The Effect of Temperature on the Respiration of 14C-Labelled Sugars in Stems of Willow

The leaves of willow cuttings were allowed to assimilate ¹⁴CO₂,and the rate of increase of activity in the sieve-tube sap,collected as aphid honeydew, was compared with the output ofrespiratory ¹⁴CO₂ from the stem at 25 °C and at 0 °C. In the case of 3–5-week-old young shoots, the relativerate of breakdown of the ¹⁴C-labelled sugars was reduced at0 °C as compared with 25 °C. With 2–4-year-oldmature stems, however, the rate of breakdown of labelled sugarsin transit through the phloem was increased at 0 °C relativeto that at 25 °C. When the labelled sugars were being respiredin isolated stem segments where no transport was taking place,then low temperatures markedly reduced the rate of breakdownin both young shoots and mature stems. These results are discussed in relation to the results of otherworkers who have studied temperature effects on sieve-tube transport.