Cold-Inhibited Phloem Translocation in Sugar Beet: II. CHARACTERIZATION AND LOCALIZATION OF THE SLOW-COOLING RESPONSE

Experiments were undertaken on a simplified sugar beet systemto characterize the phloem translocation response to slow coolingtreatments that were applied to the source leaf petiole. Inthese experiments the temperature was decreased by 4°C every16 min, such that the tissue temperature was lowered from 25°Cto 1°C over a period of 80 min. Our results indicated thatan initial slow cooling treatment, on a given test plant, causedno change in the rate of translocation. However, all subsequentslow cooling regimes that were applied to the same petiole positionelicited a characteristic step-type inhibition. This inhibitionaveraged about 10% of the original translocation rate in allcases with no recovery being observed. The data suggest thatthe initial cooling treatment induced an alteration in the petioletissue which facilitated the inhibition phenomenon during subsequentslow coolings. This alteration was shown to be localized withinthe upstream region of the chilled petiole segment, followingan initial slow cooling, or throughout the chilled petiole segmentafter an initial quick cooling from 25°C to 1°C. Resultsalso show that the alteration is a long-lived phenomenon thathas no detectable influence on the quick-cooling induced transientinhibition of translocation. Key words: Phloem, Translocation, Cooling response, Petiole     

Seed Coat Unloading in Pisum sativum--Osmotic Effects in Attached versus Excised Empty Ovules

Experiments were undertaken with embryo-less ovules of Pisumsativum to study the influence of apoplastic osmolality on seedcoat import and seed coat unloading.¹¹CO₂ pulse labelling alongwith collimated monitoring of plant tissues were used with attachedovules to measure continuously and simultaneously total podimport, import into a modified ovule and photo-assimilate washoutfrom the seed coat of the ovule into a flow-through bathingsolution.Our results indicated that seed coat import was immediatelyaffected by a change in the applied bathing solution osmolality,with a decrease in osmolality lowering seed coat import andan increase in osmolality increasing import. ¹¹C-photo-assimilatewashout from attached ovules was found to respond in a similarmanner to the apoplastic osmolality. However, the osmotic effecton ¹¹C-washout was a delayed response and it appears that themajority of this observed response was due to the alterationin seed coat tracer import. Further experiments with ¹⁴C-labelled,excised seed coat halves (i.e. no further import) supportedthis hypothesis by demonstrating that seed coat unloading (measuredas ¹⁴C-photo-assimilate washout) was actually enhanced at alow solution osmolality. PCMBS had no effect on seed coat importor washout in attached, modified ovules, suggesting that photo-assimilateunloading from seed coats of Pisum does not involve a carrierprotein. Studies of the spatial distribution of imported ¹⁴Cin Pisum seed coats further suggest that this unloading, intothe apoplast, occurs from non-phloem cell types, and that themovement of photo-assimilates from the sieve elements to theterminal unloading site occurs via symplastic transport. Key words: Pisum sativum, seed coat, seed coat unloading, phloem unloading     

Cold-Inhibited Phloem Translocation in Sugar Beet: IV. ANALYSIS OF THE COOLING-INDUCED REPARTITIONING HYPOTHESIS

Systems identification techniques were used to calculate photo-assimilatetransport characteristics in Beta vulgaris and Pisum sativum,before and after the application of localized perturbationsto the transport path. Changes in photo-assimilate partitioningto various monitored sinks were found to be promoted by slowcool (25 ?C to 1 ?C in 40 min), quick cool (25 ?C to 1 ?C in2 min)/quick warm, and apoplastic osmotic treatments of a localizedregion of the source leaf petiole in Beta. Photo-assimilatepartitioning into both intact and surgically modified (embryo-less)ovules of Pisum was also observed to change following quickcool/quick warm treatments applied to a 2.0 cm region of thepeduncle leading to the monitored pod. No changes in transportproperties were observed through the treated petiole regionof Beta during or following the slow cool treatment (i.e. transittime or system gain). High apoplastic osmolality (addition of1 000 mol m^–3 sorbitol) reduced the transit time of tracermovement through the treated petiole of Beta, while enhancingtracer washout from the bathed tissue region. Our modellingtechniques have shown that the physical or physiological basisfor the sink partitioning changes must be due to alterationsdownstream of the treated pathway zone, suggesting the involvementof physical signals transmitted from the treated region. Theseresults support the previously presented cooling-induced repartitioninghypothesis (Grusak and Lucas, 1986) and demonstrate that pathway-originatedstimuli can alter source-sink photo-assimilate partitioning. Key words: Photo-assimilate partitioning, phloem translocation, Beta vulgaris, Pisum sativum     

Short-term Effects of Heat-girdles on Source Leaves of Vicia faba: Analysis of Phloem Loading and Carbon Partitioning Parameters

Petiole heat-girdle treatments (followed by a 5 min ¹⁴CO₂ assimilation)were performed on mature leaves of Vicia faba, in order to assesstheir effect on the partitioning of photo-assimilates to theminor vein phloem. Whole leaf autoradiographic evidence indicateda high leaf-to-leaf variation in the image intensity over theminor veins (relative to the mesophyll/epidermal background)in both control and heat-girdled groups of leaves. The averagedegree of minor vein labelling in heat-girdled leaves, however,was found to be significantly lower than that in controls. Comparativeassessment of vein labelling was based on microscopic densityreadings of silver grains over veinal and interveinal regionsin autoradiographic images. Investigations into the cause ofthis alteration in vein labelling indicated no involvement ofan inhibition of apoplasmic phloem loading, as both heat-girdledand control leaves of Vicia were shown to have comparable minorvein uptake of exogenously supplied ¹⁴C-sucrose. Heat-girdlingwas shown, however, to increase significantly the partitioningof recently fixed carbon into the insoluble (mainly starch)fraction relative to the ethanol-soluble fraction, within 12min of the treatment. We suggest that this carbon partitioningchange can primarily account for the change in vein labelling,since an increase in the insoluble fraction would result in(1) more ¹⁴C-activity remaining in the leaf mesophyll and (2)less ¹⁴C-activity going into the mesophyll export pool, andthus, less ¹⁴C-sucrose being transferred to the minor vein region.Additionally, although leaf export was completely halted inheat-girdled leaves, ¹⁴C-activity was found within the majorveins as far as the point of petiole heat-girdling (followinga 5 min assimilation and 4 h chase). Apparently, continued (butlimited) solution flow within the sieve elements is maintainedby transport pathway unloading within the treated leaves. Key words: Phloem loading, carbon partitioning, heat-girdle, Vicia faba     

Recovery of Cold-Inhibited Phloem Translocation in Sugar Beet: I. EXPERIMENTAL ANALYSIS OF AN EXISTING MATHEMATICAL RECOVERY MODEL

Translocation studies were undertaken on a simplified singlesource leaf-single sink leaf sugar be system to test variouspredictions of the Ferrier and Christy (1975) mathematical recoverymodel translocation inhibited by cold. Experiments were performedby using a steady state MC-labellii system for the source leafand translocation into the sink leaf was monitored with a Geiger-Muellsystem. A specially designed Peltier apparatus enabled coolingof the source petiole to 1 ?C at vario positions on the petiole,as well as over different lengths. Results of experiments testingchanges in (1) cold block length, (2) cold block position, and(3) sink unloading rate, showed little difference in ti timecourse of recovery. These results are at variance with the predictionsof the mathematical recove model. Additional experiments wereperformed to assess the potential involvement of phloe anastomosesin the recovery response. Selective petiolar incision/excisionexperiments showed: (1) that the monitored sink obtained ¹⁴C-labelledmaterial via only a few of the vascular bundles within the sourcepetiole, and (2) that anastomoses were capable of re-establishingtranslocation. This information was used to develop a revisedmodel of translocation recovery following cold-induced inhibition,ai this model is based on the utilization of adjoining, interconnectedsieve-tubes which become integrat into the pathway that suppliesthis sink. Key words: Phloem, Translocation, Cooling, Anastomoses     

Continuous In Vivo Measurement of Carbon Partitioning within Whole Plants

A method of continuous in vivo measurement of photo-assimilatepartitioning within an intact plant is proposed. The methodis demonstrated by analysis of photo-assimilate movement betweena pea pod to a single ovule and then to solution bathing thesurgically modified seed coat Key words: Photo-assimilate partitioning, partitioning coefficient, seed coat unloading, systems identification