AblItraet. The influenceof different parameters on B 13 C-values in plants in vivo and on the 13C-isotope effect of the carboxylation reactions in vitro were examined using isotope ratio mass spectrometry.The primary fixation of carbon in photosynthesis occurs in so-called Cs-plants via the ribulosebisphosphate carboxylase (RuBP-C) reaction, and in Cs-plants via the phosphoenolpyruvate carboxylase (PEP-C) reaction. The observed difference in 813C-values" of the organic material of these two groups of plants is a consequence of the different isotope discrimination by the two carboxylation reactions. I Changes of ecological conditions during growth are accompanied by variations of the 8 13 C-values within these groups or even for a single plant species.r"In particular, rise of temperature during growth generally leads to more negative 8 I'C-values; therefore the temperature coefficient of the discrimination effect seems to be positive. On the other hand, from corresponding in vitro experiments on isotope discrimination by the carboxylation reactions of RuBP-C and PEP-C, rather contradictory results were reported.v'" Whelan et aI. 1 observed a strong decrease in isotope discrimination by RuBP-C from sorghum with increasing temperature, whereas Christeller et al.6 found temperature independence for the isotope effect of the RuBP-C-reaction (enzyme from soy beans). As methods and enzymes used in these experiments differed from each other, and as from general experience in enzyme kinetics a negative temperature coefficient of the reaction was to be expected, we investigated the influence of different parameters on 8 13C-values of plants in vivo and, in parallel experiments, on the I'C-isotope effect of the carboxylation reactions in vitro. EXPERIMENTALSamples of plant material (5 mg) were converted to CO 2 in a commercial micro-combustion apparatus in a He/02 gas flow. The CO2 was isolated in a liquid nitrogen trap and then expanded into the sample reservoir of an isotope ratio mass spectrometer with double inlet system (MM 903, VG Micromass). The B 13 C-values were determined relative to a suitable standard. The in vitro experiments with enzymes were performed on incubation mixtures with an excessof carbon dioxide. After defined partial reaction, controlled spectrophotometrically, the remaining CO 2 was transferred by a He carrier gas flowinto a liquid nitrogen trap and assayed for 13C as mentioned above; the kinetic isotope effect was determined from the change of the B13C-value of the CO 2 before and after incubation. RESULTS AND DISCUSSIONThe isotope effect of the PEP-C-reaction was klJk l, = 1.002 ± 0.0003 (25°C; pH 8.0) with a temperature coefficient of -0.OOOO3/"C; no difference was found between enzymes from wheat and maize, representing C,-and Cz-plants respectively. The low value observed for k I2/k 13 is in line with the synchronous mechanism for the carboxylation step proposed by Utter.' The dependence of the kinetic isotope effect of the PEP-C-reaction on the pH-value could be mainly correlated with the th...

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Transfer factors from feed to meat (T f), taken from literature for monogastric animals and ruminants have been correlated to their corresponding animal body mass (mb). Taking all data into account, a close relationship between both transfer factor and body mass becomes evident, yielding a regression function of T f = 8.0 x mb(-0.91) (r = -0.97). For monogastric animals (including poultry), the corresponding relationships are T f = 5.8 x mb(-0.70) (r = -0.97), for ruminants T f = 1.9 x mb(-0.72) (r = -0.78). The equations offer the opportunity to estimate the transfer factor for individual animals more precisely taking individual body masses into account. They are of interest for animals, on which no or only poor data concerning radiocesium transfer factors are available. The determination of radiocesium transfer factors are reduced to a simple weighing process.