Simple generalisation of a mesophyll resistance model for various intracellular arrangements of chloroplasts and mitochondria in C₃ leaves

The classical definition of mesophyll conductance (gm) represents an apparent parameter (gm,app) as it places (photo)respired CO2 at the same compartment where the carboxylation by Rubisco takes place. Recently, Tholen and co-workers developed a framework, in which gm better describes a physical diffusional parameter (gm,dif). They partitioned mesophyll resistance (rm,dif = 1/gm,dif) into two components, cell wall and plasmalemma resistance (rwp) and chloroplast resistance (rch), and showed that gm,app is sensitive to the ratio of photorespiratory (F) and respiratory (Rd) CO2 release to net CO2 uptake (A): gm,app = gm,dif/[1 + ω(F + Rd)/A], where ω is the fraction of rch in rm,dif. We herein extend the framework further by considering various scenarios for the intracellular arrangement of chloroplasts and mitochondria. We show that the formula of Tholen et al. implies either that mitochondria, where (photo)respired CO2 is released, locate between the plasmalemma and the chloroplast continuum or that CO2 in the cytosol is completely mixed. However, the model of Tholen et al. is still valid if ω is replaced by ω(1−σ), where σ is the fraction of (photo)respired CO2 that experiences rch (in addition to rwp and stomatal resistance) if this CO2 is to escape from being refixed. Therefore, responses of gm,app to (F + Rd)/A lie somewhere between no sensitivity in the classical method (σ =1) and high sensitivity in the model of Tholen et al. (σ =0).