Sunflower plant leaf area is very variable, even without any abiotic stress. We propose a structural-functional approach to analyse and model the variability of plant leaf area evolution and of plant architecture in response to temperature and light intensity in Helianthus annuus L. Contrasted experimental situations covering various light treatments between 2 and 40 mol m-2 d-1, including greenhouse, canopy and isolated plants layouts, with or without plant and leaves shading were studied. Radiative balance was estimated with MIR-MUSC-RADBAL software on 3D numerical plants constructed by the AMAPsim2 software. This approach enables analysing the effect of absorbed radiation on plant development, plant architecture and leaves expansion. Stable relationships related to the amount of absorbed radiation per leaf area unit were established with changes in (i) plant development pattern and (ii) in relative expansion rate of leaves. Duration of expansion was lengthened but relative growth rate was decreased in response to a reduction of absorbed radiation. These relations were used to propose an integrated model of plant leaf area with temperature and absorbed radiation as input data. The model reproduced satisfactory variations of plant development pattern and of relative expansion rates of leaves. This approach shows the existence of an adaptative response to limit the effect of a reduction of absorbed radiation on plant leaf area.