Internal conductance does not scale with photosynthetic capacity: implications for carbon isotope discrimination and the economics of water and nitrogen use in photosynthesis

A comparative study has been made of the photosynthetic physiological ecology and carbon isotope discrimination characteristics for modern-day bryophytes and closely related algal groups. Firstly, the extent of bryophyte distribution and diversification as compared with more advanced land plant groups is considered. Secondly, measurements of instantaneous carbon isotope discrimination ( Δ ), photosynthetic CO 2 assimilation and electron transport rates were compared during the drying cycles. The extent of surface diffusion limitation (when wetted), internal conductance and water use efficiency (WUE) at optimal tissue water content (TWC) were derived for liverworts and a hornwort from contrasting habitats and with differing degrees of thallus ventilation (as intra-thalline cavities and internal airspaces). We also explore how the operation of a biophysical carbon-concentrating mechanism (CCM) tempers isotope discrimination characteristics in two other hornworts, as well as the green algae Coleochaete orbicularis and Chlamydomonas reinhardtii . The magnitude of Δ was compared for each life form over a drying curve and used to derive the surface liquid-phase conductance (when wetted) and internal conductance (at optimal TWC). The magnitude of external and internal conductances, and WUE, was higher for ventilated, compared with non-ventilated, liverworts and hornworts, but the values were similar within each group, suggesting that both factors have been optimized for each life form. For the hornworts, leakiness of the CCM was highest for Megaceros vincentianus and C. orbicularis (approx. 30%) and, at 5%, lowest in C. reinhardtii grown under ambient CO 2 concentrations. Finally, evidence for the operation of a CCM in algae and hornworts is considered in terms of the probable role of the chloroplast pyrenoid, as the origins, structure and function of this enigmatic organelle are explored during the evolution of land plants.

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Genotypic Variation in Relative Growth Rate and Carbon Isotope Discrimination in Sunflower Is Related to Photosynthetic Capacity

Functional Plant Biology ◽

10.1071/pp9960227 ◽

1996 ◽

Vol 23 (2) ◽

pp. 227 ◽

Cited By ~ 29

Author(s):

JM Virgona ◽

DG Farquhar

Keyword(s):

Growth Rate ◽

Carbon Isotope ◽

Relative Growth Rate ◽

Carbon Isotope Discrimination ◽

Photosynthetic Capacity ◽

Genotypic Variation ◽

Wild Accession ◽

Assimilation Rate ◽

Relative Growth ◽

Isotope Discrimination

A previous study showed that carbon isotope discrimination (Δ) was negatively related to transpiration efficiency (W, the ratio of dry matter accumulation to transpiration) and biomass accumulation amongst sunflower (Helianthus annuus L.) genotypes. Three experiments which explore the physiological basis of relationships between Δ, relative growth rate (RGR) and leaf gas exchange characteristics are reported. Growth was analysed on seven genotypes during the early vegetative stage (up to 31 days after emergence). Carbon isotope discrimination, Δ, W, and photosynthetic CO2 assimilation rate per unit leaf area (A) at ambient concentration of CO2 were measured on plants that made up the final harvest. Six of the seven genotypes were also grown under a low nitrogen (N) regime and harvested at 30 days after emergence. Carbon isotope discrimination was negatively related to relative growth rate (RGR), net assimilation rate (NAR) and photosynthetic rate per unit area, which in turn, were all positively correlated with each other. Genotypic variation inA (51%) was greater than that in leaf conductance, g (32%) and there was no relationship between g and Δ. Under the low N regime, A and g declined by 33 and 12%, respectively, across all genotypes producing a significant rise in A of 1.10. In a second experiment, growth analysis on 13 cultivated forms of H. annuus, a wild accession and a H. argophyllus accession, also showed that there was a positive relationship between RGR and NAR, and that both growth indices were negatively related to Δ. In a further experiment, using 14 genotypes (including the wild accession) there was a highly significant negative correlation (r = -0.85, P < 0.001) between photosynthetic capacity, measured using an oxygen electrode, and Δ. These results indicate that variation in photosynthetic capacity can account for genotypic variation in both Δ and RGR during vegetative growth in sunflower. Hence, faster growing sunflower genotypes have higher rates of photosynthesis and use water more efficiently.

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Effects of Salinity on Stomatal Conductance, Photosynthetic Capacity, and Carbon Isotope Discrimination of Salt-Tolerant (Gossypium hirsutum L.) and Salt-Sensitive (Phaseolus vulgaris L.) C3 Non-Halophytes

PLANT PHYSIOLOGY ◽

10.1104/pp.95.2.628 ◽

1991 ◽

Vol 95 (2) ◽

pp. 628-635 ◽

Cited By ~ 261

Author(s):

Enrico Brugnoli ◽

Marco Lauteri

Keyword(s):

Stomatal Conductance ◽

Phaseolus Vulgaris ◽

Gossypium Hirsutum ◽

Carbon Isotope ◽

Carbon Isotope Discrimination ◽

Photosynthetic Capacity ◽

Salt Tolerant ◽

Phaseolus Vulgaris L ◽

Gossypium Hirsutum L ◽

Isotope Discrimination

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On the effect of heavy water (D2O) on carbon isotope fractionation in photosynthesis

Functional Plant Biology ◽

10.1071/fp07282 ◽

2008 ◽

Vol 35 (3) ◽

pp. 201 ◽

Cited By ~ 2

Author(s):

Guillaume Tcherkez ◽

Graham D. Farquhar

Keyword(s):

Carbon Isotope ◽

Heavy Water ◽

Carbon Isotope Discrimination ◽

Co2 Assimilation ◽

Xanthium Strumarium ◽

Phase Component ◽

Isotope Discrimination ◽

Internal Conductance ◽

On Line ◽

Co2 Addition

Internal conductance to carbon dioxide is a key aspect of leaf photosynthesis although is still not well understood. It is thought that it comprises two components, namely, a gas phase component (diffusion from intercellular spaces to cell walls) and a liquid phase component (dissolution, diffusion in water, hydration equilibrium). Here we use heavy water (D2O), which is known to slow down CO2 hydration by a factor of nearly three. Using 12C/13C stable isotope techniques and Xanthium strumarium L. leaves, we show that the on-line carbon isotope discrimination (Δ13C, or Δobs) associated with photosynthesis is not significantly decreased by heavy water, and that the internal conductance, estimated with relationships involving the deviation of Δ13C, decreased by 8–40% in 21% O2. It is concluded that in typical conditions, the CO2-hydration equilibrium does not exert an effect on CO2 assimilation larger than 9%. The carbon isotope discrimination associated with CO2 addition to ribulose-1,5,bisphosphate by Rubisco is slightly decreased by heavy water. This effect is proposed to originate from the use of solvent-derived proton/deuteron during the last step of the catalytic cycle of the enzyme (hydration/cleavage).

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