C 3 gas-exchange analyses read bundle-sheath refixation in C 3 –C 4 intermediate leaves as negative day respiration
Year: 2026
Type: preprint
Abstract: C 3 –C 4 intermediate (C 2 ) 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 C 3 leaves. I calibrated the C 3 –C 4 model of von Caemmerer against published compensation points and Laisk measurements on C 3 and C 2 Flaveria and relatives at 36 °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 CO 2 response closely but returned a day respiration ( R d ) of −5.4 µmol m −2 s −1 against a true 1.2, and underestimated maximum carboxylation and electron transport rates by 12% and 11%. The R d deficit equaled net bundle-sheath CO 2 fixation where mesophyll CO 2 equals the photorespiratory compensation point (Γ*), within 5.3% in every leaf. The Laisk method gave Γ* = 42 µbar after mesophyll correction, compared with a true 65 µbar, and the oxygen-slope method overestimated Rubisco specificity by 26–53%. Published curves of two C 3 –C 4 Moricandia species returned R d of −2.5 µmol m −2 s −1 , and their C 3 congener 1.1. An R d well below that of C 3 relatives fitted the same way, signaling bundle-sheath refixation, not a fitting failure. Highlight In C 3 fits of simulated C 3 –C 4 intermediate leaves, day respiration falls below its true value due to net bundle-sheath CO 2 fixation and turns negative; published Moricandia data agree.
Author: Emre Yazar
Institution: Kahramanmaraş Sütçü İmam University
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7222485093