Mutual shading and the photosynthetic capacity of exposed leaves of field grown soybeans

Light-saturated photosynthetic rates at air levels of carbon dioxide were measured about weekly in upper canopy leaves of two soybean cultivars grown at stand densities of 40 and 100 plants per square meter. Early in the season, when leaf area indices differed between stand densities, plants of both cultivars grown at high stand density had photosynthetic rates which averaged 23% lower than plants at low stand density. Later in the season, when there were no differences in leaf area index between stand densities, there were no differences in photosynthetic rates in the cultivar Kent, but rate differences of about 14% persisted in the cultivar Williams. In Williams mainstem leaves emerged into full sunlight later in their development at high than at low stand density. In both cultivars the oldest fully exposed leaves were photosynthetically immature for much of the season, as higher rates could be achieved by lower leaves which were shaded in situ. The results identify shading of young developing leaves and photosynthetic immaturity of fully exposed leaves as factors limiting canopy photosynthesis in soybeans, and indicate cultivar differences in how much high stand density reduces photosynthetic capacity.