AbstractThe responsiveness of photosynthesis to continuously fluctuating light intensities can provide an estimate of the net assimilation rate (A) and photosynthetic light‐use efficiency (PLUE), which cannot be measured under steady‐state conditions. This study was conducted to investigate whether the leaves of rice (Oryza sativa L.) ‘Koshihikari’ and ‘Akitakomachi’ can efficiently utilize light under constantly fluctuating incident photosynthetic photon flux density (PPFD) and also to determine the effect of leaf N concentration on the acclimation of photosynthesis to changing PPFD. To this end, the instantaneous A and continually fluctuating incident PPFD were measured for 16 youngest fully expanded leaves of individual plants from dawn to dusk at the vegetative and reproductive phases under open‐air field conditions using two portable photosynthesis systems. The results revealed that (a) daily PLUE and calculated as the slope of the linear regression of A for values of PPFD between the light compensation point and 200 μmol m−2 s−1 can be improved with leaf N; (b) rice leaves do not utilize light efficiently under constantly fluctuating light conditions; and (c) the stomatal conductance per unit PPFD decreases under higher daily incident PPFD and diurnal fluctuation. Overall, our findings will help in refining crop photosynthesis models and would assist with the selection of high‐PLUE lines in breeding programs.