Red light regulation on growth, root-leaf architecture and photosynthetic performance of rice seedlings.

Given that the problem of insufficient light intensity is prevalent in the cultivation of rice seedlings under facility conditions, and light quality plays a crucial role in regulating crop growth, it is necessary to explore the most suitable proportion of red light for improving the quality and photosynthetic capacity of the seedlings. In this study, two different types of rice varieties (conventional rice, Xiangzaoxian 24, XZX24 and hybrid rice, Huazheyou 261, HZY261) were selected as experimental materials. A systematic investigation was conducted to examine the effects of different red light ratios on the morphological development of rice seedlings, the structure development of leaves and roots, as well as the photosynthetic characteristics. Six light quality treatments were applied, including white light (W, control), monochromatic red light (R), and combined white plus red light with different ratios (WR20, WR40, WR60, WR80). The results demonstrated that appropriate compound white-red light significantly coordinated shoot and root development; under the WR40 treatment (60% white light + 40% red light), the total seedling dry weight of the two varieties increased by 25.75% and 28.56% relative to the white light control, respectively. Gradually elevated red light proportion continuously boosted leaf pigment accumulation, with WR80 achieving the maximum total chlorophyll content, while WR60 optimized the non-photochemical quenching (NPQ) photoprotective capacity of leaves. Moderate red supplementation (WR20-WR60) maintained high Rubisco activity and upregulated the transcription of OsRBCS2 and OsRBCS3, whereas the expression of OsRBCS4 showed no significant difference across all light treatments. Single monochromatic red light failed to balance root branching and carbon assimilation efficiency despite promoting vertical leaf and stem elongation. This work provides quantitative spectral reference parameters for intelligent supplementary lighting in factory rice seedling raising, and clarifies the multi-dimensional physiological and molecular response differences of rice seedlings to graded red-white composite spectra. Further field verification trials are required to validate the applicability of the WR40 scheme under fluctuating natural environmental conditions.