Enhancing rice seedling architecture and physiology in plant factories: an integrated approach via optimized light intensity and sowing density

In modern intensive agricultural production of rice, optimizing seedlings through rice seedling factories to promote rice productivity is an urgent problem that needs to be solved or improved. Although it is known that light intensity and sowing density affect the growth of rice seedlings separately, their synergistic regulatory effects and potential physiological mechanisms have not been fully explored, especially in the specific context of plant factory cultivation. This study aims to develop a synergistic regulation scheme that enhances rice seedling vigor through the combined optimization of light intensity and sowing density. The experiment used 8 rice varieties and set a combination of 3 light intensities (LL: 70, LM: 210, LH: 350 µmol m ⁻² s ⁻¹) and 3 sowing densities (D2: 2, D3: 3, D4: 4 seeds cm–2) to systematically evaluate their effects on seedling growth, photosynthetic characteristics, antioxidant system, and nitrogen metabolism. The results showed that the combination treatment of LH (350 µmol m ⁻² s ⁻¹) light intensity and D2 (2 seeds cm–2) density could synergistically optimize seedling morphology (leaf age, number of green leaves, stem base width), promote dry matter accumulation (fresh weight above ground, dry weight above ground and underground), improve root morphology (total root length, root surface area, number of intersections), and simultaneously enhance photosynthetic performance (chlorophyll content), nitrogen metabolism, and antioxidant capacity (SOD, POD, CAT). We determined leaf age as a key indicator for comprehensive evaluation of the synergistic effect of light density through grey relational analysis. Correlation analysis showed that leaf age was significantly positively correlated with soluble protein content (p < 0.01), while negatively correlated with white root number (p < 0.05), indicating that light intensity and sowing density synergistically optimize plant nitrogen metabolism and root morphology to form strong seedlings. Three-way ANOVA of variance further confirmed that leaf age is significantly regulated by all interactions between variety, light intensity, and density (p < 0.001), clarifying the synergistic effect of light density. Based on the comprehensive evaluation of membership functions and cluster analysis, ZLY819 and XZX45 were selected as the suitable varieties with the best comprehensive performance under LH-D2 optimization conditions. This study elucidated the physiological mechanism of light density synergistic regulation through multidimensional analysis, providing a theoretical basis and variety selection scheme for rice seedling cultivation in plant factories.