that Cd stress increased Cd accumulation in plants and inhibited plant growth. However, hemin alleviated the growth inhibition and improved water balance, root morphology, and root vitality under Cd stress. Additionally, hemin increased 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), total phenolic content, and phenylalanine ammonia lyase (PAL) activity to enhance tolerance to Cd stress. Hemin reduced Cd concentration, the translocation factor (TF), and the bioconcentration factor (BCF) in maize seedlings under Cd stress. Furthermore, hemin increased Cd concentrations in the cell wall and the soluble fraction of seedling roots, which helped reduce Cd transport from root to shoot. In summary, exogenous hemin could be used for alleviating adverse impacts on maize seedling induced by Cd stress.

While increasing planting density is a viable strategy for enhancing maize yield, it concurrently elevates the risks of lodging and accelerated leaf senescence due to intensified inter-plant competition, which can ultimately compromise yield stability. A field experiment was conducted in Heilongjiang Province and the study investigated two maize cultivars, JNK728 (Jingnongke 728) and SD5 (Saide 5), under high-density planting conditions (90,000 plants ha⁻¹). The treatments were arranged in a factorial design, incorporating four nitrogen levels (0, 120, 240, and 360 kg N ha⁻¹) in combination with the presence or absence of a chemical regulator (30% diethyl aminoethyl hexanoate · ethephon), with water serving as the control. Results demonstrated that the integration of 240 kg N ha⁻¹ with chemical regulation significantly enhanced photosynthetic capacity—elevating chlorophyll content (SPAD), net photosynthetic rate (Pn), and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) and phosphoenolpyruvate carboxylase (PEPCase)—while improving canopy structure through increased leaf area index (LAI) and optimized light distribution. This strategy also reinforced lodging resistance by optimizing plant morphology (reducing plant height and center of gravity), strengthening basal internodes (increasing stem diameter, dry weight per unit length, and mechanical strength), and promoting accumulation of stem structural components (cellulose, hemicellulose, lignin). Additionally, it facilitated post-anthesis nitrogen translocation to grains and up-regulated key nitrogen metabolism enzymes (glutamate synthetase-GS, glutamate dehydrogenase-GDH, and glutamate-pyruvate transaminase-GPT), thereby boosting nitrogen use efficiency. In contrast, excessive nitrogen (360 kg N ha⁻¹) suppressed these benefits and increased lodging. Consequently, the combined application of 240 kg N ha⁻¹ with chemical regulation achieved the highest yield, proving an effective approach for synergistically enhancing photosynthesis, lodging resistance, and nitrogen utilization in high-density maize systems.

Increasing planting density is an effective strategy for enhancing maize (Zea mays L.) yield. However, high density often inhibits plant growth and dry matter accumulation. Synergistic nitrogen management and chemical regulation offer an effective approach to overcoming yield limitations under high-density conditions. A two-year field experiment with two maize cultivars under high density (90,000 plants ha⁻¹), involving four nitrogen rates combined with PGR, explored their effects on root growth, yield formation, and greenhouse gas emissions. Results showed that 240 kg N ha⁻¹ significantly improved root morphological characteristics (root dry weight, root volume, root surface, root length) and physiological traits (bleeding sap rate, etc.), with chemical regulation providing additional enhancements. Additionally, nitrogen application increased the maximum grain-filling rate (V_max) and enzyme activity in grains, thereby enhancing grain weight; chemical regulation increased dry matter accumulation and its contribution to grains. Reduced nitrogen application combined with chemical regulation effectively decreased greenhouse gas emission. The highest maize yield was obtained under the application of 240 kg N ha⁻¹ combined with chemical regulation, which promoted root growth and grain formation, thereby improving yield and reducing emissions. This study indicates that the cultivation practice combining nitrogen application with chemical regulation provides an optimized approach for environmentally friendly and high-yield maize cultivation under high planting density.