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

Abstract

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 p_i/p_a (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 p_i/p_a. Additional work indicated Δ changed independently of p_i/p_a 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.