Carbon isotope discrimination as a diagnostic tool for C4 photosynthesis in C3-C4 intermediate species

Abstract

The presence and activity of the C4 cycle in C3-C4 intermediate species have proven difficult to analyze, especially when such activity is low. This study proposes a strategy to detect C4 activity and estimate its contribution to overall photosynthesis in intermediate plants, by using tunable diode laser absorption spectroscopy (TDLAS) coupled to gas exchange systems to simultaneously measure the CO2 responses of CO2 assimilation (A) and carbon isotope discrimination (Δ) under low O2 partial pressure. Mathematical models of C3-C4 photosynthesis and Δ are then fitted concurrently to both responses using the same set of constants. This strategy was applied to the intermediate species Flaveria floridana and F. brownii, and to F. pringlei and F. bidentis as C3 and C4 controls, respectively. Our results support the presence of a functional C4 cycle in F. floridana, that can fix 12-21% of carbon. In F. brownii, 75-100% of carbon is fixed via the C4 cycle, and the contribution of mesophyll Rubisco to overall carbon assimilation increases with CO2 partial pressure in both intermediate plants. Combined gas exchange and Δ measurement and modeling is a powerful diagnostic tool for C4 photosynthesis.

Keywords: C3-C4; Carbon isotope discrimination; F. brownii; F. floridana.; Flaveria; intermediate photosynthesis.

Fig. 1.

The responses of (a) CO₂ assimilation rate, A and (b) compensation point (Γ) in F. pringlei, F. floridana, F. brownii and F. bidentis to changes in atmospheric pO₂. Assimilation rate is expressed as a percentage of the assimilation rate at 19mbar O₂ (average of 28.7±1.13 μmol m⁻² s⁻¹ for F. pringlei, 24.2±1.52 for F. floridana, 20.6±1.2 for F. brownii and 21.7±0.49 for F. bidentis). Measurements were made at 25°C and 385 μbar CO₂ (R), and an irradiance of 1500 μmol m⁻² s⁻¹. Values represent averages and standard error of four replicates.

Fig. 2.

(a) In vitro Rubisco, PEPC and CA activities in F. pringlei, F. floridana, F. brownii and F. bidentis, measured from samples of the same leaves used for gas exchange and expressed on a leaf area basis. (b) PEPC to Rubisco activity ratio in these experiments. Values represent mean and standard error of four experimental replicates.

Fig. 3.

Concurrent measurements of (a, d) CO₂ assimilation rate, A, (b, e) carbon isotope discrimination, Δ, and (c, f) the ratio of intercellular to ambient CO₂, C _i/C _a, as a function of intercellular CO₂ (C _i) in F. pringlei, F. floridana, F. brownii and F. bidentis. Values represent averages and standard error of 4 replicates. Measurements were made at 19 mbar O₂, a leaf temperature of 25°C and an irradiance of 1500 μmol m⁻²s⁻¹.

Fig. 4.

Observed carbon isotope discrimination, Δ expressed as a function of the ratio of intercellular to ambient CO₂, C _i/C _a, in F. pringlei, F. floridana, F. brownii and F. bidentis. Values are the same as plotted in Fig. 3. Solid line represents the theoretical response of Δ to C _i/C _a in C₃ plants ($\Delta =4.4+\frac{C_i}{C_a}(29-4.4)$ ; (Roeske and O’Leary, 1984; Evans et al., 1994). Dashed line represents the theoretical response of Δ to C _i/C _a in C₄ plants, $\Delta =4.4+\frac{C_i}{C_a}[-5.7-4.4+\varphi \left(29-1.8\right)]$ (Henderson et al., 1992) when φ=0.25.

Fig. 5.

Comparison between modeled and measured responses of CO₂ assimilation rate, A, and carbon isotope discrimination, Δ, to variation in intercellular pCO₂, C _i, in the C₃-C₄ intermediate species F. floridana and F. brownii. Measured A (a) and Δ (b) as a function of C _i in F. floridana (empty triangles), compared with the modeled responses predicted by C₃-C₄ photosynthetic model assuming an active C₄ cycle (solid lines) or no C₄ cycle activity (dashed lines). Measured A (c) and Δ (d) as a function of C _i in F. brownii (white squares), compared with the modelled responses using the C₃-C₄ models assuming Rubisco activity in the mesophyll cells (solid lines) or a strict compartmentalization of Rubisco in the bundle sheath cells (dashed lines). Parameters used for model simulations are presented in Table 1.

Fig. 6.

Response of mesophyll conductance (g _m) to changes in atmospheric pCO₂. In F. pringlei, g _m was calculated from concurrent gas exchange and Δ measurements made at 19mbar pO₂. The values for F. floridana, F. brownii and F. bidentis were assigned assuming the same response of g _m to C _i as observed in F. pringlei, scaled from model fitting.

Fig. 7.

Biochemical fractionation (Δ _bio), as a function of intercellular CO₂ (C _i) in F. floridana, F. brownii and F. bidentis. Δ _bio was calculated from eq. 12 using the combined gas exchange and Δ measurements shown in Fig. 3. Δ _bio could not be calculated for F. pringlei because g _m is obtained from Δ measurements in this species, so both factors are not independent. Values represent averages and standard error of four replicates.

Fig. 8.

CO₂ response of the estimated contribution of the C₄ cycle and the mesophyll C₃ cycle in the intermediate species F. floridana and F. brownii, expressed as a percent of total CO₂ assimilation rate, under low pO₂.