Inhibition by light of CO2 evolution from dark respiration:: Comparison of two gas exchange methods

Two approaches to determine the fraction (mu) of mitochondrial respiration sustained during illumination by measuring CO2 gas exchange are compared. In single leaves, the respiration rate in the light (`day respiration' rate R d) is determined as the ordinate of the intersection point of A-c i curves at various photon flux densities and compared with the CO2 evolution rate in darkness (`night respiration' rate R n). Alternatively, using leaves with varying values of CO2 compensation concentration (Gamma), intracellular resistance (r i) and R n, an average number for mu can be derived from the linear regression between Gamma and the product r i.R n. Both methods also result in a number c* for that intercellular CO2 concentration at which net CO2 uptake rate is equal to -R d. c* is an approximate value of the photocompensation point Gamma* (Gamma in the absence of mitochondrial respiration), which is related to the CO2/O-2 specificity factor of Rubisco S c/o. The presuppositions and limitations for application of both approaches are discussed. In leaves of Nicotiana tabacum, at 22 degreesC, single leaf measurements resulted in mean values of mu = 0.71 and c(*) = 34 mumol mol(-1). At the photosynthetically active photon flux density of 960 mumol quanta m(-2) s(-1), nearly the same numbers were derived from the linear relationship between Gamma and r i.R n. c* and R d determined by single leaf measurements varied between 31 and 41 mumol mol(-1) and between 0.37 and 1.22 mumol m(-2) s(-1), respectively. A highly significant negative correlation between c* and R-d was found. From the regression equation we obtained estimates for Gamma* (39 mumol mol(-1)), S c/o (96.5 mol mol(-1)) and the mesophyll CO2 transfer resistance (7.0 mol(-1) m(2) s).