The continuous efforts dedicated to increase the predictive power of risk assessment for the large tritium releases imply models based on process level analysis. Tritium transfer from atmosphere to plants and the subsequent conversion into organically bound tritium strongly depend on the plant characteristics, seasons, and meteorological conditions, which have a large variability. This paper presents an inter-comparison of different models for canopy resistance and photosynthesis based on knowledge from plant physiology, agro meteorology, crop science, and atmospheric physics. The authors use Jacobs-Calvet-Ronda approach to model the canopy resistance combined with photosynthesis model and the data base taken from WOFOST crop growth model. The same photosynthesis model is used to assess the organically bound tritium production during the daytime and night time.