Elevated CO(2) induces biochemical and ultrastructural changes in leaves of the C(4) cereal sorghum

We analyzed the impact of growth at either 350 (ambient) or 700 (elevated) microL L(-1) CO(2) on key elements of the C(4) pathway (photosynthesis, carbon isotope discrimination, and leaf anatomy) using the C(4) cereal sorghum (Sorghum bicolor L. Moench.). Gas-exchange analysis of the CO(2) response of photosynthesis indicated that both carboxylation efficiency and the CO(2) saturated rate of photosynthesis were lower in plants grown at elevated relative to ambient CO(2). This was accompanied by a 49% reduction in the phosphoenolpyruvate carboxylase content of leaves (area basis) in the elevated CO(2)-grown plants, but no change in Rubisco content. Despite the lower phosphoenolpyruvate carboxylase content, there was a 3-fold increase in C isotope discrimination in leaves of plants grown at elevated CO(2) and bundle sheath leakiness was estimated to be 24% and 33%, respectively, for the ambient and elevated CO(2)-grown plants. However, we could detect no difference in quantum yield. The ratio of quantum yield of CO(2) fixation to PSII efficiency was lower in plants grown at elevated CO(2), but only when leaf internal was below 50 microL L(-1). This suggests a reduction in the efficiency of the C(4) cycle when [CO(2)] is low, and also implies increased electron transport to acceptors other than CO(2). Analysis of leaf sections using a transmission electron microscope indicated a 2-fold decrease in the thickness of the bundle sheath cell walls in plants grown at elevated relative to ambient CO(2). These results suggest that significant acclimation to increased CO(2) concentrations occurs in sorghum.

How does the C4 grass Eragrostis pilosa respond to elevated carbon dioxide and infection with the parasitic angiosperm Striga hermonthica?

Eragrostis pilosa (Linn.) P Beauv., a C₄ grass native to east Africa, was grown at both ambient (350 μmol mol^-1 and elevated (700 μmol mol^-1 ) CO₂ in either the presence or absence of the obligate, root hemi-parasite Striga hermonthica (Del.) Benth. Biomass of infected grasses was only 50% that of uninfected grasses at both CO₂ concentrations, with stems and reproductive tissues of infected plants being most severely affected. By contrast, CO₂ concentration had no effect on growth of E. pilosa, although rates of photosynthesis were enhanced by 30-40% at elevated CO₂ . Infection with S. hermonthica did not affect either rates of photosynthesis or leaf areas of E. pilosa, but did bring about an increase in root∶shoot ratio, leaf nitrogen and phosphorus concentration and a decline in leaf starch concentration at both ambient and elevated CO₂ . Striga hermonthica had higher rates of photosynthesis and shoot concentrations of soluble sugars at elevated CO₂ , but there was no difference in biomass relative to ambient grown plants. Both infection and growth at elevated CO₂ resulted in an increase in the Δ¹³ C value of leaf tissue of E. pilosa, with the CO₂ effect being greater. The proportion of host-derived carbon in parasite tissue, as determined from δ¹³ C values, was 27% and 39% in ambient and elevated CO₂ grown plants, respectively. In conclusion, infection with S. hermonthica limited growth of E. pilosa, and this limitation was not removed or alleviated by growing the association at elevated CO₂ .