Asymmetrical effects of mesophyll conductance on fundamental photosynthetic parameters and their relationships estimated from leaf gas exchange measurements

Abstract

Worldwide measurements of nearly 130 C3 species covering all major plant functional types are analysed in conjunction with model simulations to determine the effects of mesophyll conductance (g(m)) on photosynthetic parameters and their relationships estimated from A/Ci curves. We find that an assumption of infinite g(m) results in up to 75% underestimation for maximum carboxylation rate V(cmax), 60% for maximum electron transport rate J(max), and 40% for triose phosphate utilization rate T(u) . V(cmax) is most sensitive, J(max) is less sensitive, and T(u) has the least sensitivity to the variation of g(m). Because of this asymmetrical effect of g(m), the ratios of J(max) to V(cmax), T(u) to V(cmax) and T(u) to J(max) are all overestimated. An infinite g(m) assumption also limits the freedom of variation of estimated parameters and artificially constrains parameter relationships to stronger shapes. These findings suggest the importance of quantifying g(m) for understanding in situ photosynthetic machinery functioning. We show that a nonzero resistance to CO2 movement in chloroplasts has small effects on estimated parameters. A non-linear function with gm as input is developed to convert the parameters estimated under an assumption of infinite gm to proper values. This function will facilitate gm representation in global carbon cycle models.

Impact of mesophyll diffusion on estimated global land CO 2 fertilization

“…We calculated leaf-scale R (Eq. 2) for about 130 C 3 species covering all major plant functional types of the world from herbaceous temperate plants to woody tropical species (15). This obtained leaf scale R is consistent with our global analyses with CLM4.5 (Fig.…”

“…V cmax is particularly sensitive to g m and is underestimated by as much as 75% if g m is assumed infinite (15). Therefore, the phenomenological parameters used in current carbon cycle models substantially undervalue the actual biochemical capacities of the photosynthetic machinery.…”

“…These parameters include the maximum carboxylation rate (V cmax ), maximum electron transport rate (J max ), and triose phosphate utilization rate (TPU) and have typically been estimated from leaf gas exchange measurements commonly known as A/C i curves obtained under carefully controlled environmental conditions (13,14). The parameter estimation procedures used in such efforts have treated mesophyll conductance (g m ) as if it were infinitely large, even though laboratory studies indicate that it is finite and that the mesosphyll diffusion limitation on photosynthesis can be substantial (1-9, 15, 16).Without explicit consideration of mesophyll diffusion, fundamental photosynthetic parameters inferred from A/C i curves are significantly underestimated (7,15,17). V cmax is particularly sensitive to g m and is underestimated by as much as 75% if g m is assumed infinite (15).…”

“…Without explicit consideration of mesophyll diffusion, fundamental photosynthetic parameters inferred from A/C i curves are significantly underestimated (7,15,17). V cmax is particularly sensitive to g m and is underestimated by as much as 75% if g m is assumed infinite (15).…”

“…This matching correspondence was achieved via a parameter conversion function that was developed from a global leaf gas exchange dataset collected by LeafWeb (leafweb.ornl.gov) (15,20). The development of this conversion function was based on the CLM4.5 formulation of the FvCB model (SI Text).…”