δ¹³ Values of C₄ Types in Grasses

Abstract

δ¹³ values were determined for leaves of C₄ grasses (Poaceae) of different photosynthetic types. For plants grown in the same environment at the same time, mean δ¹³ values for the three C₄ types were: NADP-ME-type, - 11.350 ± 0.13 s.e. (11 spp.); PCK-type, - 11.950 ± 0.19 s.e. (11 spp.); and NAD-ME-type, -12.70 ± 0.21 s.e. (9 spp.). Although there is some overlap between the values for individual species of the three groups, the difference between any two means is highly significant [P(t) < 0.01] and is not due to taxonomic sampling bias at the subfamily level. The differences in means may suggest that C₄ types differ in rates of leakage of CO₂ and HCO₃^- from PCR tissue ('photosynthetic carbon reduction' tissue: equivalent to 'Kranz' tissue), and/or, using Farquhar's (Appendix) expression for plant δ values, that C₄ types differ in their average intercellular CO₂ concentrations (c₁). It is also possible that differences between C₄ types exist in some other, unknown, leaf fractionation process. Apoplastic CO₂ leakage from PCR tissue in NAD-ME-type C₄ grasses, which do not possess a PCR 'suberized lamella' as found in NADP-ME- type and PCK-type C₄ grasses, may give these species the most negative δ¹³ values. Expressions for C₄ plant δ¹³ values, and a model for the δ¹³ values of CO₂ and HCO₃^- in various pools and fluxes in C₄ plant leaves, are given.