Many common radiation-driven models of canopy photosynthesis, such as big-leaf and multi-layer models, rely on the assumption that leaf nitrogen (N_leaf) and thus leaf photosynthetic capacity decrease exponentially with integrated leaf irradiance.  Although this assumption may be invalid in some instances, results from many studies have demonstrated vertical gradients of N_leaf and photosynthetic capacity, especially within structurally uniform and even-aged monospecific canopies.  Furthermore, when modeling uniform monospecific canopies, violation of this assumption does not lead to substantial errors because most of the canopy photosynthate is produced in the uppermost strata of the canopy.  However, previous research has suggested that for subordinate plants grown within competitive and structurally complex canopies, vertical gradients of leaf N and photosynthetic capacity may not be well-correlated with irradiance.  Consequently, the assumption of an exponential decrease in photosynthetic capacity with irradiance may lead to substantial errors in estimates of photosynthesis of canopy sub-strata consisting of subordinate individuals, such as weeds growing within a dominant crop canopy.  Therefore, research based on field measurements of leaf gas exchange was conducted to determine the relationships between key measures of photosynthetic capacity (N_leaf; maximum rate of carboxylation, V_cmax; and maximum electron transport rate, J_max) and time-integrated leaf irradiance for two C₃ species (Brassica napus and Chenopodium album) and two C₄ species (Zea mays and Amaranthus retroflexus) for individuals grown as even-aged dominant plants in vertically-uniform monocultures and as even-aged subordinate plants in vertically non-uniform mixed-species canopies.  Preliminary results, pooled across species, demonstrated that V_cmax and J_max decreased with decreasing integrated leaf irradiance for both subordinates and dominants, but also that V_cmax decreased less with irradiance for subordinates than for dominants.  Conversely, the degree of decrease in J_max with irradiance did not differ among subordinates and dominants.