AbstractThe photosynthetic, optical, and morphological characteristics of a chlorophyll‐deficient (Chl‐deficient) “yellow” soybean mutant (MinnGold) were examined in comparison with 2 green varieties (MN0095 and Eiko). Despite the large difference in Chl content, similar leaf photosynthesis rates were maintained in the Chl‐deficient mutant by offsetting the reduced absorption of red photons by a small increase in photochemical efficiency and lower non‐photochemical quenching. When grown in the field, at full canopy cover, the mutants reflected a significantly larger proportion of incoming shortwave radiation, but the total canopy light absorption was only slightly reduced, most likely due to a deeper penetration of light into the canopy space. As a consequence, canopy‐scale gross primary production and ecosystem respiration were comparable between the Chl‐deficient mutant and the green variety. However, total biomass production was lower in the mutant, which indicates that processes other than steady state photosynthesis caused a reduction in biomass accumulation over time. Analysis of non‐photochemical quenching relaxation and gas exchange in Chl‐deficient and green leaves after transitions from high to low light conditions suggested that dynamic photosynthesis might be responsible for the reduced biomass production in the Chl‐deficient mutant under field conditions.