Digitalis purpurea, valued for its cardiotonic glycosides, remains an important medicinal species. Optimizing cultivation to enhance photosynthetic efficiency is critical for improving both biomass accumulation and metabolite yield. In this study, we compared the photosynthetic induction responses of D. purpurea from low light to high light with those of Cucumis sativus, a high-performance reference species, to identify key physiological constraints. Compared with Cucumis, D. purpurea exhibited lower net photosynthetic rate (A) and maximum carboxylation rates (V_cmax) under both induction and steady-state conditions (A_f and V_cmaxf). The time required to reach steady-state photosynthesis was substantially longer in D. purpurea, resulting in significantly lower cumulative carbon gain (20.6 vs. 28.8 mmol m⁻²) and a higher carbon loss ratio (10.7% vs. 6.8%). In addition, the averaged WUE_i during induction in D. purpurea was 20.4% lower than in Cucumis; this reduction was exacerbated by continued stomatal opening after photosynthesis stabilized, leading to further inefficiency in water use. Limitation analysis further revealed contrasting dominant constraints: biochemical limitation accounted for 88.8% of total limitation in Cucumis, whereas stomatal limitation predominated in D. purpurea (64.3%). Together, these results highlight stomatal regulation as the primary bottleneck during photosynthetic induction in D. purpurea, leading to transient carbon losses and reduced water-use efficiency, providing a physiological basis for targeted cultivation strategies to improve both productivity and cardiotonic glycoside yield.