Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

Abstract

Summary The conductance of carbon dioxide (CO2) from the substomatal cavities to the initial sites of CO2 fixation (gm) can significantly reduce the availability of CO2 for photosynthesis. There have been many recent reviews on: (i) the importance of gm for accurately modelling net rates of CO2 assimilation, (ii) on how leaf biochemical and anatomical factors influence gm, (iii) the technical limitation of estimating gm, which cannot be directly measured, and (iv) how gm responds to long‐ and short‐term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two‐resistance model of gm that describe the potential of photorespiratory and respiratory CO2 released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three‐dimensional (3‐D) reaction‐diffusion models and 3‐D image analysis that are providing new insights into how the complex structure and organization of the leaf influences gm. Finally, because most of the reviews and literature on gm have traditionally focused on C3 plants we review in the final sections some of the recent developments, current understanding and measurement techniques of gm in C4 and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO2 concentrating mechanisms (C4 and CAM, respectively) that influence how we interpret and estimate gm compared with a C3 plants.

Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>

“…(A3.13) Figure 1 A simplistic 2-D schematic of CO2 fluxes in a C3 and C4 leaf modified from (Cousins et al, 2020). During photosynthesis, ambient CO2 (ca) diffuses into the leaf intercellular spaces (ci) through the boundary layer (rb) and stomata (rs).…”

“…For detail explanation about the type of plants the reader is directed to (Ehleringer and Monson, 1993;Ehleringer and Cerling, 2002;Ubierna et al, 2018;Ubierna et al, 2019;Cousins et al, 2020). In C3 plants, the CO2 diffuses through the boundary layer surrounding the leaf, the stomata, the intercellular air space, cell wall, plasma membrane, cytosol, chloroplast envelope and stroma where it is carboxylated into C3 acid by Ribulose-1,5-bisphosphate carboxylase oxygenase RubisCO (Farquhar et al, 1982;Evans et al, 2009;Ubierna et al, 2019;Cousins et al, 2020) (Figure 1). For C4 plants, CO2 is initially carboxylated into C4 acid by phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cytosol (Sage and Monson, 1998;Caemmerer et al, 2014;Cousins et al, 2020) (Figure 1).…”

“…In C3 plants, the CO2 diffuses through the boundary layer surrounding the leaf, the stomata, the intercellular air space, cell wall, plasma membrane, cytosol, chloroplast envelope and stroma where it is carboxylated into C3 acid by Ribulose-1,5-bisphosphate carboxylase oxygenase RubisCO (Farquhar et al, 1982;Evans et al, 2009;Ubierna et al, 2019;Cousins et al, 2020) (Figure 1). For C4 plants, CO2 is initially carboxylated into C4 acid by phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cytosol (Sage and Monson, 1998;Caemmerer et al, 2014;Cousins et al, 2020) (Figure 1). The mole fraction of CO2 at the CO2-H2O exchange site (cm) is an important parameter to determine the effect of photosynthesis on the triple oxygen isotope composition of atmospheric CO2.…”

Fuelling life: recent advances in photosynthesis research

“…In the final sections, the authors provide insights into current opportunities for further research in this area. In the next review, Cousins et al () discuss the processes involved in mesophyll conductance. While g m has been the subject of several reviews in the past, only recently have methodological developments allowed proper estimations of this parameter in C 4 species.…”

“…Hence, this is the first review specifically comparing g m in C 3 , C 4 , and CAM species. In addition, Cousins et al () highlight the recent advances in moving from a classical 1‐D or 2‐D perspective to more realistic – yet more difficult to parameterize – 3‐D models for g m . They discuss several mechanisms involved in the regulation of g m , including carbonic anhydrases (CAs).…”