Comparisons of respiratory metabolism among the phloem, cambium and xylem of mature carrot root were carried out and the following results were obtained.1) The activity of O2uptake on a dry weight basis in the cambium was higher than that in the xylem. The phloem showed the lowest activity. 2) RQ was close to unity in all the three tissues. 3) The inhibition rate of O2uptake by malonate was considerably high in the three tissues. The highest inhibition was observed in the cambium. 4) In the phloem and xylem the malonic inhibition rate of14CO2release from G-U-14C was not so high as compared with that of O2uptake. In the cambium, however, those of O2uptake and14CO2output were comparable. 5) Malonate treatment did not cause any significant change in RQ. 6) The C6/C1ratio was highest in the cambium and lowest in the phloem.From these it is concluded that the participation of the TCA cycle in respiration is great in all the three tissues of carrot root, and among the three the cambium has the highest activity of the cycle. As for the PP pathway, the xylem and phloem have a higher activity than the cambium.
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