AbstractSink–source imbalance causes accumulation of nonstructural carbohydrates (NSCs) and photosynthetic downregulation. However, despite numerous studies, it remains unclear whether NSC accumulation or N deficiency more directly decreases steady‐state maximum photosynthesis and photosynthetic induction, as well as underlying gene expression profiles. We evaluated the relationship between photosynthetic capacity and NSC accumulation induced by cold girdling, sucrose feeding, and low nitrogen treatment in Glycine max and Phaseolus vulgaris. In G. max, changes in transcriptome profiles were further investigated, focusing on the physiological processes of photosynthesis and NSC accumulation. NSC accumulation decreased the maximum photosynthetic capacity and delayed photosynthetic induction in both species. In G. max, such photosynthetic downregulation was explained by coordinated downregulation of photosynthetic genes involved in the Calvin cycle, Rubisco activase, photochemical reactions, and stomatal opening. Furthermore, sink–source imbalance may have triggered a change in the balance of sugar‐phosphate translocators in chloroplast membranes, which may have promoted starch accumulation in chloroplasts. Our findings provide an overall picture of photosynthetic downregulation and NSC accumulation in G. max, demonstrating that photosynthetic downregulation is triggered by NSC accumulation and cannot be explained solely by N deficiency.