Grass species displaying the different C4 biochemical
subtypes (NAD-ME, NADP-ME and PCK) show potential for maximizing growth
under limited water. Carbon (d13C) and oxygen (d18O)
isotopic analysis provide a means of assessing the effects of water status on transpiration
and photosynthesis parameters. The
purpose of this study was to (a) determine the effects of water stress on d13C
and d18O isotope ratios and bundle sheath leakiness in two
subtypes of C4 grasses; and (b) determine the efficacy of using leaf
cellulose d18O as a proxy for stomatal behavior in C4
grasses. The species examined included
two NAD-ME grasses: switchgrass ‘Alamo'
and ‘Cave-in-Rock' and two NADP-ME: big
bluestem ‘Pawnee' and indiangrass ‘
There were significant species effects on leaf d13C. Leaf tissue was 0.81‰ more enriched with 13C in the two NAD-ME grasses, relative to the NADP-ME grasses. The NADP-ME grasses exhibited more stable bundle sheath leakiness (Φ) values with increasing water deficits compared to the NAD-ME grasses. Differences in bulk leaf water d18O in control plants between big bluestem and the other grass species were significant (P<0.05).
Water stress significantly reduced the leaf d13C of all the C4 grasses by an average of 2.45‰. Water stress significantly (P<0.05) reduced bulk leaf water d18O in all the grass species. Water stress did not significantly (P<0.05) affect either the overall or the basal and apical leaf cellulose d18O signatures in the four grass species.
The stability of bundle sheath leakiness under conditions of changing water availability suggests that variations in pi/pa accounted for most of the variation in leaf carbon isotope ratios and D (carbon isotope discrimination) (r2 = 0.89). Leaf cellulose d18O signatures cannot be reliably used as a measure of stomatal behavior under water stress in these species.