PARENT SESSION
Oral Session #69: Elevated CO₂ and Global Change. Presiding: L. Gough.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas H.

Respiratory quotient (CO₂/O₂) declines when Vitis canes are exposed to elevated CO₂.

Smart, David¹, Bloom, Arnold¹, Stockert, Christine¹, Nguyen, Tu¹, ¹

ABSTRACT- An increase in CO₂ concentration (pCO₂) around leaves of C–3 plants slows respiratory CO₂ efflux. The mechanism for this response is unknown and has eluded the scientific community for over a decade. Hypotheses advanced to explain this phenomenon can be generally divided into two categories: elevated O₂ 1) increases dark CO₂ fixation by PEP-carboxylase and other non-photosynthetic carboxylases, or 2) directly inhibits terminal oxidases or other respiratory enzymes. One line of experimental evidence that might differentiate between these alternatives involves the response of respiratory quotient (RQ), the ratio of CO₂ evolved to O₂ consumed (CO₂/O₂), to increased pCO₂. The first hypothesis would cause a downward RQ shift because net CO₂ evolved would decline while O₂ consumption by terminal oxidases would continue unabated. The second hypothesis would be supported by no change in RQ. We monitored the RQ of Vitis canes using a custom O₂ analyzer. Raising CO₂ concentration caused net CO₂ exchange to decline while net O₂ exchange remained constant. When CO₂ concentration around canes was raised from 30 to 80 Pa, RQ declined from 0.90 ± 0.02 (n = 5) to 0.83 ± 0.02. Raising pCO₂ further to 200 Pa caused the RQ to decline to 0.74 ± 0.02. No nitrate was detected in the canes, and the primary carbon source consumed was sucrose. Our investigation supports that the mechanism inhibiting respiration in Vitis cane tissue was dark CO₂ fixation.

KEY WORDS: respiration , elevated CO₂, pep carboxylase, respiratory quotient