The occurrence of various catabolic pathways of glucose in plant materials has attracted considerable interest in recent years (1,3,6). In an earlier report (2), studies on the catabolism of glucose in fruit by a radiorespirometric method were described. The radiorespirometric method previously has been successfully applied to other intact biological systems such as microorganisms and insects (11,13). Essentially, the catabolic pathways for glucose utilization in fruit were detected and estimated by comparing the rates of C140. formation from fruit administered vith C14 specifically labeled glucose samples.In applying this method to study tomato catabolism, it is important that the labeled substrate is introduced into an intact fruit by a reliable technique so that it wvill mix rapidly and evenly with a defined amount of endogeneous substrate. Fullfillment of these conditions will then make it possible to compare the rate of C140, production from a set of fruit metabolizing concurrently C14 specifically labeled substrates. The vacuum infiltration technique described in an earlier paper was found to be satisfactory by examining the reproducibility of a large number of radiorespirometric data in a series of test experiments. In order to understand better the nature of the vacuum infiltration process and to estimate the amount of fruit glucose located at the infiltrated area, isotopic dilution technique has been applied in the present work to further examine the validity of the technique described earlier for substrate administration.Results of several series of radiorespirometric experiments are also presented to provide experimental evidence in substantiating the assumptions given in the earlier paper (2) for estimating pathway participation in the glucose catabolism of tomato fruit. These assumptions, concerned with the catabolic be1Received revised manuscript March 16, 1960.

The operation of the hexose monophosphate pathway (HMP) in plants has been the object of extensive studies in recent years (1,2,4). In previous papers (3,9) it has been demonstrated that in intact tomato fruit, glucose is catabolized mainly by way of the Embden-Meyerhof-Parnas pathway (EMP) and to a limited extent via the HMP pathway. However, the catabolic fate of the pentose phosphate, derived from glucose via the HMP pathway, remains to be elucidated. The latter problem is an important one since one of the assumptions made in deriving equations for pathway estimation is that the pentose phosphate does not participate significantly in fruit respiration (3). Subsequently, from radiorespirometric studies of gluconate catabolism in tomato fruit (9) it is revealed that C-2, C-3,4, and C-6 of gluconate, in addition to C-1. were converted to some extent to respiratory CO, a fact indicating that the pentose phosphate derived from glucose via phosphogluconate was, indeed, engaged in respiratory functions.In the present work, the incorporation of gluconate-2-C14, gluconate-6-C14, and ribose-1-C14 into fruit constituents, particularly that into the fruit glucose molecules, has been studied. The isotopic distribution patterns in fruit glucose derived from the respectively labeled substrates, enables one to gain considerable insight into the catabolic mechanism responsible for the conversion of the carbon atoms of pentose phosphate to respiratory CO.,. The Materials & MethodsThe tomato fruit used in the present experiments were of the Michigan State Forcing variety and were selected on the same basis as described in earlier work (3, 9).1 Carbon-14 Labeled Substrates: Gluconate-1-C14 was purchased from the Nuclear-Chicago Corp. Gluconate-2, and -6-C14 were prepared from the correspondingly labeled glucose samples by the method of

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The glucose catabolism in several varieties of pepper fruit has been investigated by means of tracer techniques employing glucose-1, -2, -3(4), and -6-C14 as the labelled substrates. A method is presented whereby the participation of individual pathways in these fruits may be identified and estimated. It was found that 28–36% (among the varieties tested) of the glucose catabolized was routed through a direct oxidative pathway, the remainder being oxidized via the Embden–Meyerhof–Parnas and tricarboxylic acid cyclic processes. No significant difference in pathway distribution was found between green mature peppers and red mature peppers.