The 18O signature of atmospheric water vapour (d18OV) 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 (MRTLW) and sugars (MRTSugars) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRTSugars was shorter in C3 and C4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRTSugars 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 d18OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to d18OV variations, which is important for the interpretation of d18O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.