The results presented here for soybean contrast with results for alfalfa obtained using the same methodology, in which photosynthesis of alfalfa grown and measured at elevated CO2 was nearly always lower than predicted by assuming limitation by VCmax, which indicated that rates were usually limited by J [5]. Reasons
why the results presented here for soybean concerning the nature
of the limitation to A at elevated CO2 at high PPFD differ from those
for soybean in the SoyFACE system [6] are not known, but might
be related to the fact that in that study, responses of A to Ci were
measured in the laboratory on excised leaves collected as much as
two days after the field measurements [6], rather than responses
of A to Ci measured in situ. This could also account for their conclusion
that leaves at ambient CO2 were mostly also limited by J
rather than VCmax even at midday, which is highly unusual [1]. The
appearance of J limitation could occur in this type of analysis if, for
example, photosynthesis in plants in the field were reduced at the