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

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 + omega(F + R (d))/A], where omega 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 omega is replaced by omega(1-sigma), where sigma 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 (sigma =1) and high sensitivity in the model of Tholen et al. (sigma =0).