Stage-specific optimization of light intensity enhances rice seedling quality and yield potential

Light intensity critically determines rice seedling growth and yield, yet the stage-specific photoacclimation mechanisms and optimal light regimes remain unclear. We investigated optimal light intensity combinations across developmental stages to enhance seedling quality, post-transplant growth, and yield. Two experiments were conducted: Experiment 1 applied seven light combinations (0-300, 100-300, 300-300, 700-300, 300-0, 300-100, 300-700 μmol m^-2 s^-1) during 0-1 and 1-3 leaf ages, while Experiment 2 independently tested five intensities (100, 300, 500, 700, 900 μmol m^-2 s^-1) across 0-1, 1-2, and 2-3 leaf ages. Seedlings under moderate light (300-300) during the 0-1 leaf age showed 25.7% higher shoot dry matter and 14.6% higher soluble sugar content at three-leaf stage compared to 700 μmol m^-2 s^-1 (700-300). 700 and 900 μmol m^-2 s^-1 during this initial phase induced malondialdehyde (MDA) and H₂O₂ accumulation, coupled with a dysfunctional antioxidant system, triggering oxidative damage and chlorophyll degradation. During 1-3 leaf age, dry matter, soluble sugar and starch contents increased with light intensity. High-quality seedlings with elevated non-structural carbohydrate reserves provided the energy and material substrates for tiller initiation, thereby promoting post-transplant tillering and increasing grain yield by enhancing the number of effective panicles. Our findings reveal stage-dependent photoacclimation mechanisms: oxidative protection (0-1 leaf age) transitioning to productivity enhancement (1-3 leaf age). Optimal light intensity management was 300 μmol m^-2 s^-1 during initial establishment to avoid oxidative stress, followed by a progressive increase thereafter. These results provide a quantifiable benchmark for precise light management in industrial rice nurseries to maximize yield potential.