Theory is developed to explain the carbon isotopic composition of plants. It is shown how diffusion of gaseous COz can significantly affect carbon isotopic discrimination. The effects on discrimination by diffusion and carboxylation are integrated, yielding a simple relationship between discrimination and the ratio of the intercellular and atmospheric partial pressures of COZ. The effects of dark respiration and photorespiration are also considered, and it is suggested that they have relatively little effect on discrimination other than cia their effects on intercellular p(COz). It is also suggested that various environmental factors such as light, temperature, salinity and drought will also have effects via changes in intercellular p(C0,). A simple method is suggested for assessing water use efficiencies in the field.

_apleuroticCO F_._ti0n in Ca.._a_o£_ (Melzer and O'LeaD,, 1987, 1991). Although Cs plants use RuBP carboxylase to fix CO2, these plants also contain significant amounts of PEP carboxylase. The role of this latter enzyme in C_photosynthesis is unknown. The role of PEP carboxylase in photosynthesis in the C3 plant Nicotiana tabacum, has been probed by measurement of the _3Ccontent of various materials. Whole leaf and purified RuBP carboxylase are within the range expected for C.. s plants. Aspartic acid purified following acid hydrolysis of this RuBP carboxylase is enriched in t3C compared to whole protein. Carbons 1-3 of this aspartic acid are in the normal Ca range, but carbon4 (analyzed by treatment of the aspartic acid with aspartate _..decarboxylase) has an isotopic composition in the range expected for products of C4 photosynthesis (-5 o/oo), and it appears that more than half of the aspartic acid is synthesized by PEP carboxylase using atmospheric CO2/HCO3In subsequent work, we have extended this study to other species and have studied aspartate derived from protein, free aspartate, malate, and citrate. Enzymatic degradations nave been performed in all c_es to recover the carbon that might have arisen front CO 2 via PEP carboxylase. The isotopic data from tobacco, spinach, and soybean are similar, and age effects are small. The soybean data are shown in Table I. The isotopic content of whole protein is similar to that of the leaves. Carbon-4 of protein-derived aspartic acid is invariably 15.-20°/oo more positive than the protein, indicating that most of this carbon is derived from action of PEP carboxylase. This presumably happens principally during leaf development, when protein synthesis is rapid and PEP carboxylase concentration is relatively high.The isotopic content of the pool of free aspartic acid, malic acid, and citric acid indicates that in the mature leaf, these compounds are synthesized principally via the C3 cycle, although small contributions from PEP carboxylase can be seen. Thus, a principal role of PEP carboxylase in C3plants is the synthesis of four-carbon skeletons (principally aspartate) for use in protein synthesis early in development.Isotop_e__Fractionationin A___.artateSynthesi_ in .M__a.i_. We have used the same approach in maize, in order to determine the isotopic pattern of metabolites in C4plants. The results are summarized in Table I. The very positive value observed for carbon-4 of aspartate and malate in these plants is presumably due to the isotope fractionation associated with decarboxylation of four-carbon acids in the bundle sheath cells. _ ,-.uk. klM6NT t6 UNLItvt_TL-D

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