Objectives This study aims to investigate the impact of various substrate types and irrigation methods on the photosynthetic characteristics and water use efficiency (WUE) of Hydrangea macrophylla. It seeks to elucidate the plant's adaptive mechanisms to different substrate-irrigation combinations, providing insights for scientific irrigation and efficient cultivation.

Method A pot experiment was conducted using 2-year-old Hydrangea macrophylla (‘Hanatemari’) and an intelligent irrigation system was built in the greenhouse of Chenshan Botanical Garden in Shanghai. Three substrate types (S1: garden substrate, S2: 50% garden substrate + 50% garden waste, S3: 50% garden substrate + 40% garden waste + 10% biochar) and two irrigation methods (full irrigation, FI; deficit irrigation, DI) were set up. Plant height, crown width, and biomass of H. macrophylla were measured, and photosynthetic characteristics (gas exchange parameters, photosynthetic-CO₂ response curves, and relative chlorophyll content), stomatal morphological features (stomatal size and stomatal density), leaf water potential, and carbon isotope discrimination (Δ¹³C) were determined. Plant growth indices and water use efficiency were calculated, and correlation and principal component analyses were performed on the indicators.

Results Both substrate type and irrigation method significantly influenced the photosynthetic characteristics and water use efficiency of H. macrophylla (P<0.05). Compared with FI, under DI conditions, the net photosynthetic rate (P_n), stomatal conductance (G_S), transpiration rate (T_r), initial carboxylation rate (R_p), photorespiration rate (Γ), and relative chlorophyll content (SPAD) of H. macrophylla leaves were significantly reduced, but the addition of biochar (S3) significantly increased P_n, R_p, Γ, and SPAD values of leaves under DI conditions. Compared with FI, under DI conditions, the total water consumption of three substrate types showed a pattern of S1>S3>S2, saving water by 22.9%, 65.1%, and 63.1%, respectively. Under DI conditions, the plant growth index and above ground biomass of the different substrates showed a pattern of S1<S2<S3, with S2 and S3 significantly increasing the plant growth index by 16.7% and 30.1%, and above ground biomass by 0.5 times and 1.3 times compared with S1, respectively. S2 and S3 substrates maintained a high aboveground biomass while conserving water, and the addition of biochar (S3) significantly increased instantaneous water use efficiency (WUE_i), intrinsic water use efficiency (WUE_n), stomatal density, and leaf water potential, while significantly reducing Δ¹³C, maintaining the highest aboveground biomass and plant water use efficiency. Correlation analysis showed a highly significant negative correlation between Δ¹³C and both WUE_i and WUE_n, with Δ¹³C being more indicative of WUE_n; principal component analysis showed that R_p and WUE_n played a dominant role in photosynthetic characteristics and water use efficiency indicators.

Conclusions The synergistic application of DI and S3 substrate mitigates the adverse effects of water scarcity, improving leaf utilization of CO₂ , sustaining plant growth and augmenting WUE in H. macrophylla. This combination emerges as an effective cultivation strategy for enhancing the plant's water efficiency.