C3 gas-exchange analyses read bundle-sheath refixation in C3-C4 intermediate leaves as negative day respiration

Abstract

C3-C4 intermediate (C2) plants decarboxylate photorespiratory glycine in the bundle sheath, where Rubisco refixes some of the released CO2, yet researchers analyze their gas exchange with methods built for C3 leaves. I calibrated the C3-C4 model of von Caemmerer against published compensation points and Laisk measurements on C3 and C2 Flaveria and relatives at 36 {degrees}C, and analyzed the resulting leaf as an experimenter would. With its true Rubisco kinetics and mesophyll conductance, Farquhar-von Caemmerer-Berry fits described its CO2 response closely but returned a day respiration (Rd) of -5.4 mol m-2 s-1 against a true 1.2, and underestimated maximum carboxylation and electron transport rates by 12% and 11%. The Rd deficit equaled net bundle-sheath CO2 fixation where mesophyll CO2 equals the photorespiratory compensation point ({Gamma}*), within 5.3% in every leaf. The Laisk method gave {Gamma}* = 42 bar after mesophyll correction, compared with a true 65 bar, and the oxygen-slope method overestimated Rubisco specificity by 26-53%. Published curves of two C3-C4 Moricandia species returned Rd of -2.5 mol m-2 s-1, and their C3 congener 1.1. An Rd well below that of C3 relatives fitted the same way, signaling bundle-sheath refixation, not a fitting failure.