The Effect of Cooling on Translocation inSalix viminalis

Abstract

Radioactive phosphate was applied externally to a small area of the bark of the main stem below the point of insertion of a leafy branch. When photosynthesis and hence translocation was proceeding, phosphate was carried down the stem and was monitored externally by two Geiger counters spaced 10 cm apart further down the stem. Sudden cooling to 0 °C of the leaves, or the stem between the leaves and the point of application of the isotope, caused a rapid and seemingly simultaneous response by the two Geiger counters. It is concluded that cooling at one point causes an instantaneous reduction of velocity along the whole pathway. If cooling is continued for several hours the rate of translocation of phosphate recovers. The evidence suggests that this recovery is due to a compensatory increase in concentration of radioactive phosphate in the translocation stream rather than a recovery of velocity. INTRODUCTION Numerous workers, since Child and Bellamy (1919), have demonstrated a reduc tion in translocation on cooling all or part of the plant. A few workers have found an apparent recovery in rate of translocation even when the cooling is sustained. Whittle (1964), for example, found a rise in translocation rate to its original value after 2 days cooling to 5 °C in Pteridium. Geiger (1966) and Swanson and Geiger (1967) found complete recovery in Beta vulgaris between one or a few hours after cooling a 2-cm length of the petiole of the source leaf to 0 °C. Bowling (1968) found a fall followed by an increase in salt uptake (which he related to sugar transport) after 24 h cooling of a Helianthus stem to 0 °C. Swanson and Geiger suggested adaptation of respiration to the lower temperature as the cause, while Bowling suggested an increase in concentration compensating for a reduced velocity of the translocation stream. In this paper the effect of cooling on translocation in Salix viminalis was studied. Either a whole assimilating leafy side branch, or the basal portion only of the side branch, or a length of the main stem beneath the branch was cooled to 0 °C and its effect on the translocation of radioactive phosphate examined further down the stem. MATERIALS AND METHODS Twoto 3-year-old stems of Salix viminalis, 60 cm long, and 1-5-2 cm diameter were rooted in tap water for 6 to 11 weeks with an 18-h daylength. By this time they bore healthy roots and shoots. As shoots emerged from the centre 35 cm of the stem they were removed, leaving This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 06:10:31 UTC All use subject to http://about.jstor.org/terms The Effect of Cooling on Translocation 339 one or two shoots at the top. Before the experiments the roots were put in polythene bags containing vermiculite saturated with water. Two per cent agar containing 20-25 /xCi of 32P was poured into a plasticine well which surrounded an area of abraded bark. After the agar had set the stem was clamped horizontally with the agar on the lower side and with two Geiger counters underneath 14 and 24 cm away from the agar well as shown in Fig. 1. Their windows (25 mm diameter) were about 2 mm from the bark, to allow external assay of the activity. The counts were printed out every 1000 s. h Geiger-M idler tubes I I 14 cm 10 cm-X I i * * V. A ? >v'