Oxygen isotopes in carbon(3) and carbon(4) grasses

Stable oxygen isotopes offer a unique tool that elucidates physiological plant processes from the chloroplastic to global scales. The range of potential applications include paleoclimatic reconstruction to leaf-level physiological comparisons to assessment of global productivity. Central to all these applications is the oxygen isotope ratio of leaf-water and the transfer of this signal to CO₂, O₂ and plant organic material. Grasses are the dominant plant form in one quarter of the earth's vegetation and the majority of the world's grain crop species and have largely been ignored in previous oxygen isotope studies. This dissertation examines oxygen isotopes in the leaf-water, leaf-cellulose and the effects of leaf-water on atmospheric CO ₂ in C₃ and C₄ grasses.; Evaporative enrichment of heavy oxygen isotopes in the leaf-water of grass species can be significantly greater than predicted by the Craig-Gordon model, with C₄ grasses considerably more enriched than C₃ grasses. Heavy isotope leaf-water enrichment in grasses is attributable to the progressive evaporative enrichment along parallel veins (a function of both leaf length and interveinal distance), a pattern that does not occur in Dicotyledonous species.; The oxygen isotope ratio of leaf-water between C₃ and C ₄ grasses becomes less distinct as relative humidity increases and the leaf-water differences translated directly to the oxygen isotope ratio of leaf cellulose. A conceptual model is presented that was based on grass blade growth characteristics and observed patterns of progressive heavy oxygen isotope enrichment in grasses. Additionally, environmentally induced changes in the oxygen isotope ratio of leaf water were recorded in grass blade cellulose during leaf-blade expansion. This suggests that, analogously to tree-rings, grass blades represent a season-long record of local climate.; Leaf water enrichment of heavy isotopes in the C₄ grass Andropogon gerardii was shown to have a consistently small effect on atmospheric CO₂ due to low carbonic anhydrase activity. There was no significant difference between the oxygen isotope ratio of CO ₂ evolving from base and tip leaf segments where the mean oxygen isotopes of leaf water differed by more than 24‰. The consistent effect of leaf-water oxygen isotopes on atmospheric CO₂ in C ₄ grasses should result in a distinct oxygen isotope signature from C₄ grasslands.