The ¹⁸O signature of atmospheric water vapour (δ¹⁸O_V) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the ¹⁸O‐signal transfer differs among plant species and growth forms. We performed a 9‐hr greenhouse fog experiment (relative humidity ≥ 98%) with ¹⁸O‐depleted water vapour (–106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the ¹⁸O‐signal transfer by calculating the mean residence time of O in leaf water (MRT_LW) and sugars (MRT_Sugars) and related it to leaf traits and physiological drivers. MRT_LW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRT_Sugars was shorter in C₃ and C₄ plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRT_Sugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ¹⁸O_V variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ¹⁸O_V variations, which is important for the interpretation of δ¹⁸O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.