In a previous paper (3 ), we reported oni the kinietics of C'4 translocationi in sugar beet as measured in a conventional short-term labeling experiment. Evidence was presented that the rate of C14 translocation is linearly related to the specific activity of the sucrose in the source leaf, as would be anticipated in a steady-state systeml if sucrose were the domiinant source of the transport molecule.In the present study we have extended our information oIn sucrose tranislocation by long-terml labeling experiments in which both the partial pressure and specific activity of the administered C'40., were maiintainied at constant known values for the total duration of the translocationi period. Specific 4 The term steady-state is not used in the strictest sense because there is a steady increase in the assimilate content of the source leaf. This fact is taken into consideration in the model and in the discussions wh'ch follow. 942labeling period, and analytical lprocedures were essentially the samle as those used in the slhort-termii labeling experiments l)reviously relportedl (3), the only sigimificant difference being that labeled CO., wals maintained in the supply-leaf cuvette at a conlstant partial pressure of 0.38 mmin Hg (usually withini the ranige of -+ 0.08 mnIH-g) and at a constanit klnownl valuie of specific activity (2.5-6.7 jAc C14 iig-'C, de-pending on the (duration of the experiment) throughout the entire tranislocatioin period. A Sigmamotor peristaltic puml) l)rovidled a circulationl flowv rate of 800 cm3 mimiil-, giviig an average turnover time of about 20 seconds for the gas volunme in the leaf cUvette. Total C(o. concentration andl C'40(. concentration in the systemx were continuouslv mloniitoredI by a Model 15-A Beckman infrared gas an.alyzer and( a Nuclear-Chicago Dvinacoin-6000 ionization electromiieter. respectively (fig 1). Labeled CO., from the CO.) reservoir was introduced into the svstem 1v mercury displacemiient at a rate sufficient to comiipenlsate for photosynthetic uptake (approximately 14 nml CO., hr-I (1d11-2 source-leaf area), the blee(l ill rate beinlg miianutially regulated by varyiing either the pressure hea(l on the CO, reservoir or the settinig on a microconitrol valve. Proper regulation of the bleedin rate p)roved somewhat (lifficult, and significant (leviatiolls in the l)artial Dressuire of CO., from the norni,ative value of 0.38 mimi Hig occasionallv OCcurred. It should be nioted, however, that a pressure chalnge wouil(d not affect the specific activity of the Co., in the systemii.

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Summary. Fuirther stuldies are presented characterizing the time-couirse response of suicrose translocation in sugar beet (Beta vulgaris L. cv Klein Wanzleben) to low temperature inhibition. Only the temperatture of a 2 cm zone of the source-leaf petiole was varied (10 vs 250, approximately). The half-time of inhibition, defined as the time requlired for 50 % inhibition of the control or pre-cooling rate, varied from 4 to 15 minutes, and the half-time of recovery from 30 to 100 mintutes. Maximum inhibition varied from 68 to 92 %. Possible uncertainties in evaluating these parameters are discuissed. When the duiration of the low temperatuire period was suifficient to permit essentially fuill recovery, subsequent re-warming of the petiole zone to 250 to 30°effected little or no increase in the translocation rate. It is evident that the interposition between source and sink of a 2 cm petiole zone maintained at a temperatuire generally inhibitory to physiological processes resulted in little or no impairment to the translocation process, after a stuitable thermal adaptation period.Thermally adapted petiole systenms de-adapted after periods as short as 1 houir at 250.Earlier stuldies ( 1, 2, 3, 11 ) ha+ve indicated that the inhibitory effect of low temperattires on organ:c translocation rates graduially (1dm;n:ches with tim". Recently, this time factor effect has been fo-nto be highly significant in the stugar beet (12). The present paper reports the results of a studv to characterize quantitatively and in detail the timecoturse response of sucrose translocation in this species to a low temperatture block (1°-2°) applied to the petiole of the donor leaf.Becatuse of the very low temperattures employed in these studies, many phvsiological processes in the condtucting cells of the chilled petiole zonwotuld be severelv inhihited. The data contribuite, therefore, to a fuirther assessment of the var ous theories attaching significance t) the high metabolic rates of phloem tissuie with respect to the mechanism of organic translocation (cf review by Ktursanov, 7), especially the more specific theories which hold that sieve plates may fuinction as metabolic puimps (4,9).

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