Real Time Studies of Carbon-11 Translocation in Moonflower: II. THE EFFECTS OF METABOLIC AND PHOTOSYNTHEIC ACTIVITY AND OF WATER STRESS

Naturally loaded carbon-11 was used to study in vivo the dynamicsof translocation. It was found that translocation displayedstrong short-term sensitivity to source chilling, source anoxia,disturbance of the osmotic balance of stem apoplast, and apexshading but that it seemed indifferent to metabolic inhibitorsand stem anoxia. Plant-wide water stress effected a long-termdisruption of translocation which was reversed only slowly whenthe stress was relieved. These experiments generally supporta mass flow theory, but indicate that there is a close couplingbetween translocation streams from different sources.     

Analysis of Tracer Profiles with Applications to Phloem Transport

Tracer experiments are quantitatively described in a way whichis independent of the tracer profile, and hence the loadingprocess. This enables properties of the transport system tobe determined which are independent of tracer profile shapeand hence of the loading process. Comparison is made betweentransit times obtained by this approach and by measurement directlyfrom tracer fronts. The direct method is shown to give resultswhich are dependent on the shape of the tracer profile. Thismethod also enables in vivo measurement of leakage from thetransport pathway.     

The Relationship Between Spatial and Temporal Tracer Profiles in Transport Studies

The relationship between spatial and temporal tracer profilesis established, and used to re-analyse reported profiles ofboth types. Evidence of dispersive flow (i.e. a range of transportspeeds) in phloem translocation is obtained from both spatialand temporal profiles. Previous suggestions of plug flow areshown probably to be a consequence of too long a time intervalbetween successive measurements in obtaining these profiles.     

Short Term Interactions between Flows of Photosynthate

Using radioactive tracers, interactions between flows of photosyntheticassimilate were observed in vivo with several species. Changesin translocation from one source (brought about by repeatedchilling and re-warming which stops and re-starts flow) cancause an immediate and partially compensating change in translocationfrom another source. The rapidity of response implies the transmissionof a physical rather than a chemical signal. The compensationcan be sustained for at least 90 min and involves a change inthe speed of sap flow rather than a change in its concentration.A change in the rate of loading in the source (lamina) is probablyinvolved, but this is not always the case, for it seems thatthe signal calling for extra assimilate is a lowering of sievetube pressure potential and this is transmitted over only rathershort distances of stem (though over quite long distances ofpetiole), because of entry of water and assimilate (loading)from stores in close proximity to the pathway. The responseis buffered therefore and becomes less evident as one movesaway from the site of treatment. Key words: Photosynthate translocation, Phloem loading     

A Rate of Cooling Response in Phloem Translocation

A transient inhibition of phloem transport caused by coolingis found to depend on the rate of cooling. This inhibition hasbeen observed for a temperature drop of only 2.5 C?. Possiblemechanisms are discussed. Key words: Phloem, Translocation, Cooling     

Dynamics of Cold Induced Inhibition of Phloem Transport

Inhibition of phloem transport caused by chilling has been studiedby in vivo measurement of temporal tracer profiles observedboth uptstream and downstream of the chilled region. The dynamicsof the profile changes indicate that blockage is immediate butdoes not cause flow along the pathway to cease for some considerabletime (50–100 min). This behaviour is anticipated for apressure driven mass flow system. These findings demonstratethe caution needed in interpretation of sink accumulation rates. Key words: Phloem transport, Chilling     

Real Time Studies of Carbon-11 Translocation in Moonflower: I.THE EFFECTS OF COLD BLOCKS

The translocation of naturally loaded carbon-11 was studiedin the moonflower (Ipomoea alba L.) and its response to freezingand chilling determined. The immediate (within 5 min) effectsof either freezing or chilling a short length of either stemor petiole seem broadly similar: translocation in regions nearthe cold block is altered at once and translocation in regionsfar from the block can also be changed forthwith. This responseis a whole-plant response and of considerable complexity. Theresults are consistent with a mass flow mechanism.     

Phylogenetic Distribution and Mechanism of Translocation Inhibition by Chilling

Lang, A. and Minchin, P. E. H. 1986. Phylogenetic distributionand mechanism of translocation inhibition by chilling.—J.exp. Bot. 37: 389–398 A standard chilling treatment was applied to a short lengthof thc translocation path of a range of plants (86 species representativeof 50, mostly angiosperm, families) and sensitivity of the translocationprocess assessed. In all dicotyledons, but only in 30% of monocotyledons,translocation was temporanly and reversibly inhibited by chilling. Our findings emphasize that inhibition of translocation by pathwaychilling should not be assumed to occur in all species. Studies,therefore, which employ pathway chilling as a device for temporarilymodifying patterns of assimilate flow must include a carefulexamination of the translocation behaviour of the species beingused under the precise treatment conditions imposed. This hasnot always been done in the past. Although sensitivity is possibly correlated with the phylogeneticdistribution of p-protein, the response to chilling is moreplausibly explained as a perturbation of membrane function thanas a blocking and unbiocking of sieve plate pores. Key words: Chilling, Translocation, Phylogcny     

Further Errors in the Acetylene Reduction Assay: Effects of Plant Disturbance

A flow-through gas system was used to study the effects of disturbanceon nitrogenase (acetylene reduction) activity of nodulated rootsystems of soyabean (Glycine max) and white clover (Trifoliumrepens). Detopping plus removal of the rooting medium (by shaking)produced a substantial decrease in maximum nitrogenase activity.This response is due to a reduction in oxygen flux to the bacteroidscaused by an increase in the oxygen diffusion resistance ofthe nodule. The decrease in maximum nitrogenase activity wasmuch smaller for roots subjected to detopping only. Thus, theeffect of root shaking is more important than that of shootremoval. The effect of detopping plus root shaking on nitrogenase activityoccurred whether the plants were equilibrated and assayed at25°C or 15°C. However, the effect of disturbance onthe oxygen diffusion resistance of the nodules, and thus onnitrogenase activity, was greater at the higher temperature.At the lower temperature the oxygen diffusion resistance ofthe nodules had already been increased in response to the reducedrequirement for oxygen. These nodules were less susceptibleto the effects of disturbance. Thus, comparisons of the effectsof equilibration temperature on nitrogenase activity produceddifferent results depending on whether intact or disturbed systemswere used. With intact systems activity was lower at the lowertemperature but with detopped/shaken roots the lowest activityoccurred at the higher temperature. It is concluded that the use of detopped/shaken roots can producesubstantial errors in the acetylene reduction assay, which makesthe assay invalid even when used for comparative purposes. However,comparisons with rates of ¹⁵N₂ fixation and H₂ production showthat accurate measurements of nitrogenase activity can be obtainedfrom maximum rates of acetylene reduction by intact plants ina flow-through gas system. The continued use of assay proceduresin which cumulated ethylene production from disturbed systemsis measured in closed vessels must be questioned. Key words: Nodules, acetylene, nitrogenase activity