*Article* **Effect of Light Intensity on Morphology, Photosynthesis and Carbon Metabolism of Alfalfa (***Medicago sativa***) Seedlings**

**Wei Tang <sup>1</sup> , Haipeng Guo <sup>1</sup> , Carol C. Baskin 2,3 , Wangdan Xiong <sup>1</sup> , Chao Yang <sup>1</sup> , Zhenyi Li <sup>1</sup> , Hui Song <sup>1</sup> , Tingru Wang <sup>1</sup> , Jianing Yin <sup>1</sup> , Xueli Wu <sup>1</sup> , Fuhong Miao <sup>1</sup> , Shangzhi Zhong <sup>1</sup> , Qibo Tao <sup>1</sup> , Yiran Zhao <sup>1</sup> and Juan Sun 1,\***


**Abstract:** To understand how light intensity influences plant morphology and photosynthesis in the forage crop alfalfa (*Medicago sativa* L. cv. Zhongmu 1), we investigated changes in leaf angle orientation, chlorophyll fluorescence, parameters of photosynthesis and expression of genes related to enzymes involved in photosynthesis, the Calvin cycle and carbon metabolism in alfalfa seedlings exposed to five light intensities (100, 200, 300, 400 and 500 µmol m−<sup>2</sup> s −1 ) under hydroponic conditions. Seedlings grown under low light intensities had significantly increased plant height, leaf hyponasty, specific leaf area, photosynthetic pigments, leaf nitrogen content and maximal PSII quantum yield, but the increased light-capturing capacity generated a carbon resource cost (e.g., decreased carbohydrates and biomass accumulation). Increased light intensity significantly improved leaf orientation toward the sun and upregulated the genes for Calvin cycle enzymes, thereby increasing photosynthetic capacity. Furthermore, high light (400 and 500 µmol m−<sup>2</sup> s −1 ) significantly enhanced carbohydrate accumulation, accompanied by gene upregulation and increased activity of sucrose and starch-synthesis-related enzymes and those involved in carbon metabolism. Together, these results advance our understanding of morphological and physiological regulation in shade avoidance in alfalfa, which would guide the identification of suitable spatial planting patterns in the agricultural system.

**Keywords:** alfalfa; light intensity; photosynthesis; growth; adaption
