We assessed the effects of red:far-red (R:FR) ratio on net CO₂ assimilation, photosynthetic activity, and morphology of young white clover leaves developed under a simulated canopy light environment. Isolated cuttings of Trifolium repens L. were grown in controlled conditions under two irradiances in combination with two R:FR ratios. We used far-red light emitting diodes to reduce the R:FR ratio on leaves that were successively developing on the main stolon and were subject to strong competition for light within a sward. A leaf that developed under a low R:FR ratio had a lower chlorophyll content than the controls. However, this change had no consequences on the net CO₂ assimilation rate and on the biomass allocated to shoots. The low R:FR ratio modified the leaf morphology by increasing the leaf area. A leaf that developed in lower irradiance had a lower net CO₂ assimilation rate and a greater leaf area and petiole length than leaves that developed in higher irradiance. Therefore, irradiance was the main factor limiting the leaf CO₂ assimilation under a canopy. Capture of light by clover leaves was improved by FR light only via modifications of leaf morphology, because there were no significant effects of FR light on photosynthesis.