The threshold PPFD level was not examined in these experiments. Because soybean canopies often develop high leaf area indices, a large fraction of the leaves in soybean canopies spend much of the time exposed to limiting PPFDs, and even upper canopy leaves are exposed to limiting PPFDs in mornings and afternoons and under cloudy conditions. Failure of photosynthesis of leaves to be stimulated at elevated CO2 when PPFDs are limiting could have a significant impact on canopy photosynthesis at elevated CO2.
A similar lack of increase in photosynthesis measured at limiting
PPFDs also occurred in wheat plants grown at elevated CO2 [7]. This
could partly explain why the yield stimulation of wheat and soybean
by elevated CO2 is relatively small compared with cotton, for
example [25]. Understanding what limits soybean photosynthesis
at elevated CO2 at both high and low lightis importantto improving