The data presented here indicated that the type of system for growing plants at elevated CO2, FACE or OTC, did not affect the biochemical factor limiting photosynthesis at elevated CO2, when the same cultivars were compared in the same location.
Similar to prior data on soybean grown in the SoyFACE system [7] our data on photosynthesis measured in situ at low PPFD in soybeanalso indicatednonet stimulationof photosynthesis at elevated CO2 (Table 3). Our data further indicate that rates of A of plants grownat ambientCO2 were significantlyhigher atthe limiting PPFD than for leaves grown at elevated CO2 at both measurement CO2 concentrations (Table 3). This indicated that a down-regulation of photosynthesis measured at limiting PPFD occurred in soybeans grown at elevated CO2, both in the FACE systems and in the OTC,
in both cultivars. Down-regulation of photosynthesis measured at limiting PPFD during growth at elevated CO2 has been observed in
several different species [13–16,22,23], including soybean grown
in indoor chambers [24]. The occurrence of down-regulation of
photosynthesis measured at limiting PPFD can be independent of
down-regulation of photosynthesis measured at high PPFD [17]
as it was in these cases. In the soyFACE experiments, no stimulation
in photosynthesis in the field in plants at elevated CO2 was
observed at PPFD values less than about 1000 mol m−2 s−1 [7].
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