Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C4 grass are increased at high vapour pressure deficit during growth

Abstract

Bundle-sheath leakiness (ϕ) is a key parameter of the CO₂-concentrating mechanism of C₄ photosynthesis and is related to leaf-level intrinsic water use efficiency (WUE_i). This work studied short-term dynamic responses of ϕ to alterations of atmospheric CO₂ concentration in Cleistogenes squarrosa, a perennial grass, grown at high (1.6 kPa) or low (0.6 kPa) vapour pressure deficit (VPD) combined with high or low N supply in controlled environment experiments. ϕ was determined by concurrent measurements of photosynthetic gas exchange and on-line carbon isotope discrimination, using a new protocol. Growth at high VPD led to an increase of ϕ by 0.13 and a concurrent increase of WUE_i by 14%, with similar effects at both N levels. ϕ responded dynamically to intercellular CO₂ concentration (C _i), increasing with C _i Across treatments, ϕ was negatively correlated to the ratio of CO₂ saturated assimilation rate to carboxylation efficiency (a proxy of the relative activities of Rubisco and phosphoenolpyruvate carboxylase) indicating that the long-term environmental effect on ϕ was related to the balance between C₃ and C₄ cycles. Our study revealed considerable dynamic and long-term variation in ϕ of C. squarrosa, suggesting that ϕ should be determined when carbon isotope discrimination is used to assess WUE_i Also, the data indicate a trade-off between WUE_i and energetic efficiency in C. squarrosa.

Keywords: C4 photosynthesis; CO2-concentrating mechanism; carbon isotope discrimination; gas exchange; nitrogen nutrition; vapour pressure deficit..

Fig. 1.

Net CO₂ assimilation rate (A; A, B), transpiration rate (E; C, D) and stomatal conductance to water vapour (g_s; E, F) in response to short-term variation of intercellular CO₂ (C_i) under low (N1, circles; A, C, E) or high N supply (N2, triangles; B, D, F) combined with low (V1, open symbols) or high VPD (V2, filled symbols). Data are shown as the mean±SE (n=9–10). Operating conditions of gas exchange measurements were the same as conditions in growth chambers (leaf temperature 25 °C, PPFD 800 μmol m⁻² s⁻¹, VPD 0.8 kPa for V1 and 1.6 kPa for V2). The corresponding C_i values at growth CO₂ concentration (390 μmol mol⁻¹) are indicated by grey (low VPD) and black arrows (high VPD) in (E) and (F).

Fig. 2.

Carbon isotope discrimination (∆) during net CO₂ exchange (A, B) and bundle sheath leakiness (C, D) of Cleistogenes squarrosa leaves in response to short-term variation of intercellular CO₂ (C_i). Plants were grown under low (N1, circles; A, C) or high N supply (N2, triangles; B, D) combined with low (V1, open symbols, dashed lines) or high VPD (V2, filled symbols, solid lines). Operating conditions of gas exchange measurements were the same as conditions in growth chambers (leaf temperature 25 °C, PPFD 800 μmol m⁻² s⁻¹, VPD 0.8 kPa for V1 and 1.6 kPa for V2). Data are shown as the mean±SE (n=5–6). The regressions were fitted using a function of y=y₀+aln(x); all regressions have r²>0.8.

Fig. 3.

Relationships between bundle sheath leakiness and ln(C_i) under low (N1, circles; A) or high N supply (N2, triangles; B) combined with low (V1, open symbols, dashed lines) or high VPD (V2, filled symbols, solid lines). Operating conditions of gas exchange measurements were the same as conditions in growth chambers (leaf temperature 25 °C, PPFD 800 μmol m⁻² s⁻¹, VPD 0.8 kPa for V1 and 1.6 kPa for V2). Data are shown as the mean±SE (n=5–6). The data of each N level were fitted by linear regression including a dummy variable indicating VPD treatments (V) after the confirmation of parallelism of the two regressions of VPD levels: with V=0 for low VPD and V=1 for high VPD (for equations see Table 2).

Fig. 4.

A _sat (A), carboxylation efficiency (CE; B) and the relationship between leakiness measured under growth conditions (390 μmol mol⁻¹) and A_sat/CE (μmol mol⁻¹; C) under low or high N fertilizer supply (N1, circles; or N2, triangles) combined with low or high VPD (V1, open symbols; or V2, filled symbols). Data are shown as the mean±SE (n=9–10 for A_sat and CE, n=5–6 for leakiness). * indicates the treatment effect was significant at the P-level of 0.05, while ns indicates no significant effect.

Fig. 5.

Correlations between N_mass and A_sat (A), CE (B) and A_sat/CE (C) of leaves grown at low or high N fertilizer supply (N1, circles; or N2, triangles) combined with low or high VPD (V1, open symbols; or V2, filled symbols). Each symbol represents a data point of an individual plant.

Fig. 6.

Correlations between WUE_i and A_sat (A), CE (B), A_sat/CE (C), and leakiness (D) of leaves grown at low or high N fertilizer supply (N1, circles; or N2, triangles) combined with low or high VPD (V1, open symbols; or V2, filled symbols). Each symbol represents a data point of an individual plant.