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

<p>The classical definition of mesophyll conductance (g_m) represents an apparent parameter (g_m,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 g_m better describes a physical diffusional parameter (g_m,dif). They partitioned mesophyll resistance (r_m,dif = 1/g_m,dif) into two components, cell wall and plasmalemma resistance (r_wp) and chloroplast resistance (r_ch), and showed that g_m,app is sensitive to the ratio of photorespiratory (F) and respiratory (R_d) CO₂ release to net CO₂ uptake (A): g_m,app = g_m,dif/[1 + ω(F + R_d)/A], where ω is the fraction of r_ch in r_m,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 r_ch (in addition to r_wp and stomatal resistance) if this CO₂ is to escape from being refixed. Therefore, responses of g_m,app to (F + R_d)/A lie somewhere between no sensitivity in the classical method (σ =1) and high sensitivity in the model of Tholen et al. (σ =0).