To document the tritium uptake following chronic atmospheric exposure, potted lettuces (Lactuca sativa L.) were cultivated outdoors in the vicinity of a nuclear facility. During two sets of experiments performed in spring and autumn, the plants were continually exposed to low tritium levels throughout growth. These experiments factored in meteorological data, tritium monitoring in air (HT and HTO) and analysis of Tissue Free Water Tritium (TFWT) and Organically-Bound Tritium (OBT) in biological material. The global conversion rate of HTO to OBT in plants averaged 0.16% ⋅ h-1 over the growing period, but marked variations were observed during growth. In particular, a significant increase appeared at the exponential growth stage. Consequently, the usual predictive models (for cases of accidental or chronic exposure to tritium) should factor in the conversion rate according to the biological stage. Moreover, as plant growth is closely correlated to total energy received, meteorological parameters (temperature, light intensity) may be considered reliable indicators of growth.