Accordingly, tolerance to shade stress increased at high net photosynthetic rate in C 3 plants ( Su et al., 2014). Reductions in light intensity could affect carbon balance of crop plant because the carbohydrate demand increases while its production decreases: rates of physiological processes rise while the photosynthetic yield reduces ( Lichtenthaler et al., 1981). Indeed, the numerous plant processes improve with increasing light intensity up to a moderate level which bring dramatic developmental and physiological changes to occur, leading to the rapid increase of these processes ( Yang et al., 2015 Wu et al., 2016). Taken together, light intensity is the main factors which controls the central process of plants such as germination, leaf proliferation and expansion, photosynthesis, buds and flower initiation, and cell division ( Kong et al., 2016 Wu et al., 2018 Yang et al., 2018b). However, shading environments increased the plant height and lodging rate which hinders the transportation of nutrients, water, and photosynthetic products and ultimately causes huge losses to agriculture production. In addition, crop plants produce smaller and thinner leaves under low light conditions than corresponding leaves in full sunlight conditions ( Wu et al., 2017). According to past comparative studies, the dry matter of roots, stems, leaves, and whole plant as well as the photosynthetic rate, transpiration, and stomatal conductance, and the stem diameter decreased in low light conditions ( Yang et al., 2014, 2017). For most crop plants, even a slight increase or decrease in light intensity leads to considerable changes in leaf morphology and structure ( Wu et al., 2017). Light intensity and quality are among the most critical environmental factors for crop physiology and biochemistry ( Yang et al., 2018a). Overall, these results suggested that 400 and 500 μmol m −2 s −1 is the optimum light intensity which positively changed the leaf orientation and adjusts leaf angle to perpendicular to coming light, consequently, soybean plants grow well under prevailing conditions. Furthermore, sucrose synthesis-related genes were also up-regulated by increasing light intensity, and the highest seed yield and yield related parameters were recorded in the L 400. The same trends were observed in the enzyme activity of sucrose-synthase, sucrose phosphate synthase, starch synthase, rubisco, phosphoenol pyruvate carboxykinase, and phosphoenol pyruvate phosphatase. Compared to L 100, the content of starch granules increased by 35.5, 122.0, 157.6, and 145.5%, respectively in L 400. Moreover, the cross-sectional area of chloroplast (C) outer membrane and starch grains (S), and sectional area ratio (S:C) was highest under L 400 and L 500, respectively. Leaf microstructure and chloroplast ultrastructure positively improved with increasing light intensity, and leaf-thickness, palisade, and spongy tissues-thickness were increased by 105, 90, and 370%, under L 500 than L 100. In addition, the cytochrome content (Chl a, Chl b, Car), net photosynthetic rate, chlorophyll fluorescence values of F v/ F m, F v ′/ F m ′, ETR, Φ PSII, and qP were increased as the light intensity increased, and higher values were noted under L 400. Leaf petiole movement and leaf hyponasty in each treatment has presented a tendency to decrease the leaf angle from L 500 to L 100. Compared with L 100, plant height, hypocotyl length, and abaxial leaf petiole angle were decreased, biomass, root:shoot ratio, and stem diameter were increased, extremum was almost observed in L 400 and L 500. A pot experiment was set up in a growth chamber under increasing light intensity treatments of 100 (L 100), 200 (L 200), 300 (L 300), 400 (L 400), and 500 (L 500) μmol m −2 s −1. An integrated approach combining morphology, physiology, biochemistry and genetic analysis was undertaken to study the light intensity effects on soybean growth and development. Therefore, it is an urgent need to determine the threshold light intensity to ensure sustainable soybean production under these systems. In intercropping systems shading conditions significantly impair the seed yield and quality of soybean, and rarely someone investigated the minimum amount of light requirement for soybean growth and development. 4College of Resources, Sichuan Agricultural University, Chengdu, China.3Maize Research Institute, Sichuan Agricultural University, Chengdu, China.2China Key Laboratory of Crop Ecophysiology and Farming System in Southwest, Ministry of Agriculture, Chengdu, China.1College of Agronomy, Sichuan Agricultural University, Chengdu, China. Lingyang Feng 1,2 † Muhammad Ali Raza 1,2 † Zhongchuan Li 1,2 † Yuankai Chen 1,2 † Muhammad Hayder Bin Khalid 1,3 Junbo Du 1,2 Weiguo Liu 1,2 Xiaoling Wu 1,2 Chun Song 1,2 Liang Yu 1,2 Zhongwei Zhang 4 Shu Yuan 4 Wenyu Yang 1,2 * Feng Yang 1,2 *
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