Earlier studies have shown that the retarding effect of low petiolar temperatures on sucrose transport through sugar beet (Beta vulgaris L.) petioles is markedly time-dependent. Although the initial effect of chilling the petiole to near 0 C is severely inhibitory, translocation rates soon recover (usually within about 2 hours) to values at or near the control rate. In the present studies, selected metabolic parameters were measured simultaneously with translocation. No stoichiometric relationships among petiolar sucrose transport, petiolar respiration (CO2 production), and calculated petiolar ATP turnover rates were evident. It appears that the major sources of energy input energizing carbohydrate transport in sieve tubes function mainly at either loading or unloading sites and not at the level of individual sieve-tube elements.Earlier studies have shown that the retarding effect of low petiolar temperatures on sucrose transport through sugar beet petioles is markedly time-dependent (6, 11). Although the initial effect of chilling the petiole to near 0 C (0.1-2.5 C) is severely inhibiting, the degree of inhibition diminishes rapidly with time, and the translocation rate usually recovers to values at or near (and frequently above) the control rate within 1.5 to 2.5 hr. If the "cold-adapted" petiole is then rewarmed to room temperatures, and again rechilled, the cycle of inhibition-deinhibition is repeated (11). Thus, sucrose translocation in sugar beet petioles is essentially a homeostatic process.This homeostasis is interesting, since if sucrose translocation depends on production and use of metabolic energy along the path between source and sink, the reversal of low temperature inhibition with time should be reflected in similar time-dependent changes in various metabolic parameters of the chilled petiole.The experiments reported here permit comparisons to be made among the following variables at both normal (22-25 C) and low (0.7-2.5 C) petiolar temperatures: (a) the translocation rate; (b) the rate of CO2 evolution from the petiole; (c) changes in the molar concentration of ATP in the petiole; and (d) The day before each experiment, the hypocotyl of the test plant was girdled with an electrically-heated nichrome wire about 1 cm below the crown to restrict the translocation sink to the young shoot. These plants, therefore, differed significantly from those used in earlier studies in that the ratio of accessible sink tissue to source tissue was considerably smaller because of the exclusion of the root system as a translocation sink. Figure 1 presents a flow diagram of the analytical system used for measuring translocation to the sink leaf, photosynthesis in the donor-leaf blade, and respiration in the petiole zone subjected to cold treatment. Steady-state labeling of assimilates was obtained by circulating CO2 at a constant concentration and specific activity through the Plexiglas cuvette (B). The desired concentration of about 600 pd/1 was maintained within approximately 1% by means of a motorized ...

Translocation profiles along the path were studied using a modified flap-feeding technique for the simultaneous application of THO and "4C-sucrose. A re-evaluation of a mathematical model for phloem transport with reversible lateral exchange of tracer along the path indicates that lower apparent velocities for THO as compared to labeled carbohydrate are primarily due to extensive lateral exchange of THO along the conduction path. Path-chilling experiments support the concept that THO and "C-sucrose exhibit different lateral exchange characteristics. The data presented are consistent with a solutionflow mechanism.The mechanism underlying long distance transport of photosynthate has been a subject of controversy for many years (6, 21, 24). Points of contention center on the fine structure of the sieve tube, the extent and importance of metabolic energy, as well as the underlying driving potential which enables a sourcesink system to function. The most prevalent of the proposed mechanisms is the mass-flow hypothesis adopted from Munch (14). Since mass-flow is set apart from some other proposed mechanisms by its requirement for a "bulk flow" of solution, many workers have looked to isotopic water as a tool in evaluating mass flow; however, experimental results have been contradictory. In the present investigations, the problem of "solution-flow" along the path is approached through the simultaneous appli-

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Experiments are described in which bark strips of willow were sealed to polythene tubes having two compartments. This allowed investigations to be made of the transport along the sieve tubes of tritiated water, "C-Iabelled sugars, and '-P-phosphates from one compartment, towards a stylet situated in the bark over the other compartment.Although activity from both "C and ^^p was detected in the stylet exudate usually within 1 hour from isotope application, tritium activity was never detected even after a period of 8 hours in most experiments, though in certain cases, very low activities were detected after 4 hours. Suhsequent experiments in which stylets were sited over both compartments showed that tritium activity moved laterally into the punctured sieve element more rapidly than either '''C or '-P. Experiments vising both live and dead bark in which stylets were not employed, showed that within 4 hours tritium activity had moved by diffusion along the whole length of a bark strip, therefore after this time tritium activity could have moved into the stylet exudate by a diffusional process.The lack of rapid longitudinal movement of ti-itiated water along the sieve tubes, indicates that the transport process is unlikely to be a mass flow of solution.