The response of mesophyll conductance to short-term variation in CO2 in the C4 plants Setaria viridis and Zea mays

Abstract

Mesophyll conductance (gm) limits rates of C3 photosynthesis but little is known about its role in C4 photosynthesis. If gm were to limit C4 photosynthesis, it would likely be at low CO2 concentrations (pCO2). However, data on C4-gm across ranges of pCO2 are scarce. We describe the response of C4-gm to short-term variation in pCO2, at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite gm calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO2. In both species, gm increased with decreasing pCO2. Including a finite gm resulted in either no change or increased ϕ compared with values calculated with infinite gm depending on whether the observed 13C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of gm and ϕ. At pCO2 below ambient, the photosynthetic rate was limited by CO2 availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO2, CA further constrained photosynthesis. High gm could increase CO2 assimilation at low pCO2 and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.

Fig. 1.

Responses of (A, B) photosynthetic rate (A) and (C, D) observed ¹³C photosynthetic discrimination (${\Delta }_{obs}^{13}$) to variation in the CO₂ partial pressure inside the leaf (C_i) in Setaria viridis (circles) and Zea mays (squares). In Setaria, three leaf temperatures (T_L) were measured: 40, 25, and 10 °C, as indicated in the key. Measurements in Zea were at T_L=25 °C. Values are means ±SE; n=4 in Setaria and n=3 in Zea.

Fig. 2.

The ratio of carbonic anhydrase-limited mesophyll conductance (CA-lim g_m) to CA-saturated g_m (CA-sat g_m) at different pCO₂ inside the leaf (C_i) in (A) Setaria viridis (circles) and (B) Zea mays (squares). Setaria was measured at three leaf temperatures (T_L): 40, 25, and 10 °C, as indicated in the key. Zea was measured at T_L=25 °C. Values are means ±SE; n=4 in Setaria and n=3 in Zea. An asterisk inside a symbol indicates CA-lim g_m/CA-sat g_m ≠ 1 with P<0.05. The arrows indicate the values at ambient pCO₂ and at 40 °C (black arrow), 25 °C (grey arrow), and 10 °C (dashed arrow).

Fig. 3.

The response of carbonic anhydrase-limited mesophyll conductance (CA-lim g_m) to changes in pCO₂ inside the leaf (C_i) in (A, C) Setaria viridis (circles) and (B) Zea mays (white squares). Setaria was measured at three leaf temperatures, as indicated at the top of the figure. Zea was measured at T_L=25 °C. For comparison, the available literature reports for Δ¹⁸O-g_m for different species and temperatures are included, as indicated in the keys: Ubierna et al. (2017)Setaria measured at T_L=40 °C, T_L=25 °C, and T_L=10 °C; Osborn et al. (2017)Setaria measured at T_L=25 °C; Barbour et al. (2016)Setaria measured with block temperature of 30 °C; Ubierna et al. (2017)Zea measured at T_L=25 °C; Barbour et al. (2016)Zea measured with block temperature of 30 °C. For all species and temperatures CA-lim g_m significantly varied with pCO₂. (D–F) The CO₂ response of normalized g_m, calculated by dividing individual values by the g_m at ambient pCO₂ at each temperature. Values are means ±SE; n=4 in Setaria, n=3 in Zea. The arrows indicate the values at ambient pCO₂: black, Setaria; grey, Zea.

Fig. 4.

Diffusional limitation to photosynthetic rate (A) imposed by stomatal resistance (L_s, Eqn 11, solid line), mesophyll resistance (L_m, Eqn 12, dashed line), and carbonic anhydrase (L_CA, Eqn 13, dotted line) as a function of the CO₂ supply (pCO₂) available for each (C_a, C_i, and C_m for L_s, L_m, and L_CA, respectively). (A) Setaria viridis at T_L=40 °C, (B) Setaria viridis at T_L=10 °C, (C) Setaria viridis at T_L=25 °C, and (D) Zea mays at T_L=25 °C.

Fig. 5.

Effect of different parameterizations of models of photosynthesis in the calculation of leakiness (ϕ) in (A) Setaria virids and (B) Zea mays at 25 °C and over a range of pCO₂ inside the leaf intercellular spaces (C_i). Model 1 (solid black line) uses in vitro V_pmax and g_m finite and equal to the values presented in Fig. 3; Model 2 (dashed grey line) uses in vivo V_pmax (which is variable with pCO₂, see Supplementary Fig. S2) and g_m infinite; Model 3 (dotted grey line) uses the same as Model 1 but the solution was only constrained by A and not Δ¹³C; Model 4 (dashed black line) uses the same as Model 1 but introducing V_pr (= 64 and 59 μmol m^–2 s^–1 in Setaria and Zea, respectively) in the calculation of V_p (Eqn 6). The rest of the variables included in these models were calculated as explained in the Methods section: values for some of them can be found in Supplementary Fig. S2. In Models 1, 2, and 4, ϕ was calculated with Eqns 14 or 16 (same result) and in Model 3, ϕ was calculated with Eqn 14. The symbols indicate the value of ϕ at ambient air pCO₂ calculated with the simplified Eqn 19 assuming either g_m finite (solid symbols) or infinite (clear symbols). Values are means ±SE; n=4 in Setaria, n=3 in Zea.

Fig. 6.

Δ¹³C (Eqn 9) as a function of C_m/C_a for different ϕ values (indicated by the numbers at the end of each line). For calculations we used values of 37, 36, 20, and 1364 Pa for C_a, C_L, C_i, and C_bs, respectively; t=0.0058, b₄=–4.49‰, and b₃=29.87‰. These values correspond to the mean values measured or calculated in Setaria at 25 °C and ambient pCO₂. Black symbols represent data for Zea and grey symbols for Setaria. For both species, ϕ was calculated assuming either g_m infinite (triangles) or g_m=2.00 and 2.43 μmol m^–2 s^–1 Pa^–1 in Setaria and Zea, respectively (circles).