'C/"C ratios have been determined for plant tissue from 104 species representing 60 families. Higher plants fall into two categories, those with low 8PDBI "C Values (-24 to -34%,) and those with high a "C values (-6 to -19%o Carbon isotope fractionation is associated with photosynthesis. This fractionation results in lowering the "C/"C ratio by about 20 per mille for land plants and 10 per mille for marine plants relative to atmospheric CO. A model has been proposed by Park and Epstein (12) to delineate the processes associated with this fractionation. To understand more fully the carbon isotope record and its implications for plant physiology, a more extensive investigation of the "C/C ratio in plants was undertaken. One hundred and four species representing 60 families have been investigated and the "C/"C ratio for these samples shows a much wider variation than previously reported. These results bear on more recent ideas regarding the biochemical mechanisms or pathways of carbon fixation as well as showing the relevance of C/P'C studies to biological processes.

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The temperature dependence of metabolic rates determines how plant growth rates vary with temperature. This paper shows that equations on physiological relations between respiration rates (i.e. rates of heat loss and CO 2 evolution) and growth rates can be used to describe temperature effects on plant growth rate. Incorporating measured values of plant respiratory heat and CO 2 rates at a few temperatures into the equations allows description of growth rates as a function of temperature and provides a physiological basis for understanding the effects of temperature on growth rate. The paper presents data on cabbage (Brassica oleracea L. Capitata) and tomato (Lycopersicon esculentum Miller cv. Ace) as model cool-climate and warm-climate cultivars to illustrate application of the methods in determining optimal growth climates for different cultivars, accessions, and ecotypes. The respiration-based calculations of growth rate vs. temperature yield curves for both species that are consistent with known temperature-growth requirements. We conclude that plant responses to temperature can be accurately predicted in detail from respiration rate measurements and the growth-respiration model. These studies demonstrate that the temperature dependence of growth rates is a function of the temperature dependencies of both metabolic rates and metabolic efficiency, which change continuously with temperature. The ultimate cause of high-and low-temperature growth limits is commonly not membrane phase transitions or enzyme denaturation as has been supposed, but is loss of substrate carbon conversion efficiency. The results show that ''plant temperature stress'' has been misunderstood and must be redefined because there is no ''nonstressfull temperature''.Abbreviations and symbols: R CO2 = rate of CO 2 production; R sq = specific mass-growth rate; q = metallic heat rate (rate of energy loss; p = mean chemical oxidation state of the substrate carbon; ∆H B = enthalpy change for the formation of biomass from photosynthat and minerals; = substrate carbon conversion efficiency Correspondence to: B.N. Smith;

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Deuterium to hydrogen ratios of 14 plant species from a salt marsh and lagoon were 55%0 depleted in deuterium relative to the environmental water. Carbon tetrachlorideextractable material from these plants was another 92%o depleted in deuterium. This gave a fractionation factor from water to CCl4 extract of 1.147. This over-all fractionation was remarkably constant for all species analyzed. Plants also discriminate against 13C, particularly in the lipid fraction. Data suggest that different mechanisms for carbon fixation result in different fractionations of the carbon isotopes. Herbivore tissues reflected the isotopic ratios of plants ingested. Apparently different metabolic processes are responsible for the different degrees of fractionation observed for hydrogen and carbon isotopes.Plants remove hydrogen from wager and transfer it to organic compounds. Although many organic hydrogen atoms are exchangeable with environmental water, once a carbon-hydrogen bond is formed in an organism, the hydrogen is no longer readily exchanged. Early studies on the natural abundance of deuterium and hydrogen in organic matter made no attempt to relate the D/H ratios from organic matter to the hydrogen source, i.e., environmental water (3, 18). Zborowski et al. (22) determined D/H ratios from fatty acids of fish and rats and marine sediments.They found the fractionation to be constant between fatty acids and water removed from the tissues and sediments by lyophilization. Epstein and Weiss (unpublished) collected plants at successive elevations in the Sierra Nevada. The D/H ratio of total organic matter varied with altitude, as did ground water at each collection site. Schiegl and Vogel (20) recently attempted to correlate organic D/H ratios with waters precipitated from the atmosphere in Europe and South Africa. We studied several species of plants and animals growing in close proximity in salt marsh, lagoon, and intertidal habitats utilizing the same water source. Since the D/H ratio in ocean waters from different geographic areas is nearly constant (9, 10) our samples were utilizing a common isotopic pool. Carbon isotope abundance in organisms is somewhat better understood (4, 16, 21) than hydrogen isotope abundance, so we have included carbon data for the same samples.