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

Online carbon isotope discrimination (Delta) and leaf gas exchange measurements were made with control and salt-stressed Zea mays and Andropogon glomeratus, two NADP-ME type C(4) grasses. Linear relationships between Delta and p(i)/p(a) (the ratio of intercellular to atmospheric CO(2) partial pressure) were found for control plants which agreed well with theoretical models describing carbon isotope discrimination in C(4) plants. These data provided estimates of phi, the proportion of CO(2) 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 Delta for the same or less range in p(i)/p(a). Additional work indicated Delta changed independently of p(i)/p(a) in both water- and salt-stressed plants, suggesting a possible diurnal change in phi as plant water status changed linked to a decrease in the activity of the C(3) photosynthetic pathway relative to C(4) pathway activity. The possible effect of stress-induced changes in phi on organic matter delta(13) C of C(4) plants is apt to be most apparent in chronically stressed environments.