Mesophyll conductance response to short-term changes in pCO2 is related to leaf anatomy and biochemistry in diverse C4 grasses

Abstract

Mesophyll CO₂ conductance (g_m ) in C₃ species responds to short-term (minutes) changes in environment potentially due to changes in leaf anatomical and biochemical properties and measurement artefacts. Compared with C₃ species, there is less information on g_m responses to short-term changes in environmental conditions such as partial pressure of CO₂ (pCO₂ ) across diverse C₄ species and the potential determinants of these responses. Using 16 C₄ grasses we investigated the response of g_m to short-term changes in pCO₂ and its relationship with leaf anatomy and biochemistry. In general, g_m increased as pCO₂ decreased (statistically significant increase in 12 species), with percentage increases in g_m ranging from +13% to +250%. Greater increase in g_m at low pCO₂ was observed in species exhibiting relatively thinner mesophyll cell walls along with greater mesophyll surface area exposed to intercellular air spaces, leaf N, photosynthetic capacity and activities of phosphoenolpyruvate carboxylase and Rubisco. Species with greater CO₂ responses of g_m were also able to maintain their leaf water-use efficiencies (TE_i ) under low CO₂ . Our study advances understanding of CO₂ response of g_m in diverse C₄ species, identifies the key leaf traits related to this response and has implications for improving C₄ photosynthetic models and TE_i through modification of g_m .

Keywords: C4 photosynthesis; CO2 response of mesophyll conductance; PEPC affinity for bicarbonate (Km); carbonic anhydrase (CA); leaf anatomy; mesophyll cell wall thickness; phosphoenolpyruvate carboxylase (PEPC); water-use efficiency.

Fig. 1

Response of mesophyll conductance (g _m) to changes in the partial pressure of CO₂ (pCO₂) inside the leaf chamber (C _a) in 16 diverse C₄ grasses. Data for each of the species are shown separately from panels (a) to (p) along with the CO₂ response of g _m (black solid line) modelled using the equation, g _m = a × (34/C _a) ^b , where coefficient ‘a’ is the value of g _m at 34 Pa pCO₂ and coefficient ‘b’ is the rate of change in g _m with change in C _a. Mean ± SE values for the model constants (a and b) for each species are shown in Supporting Information Table S3. Measurements were performed at constant light (photosynthetic photon flux density (PPFD) = 1200 μmol m⁻² s⁻¹) and leaf temperature (25°C). Values in each panel represent the mean ± SE (green colour) with n = 3–6. Grey points indicate the replicate values for each species and CO₂ level. Response of g _m to changes in pCO₂ for each species is plotted in separate panel of Fig. 1. Species code has been indicated as the first letter of the genus and first three letters of the species (please refer to Table 1 for full names of species). P‐values from one‐way ANOVA along with Tukey's letters are shown.

Fig. 2

Relationship of coefficient b (or sensitivity of g _m, unitless) with (a) mesophyll cell wall thickness (T _CW), (b) mesophyll surface area exposed to intercellular air spaces (S_mes), (c) ratio of T _CW : S_mes, (d) stomatal ratio (SR), (e) stomatal density adaxial (SD_ada) and (f) leaf thickness among the 16 C₄ grasses measured in current study. Linear models were used for deriving regression coefficients (R ²) in all panels, except panels (a) and (c) for which we used a polynomial model. Significance of R ²: *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001. Each circle represents the mean ± SE value for each species (n = 3–6). Species names are indicated by the codes given in Table 1.

Fig. 3

Relationship of coefficient b (or sensitivity of g _m, unitless) with (a) phosphoenolpyruvate carboxylase (PEPC) activity, (b) Rubisco activity, (c) carbonic anhydrase (CA) activity expressed as k _CA, (d) PEPC affinity for HCO₃ ⁻ (K _m), (e) maximum photosynthetic capacity (A _max) and (f) leaf N content (N_area) among the 16 C₄ grasses measured in current study. Linear models were used for deriving regression coefficients (R ²) in all panels. In panel (c) R ² = 0.22⁺ after removing influential species pvir. Significance of R ²: +, marginally significant; *, P ≤ 0.05. Each circle represents the mean ± SE value for each species (n = 3–6). Species names are indicated by codes given in Table 1.

Fig. 4

Relationship of coefficient b (or sensitivity of g _m, unitless) with ratio of mesophyll cell wall thickness (T _CW) to (a) phosphoenolpyruvate carboxylase (PEPC) activity (b) Rubisco activity (c) PEPC affinity for HCO₃ ⁻ (K _m), (d) carbonic anhydrase (CA) activity expressed as k _CA, (e) maximum photosynthetic rates (A _max) and (f) leaf N content (N_area) among the 16 C₄ grasses measured in current study. Polynomial models were used to derive regression coefficients (R ²) in all panels. Significance of R ²: *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001. Each circle represents the mean ± SE value for each species (n = 3–6). Species names are indicated by codes given in Table 1.