For the measurements at 2% O2, a gas cylinder containing
a mixture of 2% O2 and 98% N2 was used. Gas from the cylinder
was humidified and supplied to the Li-Cor 6400 where CO2 was
blended with the gas. CO2 exchange data where the set Ca values
were lower than the ambient air value were corrected for leakage of CO2
into the leaf cuvette, using measurements with heat-killed leaves