Drivers and mechanisms of 13C discrimination in Cleistogenes squarrosa (C4) - reducing uncertainties on bundle sheath leakiness
Final Report 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 (WUEi). Δ13C was suggested as a potentially promising screening tool for breeding and improvement of C4 crops. However, such endeavours require knowledge of the magnitude and variability of ϕ and relevant environmental drivers. Leakiness is generally quantified using a online carbon isotope approach based on the model of carbon isotope discrimination of C4 photosynthesis (Farquhar, 1983). However, measuring online Δ13C is a difficult and laborious task. We found that measurement artefacts, i.e. CO2 leak fluxes between the leaf cuvette and the surrounding air and isotopic disequilibria between photosynthetic and respiratory CO2 fluxes, can lead to very substantial errors of measured Δ13C. We conducted experiments in specialized 13CO2/12CO2 gas exchange mesocosms and we used advanced protocols to measure online Δ13C of C. squarrosa, an important C4 grass in the Eurasian grassland. Growth at high VPD led to an increase of ϕ by 0.13 and a concurrent increase of WUEi by 14%, with similar effects at both N levels. ϕ responded dynamically to intercellular CO2 concentration (Ci), increasing with Ci. 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/PEPc) indicating that the long-term environmental effect on ϕ was related to the balance between C3 and C4 cycles. The study revealed considerable dynamic and long-term variation in ϕ of C. squarrosa, showing that ϕ must be determined when Δ13C is used to assess WUEi. Also, the data indicate a trade-off between WUEi and energetic efficiency in C. squarrosa. Our data also showed a clear ontogenetic variation in Δ13C in leaf dry matter, which suggested that Δ13CDM is not ideal for estimating ϕ, at least in C. squarrosa. As discussed by many authors, using Δ13CDM to quantify ϕ has considerable uncertainty related to postphotosynthetic fractionation and the temporal mismatch between biomass formation and gas exchange measurements. Via the addition of two CSC funded PhD student to the project, we were able to expand the scope of this project to include analyses of morphological development and 18O signals in leaf water and cellulose that provided supporting information for the analysis of the primary project questions. In that, we assessed a putative limitation (and effect) of in vivo carbonic hydrase activity, and nitrogen (N) fertilizer supply and vapor pressure deficit (VPD) effects on δ18OCel, 18O-enrichment of leaf water (Δ18OLW) and cellulose (Δ18OCel) relative to source water, and pexpx, the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis. We found that δ18OCO2 and N supply had no effect on δ18OCel, Δ18OLW, Δ18OCel and pexpx; Δ18OCel and Δ18OLW increased with VPD while pexpx decreased. Also, as carbonic anhydrase activity appeared non-limiting, a carbonic anhydrase effect on Δ13C seemed also non-likely. Since Δ18OCel is an effective indicator of VPD, capitalizing on the short and predictable leaf appearance interval of C. squarrosa, we suggest that successively produced leaf blades of C. squarrosa can be used to provide highly time-resolved δ18OCel records of VPD.
Publications
- (2015) 13CO2/12CO2 exchange fluxes in a clampon leaf cuvette: disentangling artifacts and flux components. Plant, Cell and Environment 38, 2417-2432
Gong XY, Schäufele R, Feneis W, Schnyder H
(See online at https://doi.org/10.1111/pce.12564) - (2016) Bundle-sheath leakiness and intrinsic water-use efficiency of a perennial C4 grass are increased at high VPD during growth. Journal of Experimental Botany
Gong XY, Schäufele R, Schnyder H
(See online at https://doi.org/10.1093/jxb/erw417) - (2016) Effects of nitrogen and vapour pressure deficit on phytomer growth and development in a C4 grass. AoB Plants
Yang F, Gong XY, Liu H, Schäufele R, Schnyder H
(See online at https://doi.org/10.1093/aobpla/plw075) - (2016) Nitrogen fertilization and δ18O of CO2 have no effect on 18O-enrichment of leaf water and cellulose in Cleistogenes squarrosa (C4) – is VPD the sole control? Plant, Cell and Environment 39, 2701-2712
Liu HT, Gong XY, Schäufele R, Yang F, Hirl RT, Schmidt A, Schnyder H
(See online at https://doi.org/10.1111/pce.12824)
