Short-Term Changes in Leaf Carbon Isotope Discrimination in Salt- and Water-Stressed C4 Grasses

Online carbon isotope discrimination (Δ) and leaf gas exchange measurements were made with control and salt-stressed Zea mays and Andropogon glomeratus, two NADP-ME type C4 grasses. Linear relationships between Δ and pi/pa (the ratio of intercellular to atmospheric CO2 partial pressure) were found for control plants which agreed well with theoretical models describing carbon isotope discrimination in C4 plants. These data provided estimates of φ, the proportion of CO2 fixed by phosphoenolpyruvate carboxylase which leaks out of the bundle sheath and the component of fractionation due to diffusion in air. Salt-stressed plants had wider variation in Δ for the same or less range in pi/pa. Additional work indicated Δ changed independently of pi/pa in both water- and salt-stressed plants, suggesting a possible diurnal change in φ as plant water status changed linked to a decrease in the activity of the C3 photosynthetic pathway relative to C4 pathway activity. The possible effect of stress-induced changes in φ on organic matter δ13 C of C4 plants is apt to be most apparent in chronically stressed environments.

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