Increasing water use efficiency along the C3 to C4 evolutionary pathway: a stomatal optimization perspective

Abstract

C4 photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO2 concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C3 to C4 photosynthesis at current (400 μmol mol(-1)) and low (280 μmol mol(-1)) atmospheric CO2 conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C3 to C4 species within a unified framework. Data on C3, three categories of C3-C4 intermediates, and C4 Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO2-concentrating mechanism (or C4 pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C3 to C4 transformation. Neither the marginal water use efficiency nor the C4 pump strength changed significantly from C3 to early C3-C4 intermediate stages, but both traits significantly increased between early C3-C4 intermediates and the C4-like intermediates with an operational C4 cycle. At low CO2, net photosynthetic rates showed continuous increases from a C3 state, across the intermediates and towards C4 photosynthesis, but only C4-like intermediates and C4 species (with an operational C4 cycle) had higher water use efficiencies than C3 Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4 Flaveria to improve their photosynthesis and water use efficiency compared with C3 species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related.

Keywords: C3 photosynthesis; C3–C4 intermediates; leaf gas exchange; photosynthetic model; stomatal conductance; water use efficiency..

Fig. 1.

(A) Responses of net CO₂ assimilation rates (A _net) to increases in intercellular CO₂ concentration (c _i), relativized to maximum A _net (=A _max) for each photosynthetic type, as commonly presented in the literature. (B) Estimated η for each photosynthetic type. Means ±SE, n indicated at the top, dotted line indicates η=1. C₃ species, purple circles and solid line; Type I species, blue diamonds and dashed line; Type II species, green triangles and dotted line; C₄-like species, yellow inverted triangles and dashed-dotted line; C₄ species, red squares and solid line.

Fig. 2.

Relationships between the pump strength of the CO₂-concentrating mechanism (η) and: (A) maximum carboxylation capacity of Rubisco (V _c,max); (B) $K_{cair}=K_c\left(1+O/K_o\right)$ ; and (C) the CO₂ compensation point in the absence of mitochondrial respiration (Γ*) for C₃, C₄, and C₃–C₄ intermediate species. Solid lines are fit to data; vertical dotted lines indicate η=1. C₃ species, purple circles; Type I species, blue diamonds; Type II species, green triangles; C₄-like species, yellow inverted triangles; C₄ species, red squares.

Fig. 3.

Relationships between stomatal conductance to CO₂ (g _s) and net CO₂ assimilation rate (A _net) in Flaveria species across the C₃ to C₄ photosynthetic range. Data points are from Vogan and Sage (2011); lines are obtained by analytical least-square fitting of the water use efficiency λ, employing a linearized version of the stomatal optimization model (Manzoni et al., 2011) for analytical tractability.

Fig. 4.

(A) Modelled relationships between net CO₂ assimilation rate (A _net), marginal water use efficiency (λ), and the CO₂-concentrating pump strength (η) modelled at current CO₂ concentrations (400 μmol mol^–1); V _c,max, K _cair, and Γ* vary with η according to the relationships in Fig. 2; vapour pressure deficit (D) was set to 1.5 kPa, leaf temperature to 30 °C, Q to 1500 μmol m^–2 s^–1. Mean values of λ and η for each of the five photosynthetic types are indicated on the surface. (B) The ratio of A _net at current atmospheric CO₂ levels versus A _net of C₃ Flaveria at low atmospheric CO₂ concentrations (280 μmol mol^–1) (A _net,₄₀₀/A _net ^C3,₂₈₀) for each photosynthetic group; means ±SE across species; filled symbols refer to constant λ, open symbols to λ increasing linearly with c _a; the dashed-dotted line indicates a ratio of 1. C₃ species, purple circle; Type I species, blue diamond; Type II species, green triangle; C₄-like species, yellow inverted triangle; C₄ species, red square.

Fig. 5.

Modelled relationships between instantaneous water use efficiency (WUE_i, the ratio of A _net to E), marginal WUE (λ), and the CO₂-concentrating pump strength (η) modelled at current CO₂ concentrations (400 μmol mol^–1); V _c,max, K _cair, and Γ* vary with η according to the relationships in Fig. 2; vapour pressure deficit (D) was set to 1.5 kPa, leaf temperature to 30 °C, Q to 1500 μmol m^–2 s^–1. Mean values of λ and η for each of the five photosynthetic types are indicated on the surface. (B) The ratio of WUE_i at current atmospheric CO₂ levels versus the WUE_i of a C₃ Flaveria at low atmospheric CO₂ concentrations (280 μmol mol^–1) (WUE_i400/WUE^C3 _i280) for each photosynthetic group; means ±SE across species; filled symbols refer to constant λ, open symbols to λ increasing linearly with c _a; the dashed-dotted line indicates a ratio of 1. C₃ species, purple circle; Type I species, blue diamond; Type II species, green triangle; C₄-like species, yellow inverted triangle; C₄ species, red square.

Fig. 6.

Comparison of (A) modelled photosynthetic rates and (B) modelled WUE_i among Flaveria species with different photosynthetic types at c _a=280 μmol mol^–1, expressed as ratios over the mean A _net and WUE_i for C₃ species at c _a=280 μmol mol^–1. Symbols represent means ±SE across species (for fixed C₃ A _net and WUE_i values); filled symbols refer to constant λ, open symbols to λ increasing linearly with c _a; the dashed-dotted line indicates a ratio of 1. Other parameters are as in Figs 4 and 5.