AND · TRANSLOCATION IN ZEA MAYS LEAVES parameter in translocation because it is required for...

Plant Physiol. (1977) 59, 808-820

Relations between Light Level, Sucrose Concentration, andTranslocation of Carbon 11 in Zea mays Leaves

Received for publication April 6, 1975 and in revised form October 16, 1975

JOHN H. TROUGHTON AND B. G. CURRIEPhysics and Engineering Laboratory, Department ofScientific and Industrial Research, Private Bag, LowerHutt, New Zealand

F. H. CHANGBotany Department, Victoria University, Wellington, New Zealand

ABSTRACT

The mechanism of carbon transport in Zea mays leaves was investi-gated using carbon 11 which is a short lved (half-lfe 20.4 min) positron-emitting isotope. The gamma radiation produced on annihilation allowsin vivo or nondestructive measurement of the isotope and the short half-life allows many measurements of trandocation to be made on the sameleaf within the same day.

Carbon 11 produced by the 10B (d,n)"C nuclear reaction was con-verted to "CO2, fed to a leaf as a short pulse, and assimilated duringphotosynthesis. The progress of the radioactive pulse along the leaf inthe phloem was monitored in severd positions simultaneously withcounters. The counters were Nal crystals with photomaltipliers and theoutput was amplified, passed to single channel analyzers, and the countsaccumulated for 20 seconds every 30 seconds. Corrections were madefor the half-life and background radiation by computer, and the resultswere displayed on a high speed plotter. Information derived from thecorrected data induded the speed of translocation, the shape of theradioactive carbon pulse, and the influence of light and distance alongthe leaf on these parmeters. The plants were kept under controlledenvironment conditions during all measurements.A speed was derived from the time displacement of the midpoint of

the front of the pulse, measured at two positions along the leaf. This wasan apparent mean speed of translocation because it averaged a variationin speed with distance, variation in speed between or within sieve tubes,and it averaged the mean speed of all of the particles in the pulse.A wide range of speeds of translocation from 0.25 to 11 cm min-' was

observed but most of the variability was due to the variation in lightavailable to the leaf. For example, the speed of translocation was pro-portional to the light level on either the whole plant or individual leaf.Shading of the leaf established that the light effect was not localized ineither the feeding area or in the portion of the leaf on which themeasurements were made. It was proposed that the speed was depend-ent on the proportion of the leaf in the light upstream from the lastcounter. The speed of translocation was relatively independent of thestage of growth of the plant, age of the leaf, and the time during thediurnal light cyde.

Data obtained on the level of the reducing sugars, starch, and sucrosein the leaf were related to the speed of translocation. A biphasic relation-ship between speed and sucrose concentration in the leaf was establishedand the high speeds measured during experiments only occurred whensucrose concentrations in the leaf exceeded 8% of the dry weight.The shape of the pulse loaded into and translocated in the phloem was

estimated from the half-width of the pulse. The half-width was primarilydetermined by loading phenomena which resulted in an increase in thehalf-width from 2 minutes when fed to the leaf to more than 40 minutesin the phloem. In many examples, the pulse continued to broaden withdistance along the leaf from the fed region. The half-width was inde-pendent of the speed but highly dependent on the light level.

Carbon translocation is an integral component of the growth-and yield-determining processes in higher plants. To character-ize long distance C transport, it will be necessary to measure thespeed and sugar concentration of the solution and the cross-sectional area of the sieve tubes active in translocation. Otherdata, such as the turgor pressure and the water, ion, and sugarexchange rates of the sieve tubes, are required to establish themechanism of C transport and explain the cause of the variationin C translocation between species, or between plants, underdifferent environmental conditions.

Routine measurement of some of these translocation parame-ters has not been easy. Through the development of techniquesto produce 'IC in this laboratory, we are now able to measureroutinely the speed of translocation and the shape of the pulse inthe phloem (36). Carbon 11 is a short lived isotope with a half-life of 20.4 min that decays by emission of 0.96 Mev positrons,which on annihilation yield two 51 1-kev gamma rays. The highenergy gamma radiation allows nondestructive monitoring of theisotope in vivo and, by use of external detectors, the distributionwith time of the isotope throughout the plant can be studied. Theshort half-life of the isotope restricts its use to about 3 hr in ourapplication, but is advantageous because the same leaf can bereused for many days, and at least six experiments are possibleon the same leaf within the same day. The ease and efficiency ofdetection of the gamma radiation allow the use of small amounts(10-13 g) of C, and short counting periods (less than 1 min) canbe used which allows dynamic studies of isotope transport. Thelow levels of radiation and the short half-life of carbon 11 wouldprevent radiation damage to the plants.

Continuous feeding of carbon 11 to the plant is possible if anaccelerator or cyclotron is used on-line. Under many conditions,it will be more practical to use pulse-feeding as practiced in thislaboratory. From pulse-feeding, an immediate estimate of thespeed of translocation and the half-width pulse can be made. Thespeed is obtained by determining the time taken for the pulse topass between two points a known distance apart and the half-width is measured directly from each pulse.The role of speed in translocation has yet to be defined,

although two aspects are of special importance. The relationshipbetween speed and mass flow of sugar will be critical informationfor understanding the physics of flow in the sieve tubes (2) and,for plant productivity studies, the time delay between C fixationand its utilization in new leaf production will be important tocrop growth rate. Canny (2) reports about 30 measurements ofthe speed of translocation in different species under differentconditions and using different techniques. The range was 0.1 to2.5 cm min-'. There have been few attempts to relate speed toany genetic, physiological, or environmental parameters.

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TRANSLOCATION IN ZEA MAYS LEAVES

parameter in translocation because it is required for photosyn-thesis and, if the driving force for translocation is osmotic, thenphotosynthesis provides one source of osmotically active sub-stances. For example, the rate of translocation (mass moved perunit time per unit cross-sectional area of the sieve tubes) hasbeen shown to be greater in the light than the dark (13-15, 21,25, 28, 31). Photosynthesis may stimulate translocation in sev-eral ways: (a) increasing ATP (and NADPH) levels which mayinfluence CO2 fixation, distribution of C between products,sucrose transport, sieve tube loading, and transport in the sievetubes; and (b) increasing the level of C compounds (primarilysucrose) which would increase the respiration rate and/or changeosmotic conditions in the leaf.The rate of translocation would not be expected to be corre-

lated with the instantaneous rate of photosynthesis, as transloca-tion can occur in the dark and in nonphotosynthetic tissues.Translocation must, at least partly, be independent of a directeffect of the cyclic and the noncyclic photophosphorylation reac-tions, even though these processes may stimulate some aspectsof translocation. It is more likely that a product of the C cycle isinvolved and it has been proposed that the level of sucrose in theleaf determines the rate of translocation (11). At least as aninitial proposition, we predict sucrose concentration will also belikely to be important in influencing the speed of translocation,under conditions of nonlimiting water and nutrients and at opti-mum temperatures. Light may also influence the rate of evapo-ration of water from the leaf and thereby influence translocationthrough an effect on the water relations of the plant.

Experiments reported here establish the relationships betweenthe environmental parameter light, sucrose levels in the leaf, andthe speed of translocation. Four aspects were investigated: (a)the quantitative relationship between light level and the speed oftranslocation; (b) localization within the leaf of the light effecton speed; (c) the relationship between the speed of translocationand the levels of sucrose in the leaf; and (d) the effect of lightlevel and position in the leaf on the shape of the isotope pulse.The plant species used in experiments was maize. It has a lack

of CO2 evolution in the light and a high affinity for CO2 (33, 34)which is characteristic of a C4 plant. Sucrose has been shown tobe the major C compound translocated in maize (16).

MATERIALS AND METHODS

Plant Materials and Environmental Conditions. Zea mays (cv.Morden) plants were grown from seed in controlled environmentcabinets at 28 + 1 C, 80 ± 10% RH, 12-hr day length and at alight level of 240 w m2 (400-700 nm) produced by 400 wHPLR mercury vapor lamps. The plants were grown in perliteand regularly fed a modified Hoagland nutrient solution. Acomparison was made of translocation in plants at differentstages of growth but most plants were about 4 weeks old and hadfive fully expanded leaves. All experiments were conducted in acontrolled environment cabinet kept at the same conditions asgiven above. The light level was varied by raising or lowering thelamps. The thermal environment was dntinuously monitored ineight positions by copper-constantan thermocouples with theoutput read on a multichannel digital voltmeter. The tempera-ture in all parts of the system was 28 t 1 C at all times (leaftemperatures 28 ± 2 C), even during periods of transition fromlight to dark and when parts of the plant were in the dark.A small leaf chamber enclosed part of the leaf. Temperature

control was by a water jacket. Normal air was humidified andpassed through this chamber at approximately 5 liters min-' tomaintain 300 ± 20 1l/l CO2 concentration in the chamber at alltimes. The area of leaf enclosed by this chamber was about 15cm2. The organization of the equipment has changed consider-ably over the period of the experiments reported here, and adescription of equipment used in the early stages is given else-where (36).

Carbon 11 Production and Counting Procedures. The isotopecarbon 11 was produced by the '0B (d,n) 'IC reaction usingdeuterons from a 3-Mev Van der Graaff accelerator (22). Asolid enriched 'OB target was bombarded with deuterons and 02gas was passed across the face of the target. The 02 reacted with'IC produced in the nuclear reaction to form "1CO and "CO2which were recovered from the target area by a differentialpumping system. The "1CO was converted to "1CO2 by heating to600 C in the presence of CuO wire and any H20 was removedfrom the air stream by a Dry Ice-alcohol trap. A liquid 02 trapwas used to remove "CO2, and contaminant gases, such asexcess 02 and N2, were exhausted to the atmosphere. Thistechnique has the potential for producing "CO2 without contam-ination from any other gases, including 12CO2 or 13CO2. The"1CO2 was recovered by warming the liquid 02 trap to roomtemperature and the gas was transferred to a syringe ready forinjection into the leaf chamber.The "CO2 was fed into a leaf chamber enclosing a 2-cm wide

strip of the maize leaf. The leaf was exposed to normal airbetween isotope feeding periods. The radioactive gas was fed asa pulse of between 3 and 5 min duration and was flushed fromthe system within 1 min by using normal air. The natural photo-synthetic reaction was used to incorporate the "1CO2 into "IC-labeled sucrose. During translocation experiments in the dark, itwas necessary to expose the leaf to light for about 5 min. Thisinfluenced the results, but the ratio of light to dark was 1:36 andthe light period would only have a small effect on the results.The procedure was the same for all dark measurements, so atleast the treatment results can be compared.The activity of 'IC in vivo was monitored externally with

counters set at 10-cm intervals downstream (i.e. toward the leafbase) from the feeding chamber. Three or four counters wereused. The relationship between the fed region, sieve tubes,counters, and leaf base is shown diagramatically (Fig. 1). Eachcounter was shielded by lead to collimate the counts to a speci-fied segment of the leaf.The detectors were Nal crystals (5 x 2.5 cm) coupled to a

photomultiplier and preamplifier. The output was then passed toan amplifier and single channel analyzer set at 511 kev. Thecounts were accumulated on blind scalers and at intervals thecounts were printed and punched onto paper tape using a tele-type. The paper tape was then fed to a computer and the datacorrected for background counts and the half-life of the isotope.The period required to accumulate sufficient counts for accu-

rate analysis depends on numerous physical and physiologicalaspects of the system. The parameters that can be altered includethe activity of the isotope fed to the leaf, the length of thefeeding period, the width of the fed areas, the width of thecollimator slit, the distance between counters, the sensitivity ofthe detectors, and the counting period. Physiological attributesare usually under investigation and therefore any limitations inthese components have to be accepted. Plant species and envi-ronmental conditions will influence the loading characteristicsand the speed of translocation, and will necessitate continualadjustment of the settings for efficient counting. For our experi-ments, the counts were accumulated over 20 sec, with twocounting periods/min. To improve the counting statistics at lowspeeds, and to standardize the data, the mean count rate/minwas derived from two 20-sec counting periods. The correcteddata were normalized and the output from each counter re-corded, using a high speed plotter.Time zero in all results was the time of injection of the isotope

into the feeding chamber. For the feeding and flushing periods,there was high isotope activity in the vicinity of the counters andon some occasions this gave spurious counts during the first 4min. After flushing, the isotope activity in these regions dimin-ished and its effect on the counters was negligible. Curve fittingor smoothing of the data was unnecessary.Two parameters were derived from the isotope data: the speed

Plant Physiol. Vol. 59, 1977 809

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TROUGHTON, CURRIE, AND CHANG

DIAGRAMMATIC REPRESENTATION OF A MAIZE LEAF

CO;PHOTOSYNTHESI S

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FIG. 1. Diagram of the relationship between the fed region of the maize leaf, the sieve tubes, and the counters.

of translocation, and the half-width of the pulse. Both parame-ters were obtained by direct measurement from the plotted data.The speed was taken from the time delay in the arrival of themidpoint of the front of the pulse between any two counters. Thehalf-width was the width of the pulse (expressed as time) at halfthe maximum height of the pulse at each counter.

Carbohydrate Determination. The plants used for carbohy-drate measurements were kept under identical conditions, in-cluding pretreatment light or dark as previously reported (36).This allowed direct correlation to be made between speedsmeasured previously with carbohydrate levels reported here.

Small segments or all of the leaf (150-200 mg, fresh wt) washarvested and plunged into 20 ml of boiling 80% ethyl alcohol.Three extractions with 80% boiling ethyl alcohol were made andthe combined supernatants kept for soluble sugar estimation andthe residues were kept for starch estimations.

For soluble sugar determinations, Chl and lipids in the super-natants were removed by adding chloroform and equal volumesof H20. The solution containing sugar was evaporated to drynesson a rotary evaporator. Twenty ml of distilled H20 was addedand an aliquot was taken for analysis of both the reducing andthe total soluble sugar. Sucrose was taken as a difference be-tween total soluble sugar and reducing sugar.

Reducing sugar levels were estimated using the neocuproine-HCl method (5). Reducing sugar was measured using A at 425nm and by comparison with a standard curve for reducing sugarobtained using glucose.

Total soluble sugars were determined using the anthrone re-agent (32, 37). Absorbance was measured at 625 nm and thetotal soluble sugar level obtained by comparison with a standardset of glucose samples.

Starch in the leaf tissue was analyzed using MacRae's enzymicmethod (20). The residue obtained from the leaf tissue afterextraction with boiling 80% ethyl alcohol was ground and sus-pended in distilled H20. This suspension was heated for 30 minat 100 C and the volume of the sample was kept constant. Thisgelatinized the starch. The starch was hydrolyzed with amyloglu-

cosidase at 60 C for 40 hr and resulted in the production ofglucose. A layer of paraffin oil prevented oxidation of the glu-cose.

Glucose was estimated by the glucose oxidase method (17).The tissue extract was incubated with distilled H20 and glucoseoxidase reagent at 37 C for 1 hr. After the addition of 18 N H2-S04, the absorbances were read at 540 nm against a water blankand were compared with results from a standard curve usingdifferent amounts of glucose. The starch content of the leaf wascalculated from knowing the glucose concentration and the origi-nal weight of the starting tissue.

RESULTSGENERAL FEATURES OF CARBON 1 1 TRANSLOCATION

Direction of Photosynthate Movement in the Zea mays Leaf.The pulse of 1'CO2 was fed to a 2-cm wide leaf segment, two-thirds of the distance from the base to the tip of the leaf. Thedirection of transport of the isotope was monitored by counters(5 x 2.5 cm) 10 cm either side of the fed region. The isotopepredominantly moved toward the leaf base. The detector (5 x2.5 cm) upstream from the fed region was replaced with a crystal(7.5 x 7.5 cm) for seural experiments, and, with the highersensitivity of this detector, it was possible to detect counts in theleaf tip on some occasions. A pulse was observed in severalexperiments (Fig. 2). The total counts moving acropetally wereabout 2% of the counts in the pulse moving toward the leaf baseover the same period. It is also possible that the acropetalmovement was in the xylem. In view of the highly directionalnature of C transport in maize leaves under these physiologicalconditions, the results in this paper will only refer to transloca-tion toward the leaf base.

Shape of the Efflux Curves. A counter beneath the fed regionmonitored the loss of counts from that leaf segment. There islittle loss of CO2 in the light from photorespiration in maize (34),and therefore the change in counts was primarily due to translo-cation. Differentiation of this efflux curve yields the shape of the

810 Plant Physiol. Vol. 59, 1977




pulse of isotope loaded into the phloem. Two contrasting effluxcurves are illustrated in Figure 3A. In one example, 50% of the'IC fed to the leaf as a 4-min pulse was still in the leaf after 30min, and in the other example, the same amount was presentafter 90 min. There was a considerable and variable delaybetween C fixation and translocation.One implication of this delay is that there may not be a close

relationship between the rate of translocation and the rate ofphotosynthesis unless there is a constant and irreversible loss ofC to the nonsucrose pool and a rapid turnover of the sucrosepool. For example, at the beginning of the light period, photo-synthesis may quickly attain its maximum value (within 5 min),

LI)HzD0U

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FIG. 2. Evidence for acropetal transport in a Z. mays leaf. A NaIcrystal (7.5 x 7.5 cm) 10 cm toward the leaf tip from the fed regionmonitored a pulse of 'IC moving acropetally. Total number of counts inthe pulse was less than 2% of the counts moving toward the base. Peakof the pulse at about 20 min indicates a speed of about 0.5 cm min-'.

whereas 50% of the labeled C assimilated during the period maynot have been translocated 30 min (or more) later.

In C4 plants, such as maize, it was anticipated that lightactivation of some of the photosynthetic enzymes may causedelays to CO2 fixation during a dark-light transient. This waspreviously tested using Atriplex spongiosa, and the delays, al-though present, were not significant (33).The maximum slope of the efflux curve was measured in

numerous experiments but it was highly variable and the varia-bility difficult to explain. The plant was transferred from dark(16 hr) to light and kept in 240 w m-2 of light for 48 hr, underconstant environmental conditions. The same position in the leafwas fed each experiment. The value in the dark was about themean of the values determined in the light. The low values earlyin the light period may be associated with the increase in thetotal carbohydrate of the leaves (see Fig. 1 1A). There was nodirect relationship between the speed of translocation and themaximum slope of the efflux curve, at least in this example, asthe speed was constant at 3.5 ± 0.5 cm min-'.

Pulse Shape and Estimation of an Apparent Mean Speed ofTranslocation. A feature of all results was that the half-width ofthe isotope pulse increases from 2 min for the pulse fed to theleaf as "CO2 to greater than 40 min at the first counter. Thebroadening occurred in the C metabolism between photosyn-thesis and translocation. Although the feeding period was keptconstant, the half-width of the pulse in the leaf was highlyvariable. Characteristic shapes of the pulses at three countersalong the leaf are shown in Figure 4. Examples of a pulse for afaster (Fig. 4B) and a slower moving stream (Fig. 4C) are alsoillustrated.

It was not possible to define the speed as being the mean speedfor all of the particles in the pulse because the tail was occasion-ally diffuse and had long delays. For a slow moving stream and atlarge distances from the fed region, the time taken for the tail toreach the last counter was beyond the time scale of the experi-ment (e.g. last counter in Fig. 4C). Lateral movement of isotopeout of the sieve tube could also influence the measured speed,but the effect is likely to be least in the front of the pulse. As aresult of these considerations, the speed was estimated from themidpoint of the front of the pulse. A speed estimated in this waycan only be an apparent mean speed of translocation, although it

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TIME IN THE LIGHT (hrs.)FIG. 3. A: Change in counts in the fed region as a function of time (an efflux curve, ref. 36). Differentiation of these curves indicates the shape of

the isotope pulse loaded into the phloem. Data illustrate the variation that can occur and are evidence that the half-width of a pulse fed to the leaf is

substantially altered before entering the phloem. B: Variation in the maximum slope of the efflux curve on a leaf under constant environmentalconditions and 240 m-2 light level. The plant was pretreated with 16 hr darkness.

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Plant Physiol. Vol. 59, 1977 811

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were corrected for half-life and background and the results are photographed from computer plots. No smoothing of the results was necessary. Eachdot in A is for a half-min interval; in B and C, they are for 1-min intervals. For comparison of results, the data have been normalized to the maximumcount rate. In all cases, the curves represent in order of increasing time from time zero (time of injection) the counts on counters 1, 2, 3, and 4,respectively. Distance between counters was constant at 10 cm, with the first counter 10 cm from the fed region.

will be referred to in this paper as the speed of translocation. It isnot possible to compare directly the results calculated in this waywith other work, unless the techniques for analysis are similar.The shape of the pulse also had implications for the maximum

period between measurements. This period was determined bythe rate of fall off in the tail of the pulse and the natural decay inisotope activity. The period between measurements need not

exceed 3 hr and under many conditions was 2 hr.Physiological Age of the Plant and the Speed of Transloca-

tion. The plants used during the period of these experimentsincluded a wide range of physiological ages and stages of growth.The influence of the physiological state of the plant on speed was

investigated by selecting measurements made under similar con-

ditions, i.e. over a 20-cm path length, a temperature of 28 +

2 C, and a light level of 240 ± 40 w m-2. There may have beensome effect on the results of an improvement in the experimentaltechniques during the period and the variation in the environ-ment between treatments. By standardizing the growing condi-tions of the plant, the species used, the environment, and ar-

rangement of the leaf during the measurements, it was possibleto sort the results according to the stage of growth.The results in Table I give evidence for a small (less than 10%)

increase in speed during the period of stem elongation and cobformation, but this effect is unlikely to be important. During thegrowth periods investigated, there was apparently little effect onthe speed of translocation of potential variations in sink strengthdue to the stage of growth, in a fully expanded leaf. Othermeasurements in this paper primarily refer to plants in theintermediate stage of growth, but the results would not besubstantially affected by some variation in the physiological ageof the plants.

Factors Determining the Speed of TranslocationLight Level. The light level on a whole maize plant was varied

and the speed of translocation monitored over 20 cm on one

leaf. The light was measured in the position of this leaf and thelight level was kept constant along the length of the leaf. Theplant was kept at each light level for a 12-hr day and until threeconsecutive measurements of the speed were constant (to ±0.2cm min-'), to ensure that the plant was in equilibrium at eachnew light level. The same leaf was used for all experiments and a

Table I. The Speed of Translocat ion and the PhysiologicalAge of the Leaf and Plant

No. Stage of Growth Leaf Position leasurements Range of Mean SpeedNo. Seelds

cm minm1 cm min-11 Young (<3 weeks) 4th of 7 5 3.5 - 4 3.8 + 0.2

Vegetative2 Yo.Mg 5th of 7 6 3.7 - 4 3.9 0.1

Vegetative3 Intermediate (3-5 5th of 7 3 3.6 - 3.9 3.8 0.1

weeks) Stemelongation

4 Intermediate 4th of 6 3 4.1 - 4.8 4.5 + 0.3Stem elongation

5 Mature (>5 weeks) 5th of 9 5 3.9 - 4.1 4.0 ± 0.1Cob formation

6 Mature (>5 weeks) 6th of 10 10 3.5 - 4.3 3.9 0.2Cob formntion __

speed was measured in the dark at the beginning of each day totest that there were no carry-over effects between days or treat-ments.The dark values before each light period were 1.1 + 0.2 cm

min-'. These values, shown with the diurnal variation in speedfor two light levels (Fig. SA), indicate there were no majorcarry-over effects. At low light, the speed reached an equilib-rium value soon after the light was turned on, but at high lightthere was a lag of several hr, usually 3, before the speed was

constant. As is evident in Figure 5A, light level influenced thespeed of translocation. There was an approximately linear rela-tionship between light level and the speed of translocation (Fig.SB).The relationship between light level and speed, extrapolated

to zero light, yields a value for the speed in the dark. This valuewas 1.1 cm min-1, which is similar to the value measured after 12hr of darkness (Fig. 5A).Speed of Translocation during Transfer from Light to Dark

Conditions. The light level experiments indicated that the timeconstant for adaptation of the speed of translocation to high lightwas long (greater than 1 hr). In contrast, photosynthetic re-

sponses to a change in light are short (less than 5 min). The timeconstant for adaptation of the speed of translocation to darknesswas investigated. The same experiments identified the site of thelight effect on translocation. These investigations only alteredthe light on the leaf on which the speed was measured. The rest

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Plant Physiol. Vol. 59, 1977

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FIG. 5. Influence of light level on speed of translocation. A: Daily course of the speed of translocation in the same leaf at two light levels during a

dark-light transition. Arrows indicate light on; (4', 0): 70 w m-2; (*, *): 230 w m-2. Speed in the dark was 1.1 + 0.1 cm min-' and was constant forthree values at each light level. Speed was measured over a 20-cm length of leaf. B: Relationship between light level and speed of translocation inmaize. All values were determined on the same leaf under constant environmental conditions, except for light level. Speed was measured over 20 cmand the reading at each light level was the mean of three values which were constant to within 0.2 cm min-'.

813

of the leaves on the plant and the feeding chamber were kept athigh light.The results of the darkening treatment are given in Figure 6A.

Speed was measured at intervals over a 12-hr period, with thefirst 6 hr at 240 w m-2 and the remaining time in the dark.Measurements were made at four positions downstream fromthe feeding chamber and, from these measurements, the changesof speed with distance were derived. The speed of translocationwas in equilibrium with the light level before the leaf was

darkened. After 6 hr in the dark, the speed had dropped fromabout 3.8 cm min-' to 1 cm min-', with a time constant of about1.5 hr.

In the light, the speed was faster over the last two 10-cmsegments than in the first segment, but in the dark, any influenceof distance from the fed area on speed was eliminated.

Localized Shading Treatments and the Speed of Translocation.Shading treatments were applied to identify the site of the effectof light on the speed of translocation. Initially, a 20-cm length ofthe leaf directly over the counters was shaded, and the leaf area

involved was approximately half of the area of the leaf upstreamfrom the last counter. The measurements reported in Figure 6Bwere made over 20 cm on two different maize leaves. The leafwas allowed to come to equilibrium with the light level for 6 hrbefore the shading treatment was applied. Shading caused the

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FIG. 6. A: Influence of a light-to-dark transition on the speed of translocation at three positions along the leaf 10 to 20 (0), 20 to 30 (x), and 30to 40 (A) cm from the fed region. Time zero is lights on and the arrow is lights off. Differences in speed with distance along the leaf are removed after4 hr dark. The whole plant was kept at 240 w m-2 and only the leaf on which the measurements were made was darkened. The fed region was kept inthe light. B: Influence of shading 50% of the area of a leaf upstream from the counters on the speed of translocation over 20 cm. Two different leaveswere used (0, *) and (4, -*) refer to lights off for each leaf. The rest of the plant was in high light.

speed to decline from 3.9 cm min-' to a new equilibrium value of2.6 cm min-'. Assuming the speed in the dark would be about 1cm min-', shading 50% of the leaf reduced the speed by about45%. These results suggest that the reduction in speed was

approximately proportional to the area of the leaf that was

shaded upstream from the last counter.Comparison of Figure 6, A and B establishes that there is a

contribution from outside the shaded area to the componentsdetermining the speed. The reduction in speed with shadingcould be a consequence of a reduction in photosynthesis.

A 10-cm length of leaf between two counters was shaded, andthe speed was measured in that segment and in two 10-cmsegments immediately upstream and downstream from theshaded portion. Measurements of speed in this example includethe effect of distance along the leaf on the speed. The effect ofdistance on the speed was investigated in high light and beforethe shading treatment was applied (Fig. 7). There was a largeincrease in the speed between the first two segments and only a

small effect of distance on speed between the last two segments.The shading treatment decreased the speed by less than 20% at

6A

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TIME (hrs.)FIG. 7. A: Ten-cm length of leaf between two counters was shaded. Speed over 10 cm was measured immediately upstream (0), and immediately

downstream (A) from the shaded region and in the shaded region (x). The whole plant was kept at 240 ± 10 w m-2 throughout the experiment.

all positions, which was in proportion to the area shaded.The effect of shading different positions in the leaf was investi-

gated in a further series of experiments. The same leaf was usedfor all measurements. The area shaded was approximately 10%of the area upstream from the counters, and shading was either a10-cm length upstream or downstream from the fed area or a 10-cm length over the counters. Each treatment occupied a day.The mean speed over 20 cm was the same on 3 consecutive daysafter 4 hr in high light (Fig. 8A).

Shading upstream from the fed region or shading a 10-cmlength over the counters had a small effect on the speed oftranslocation over a 20-cm segment. In contrast, shading the leafbetween the fed region and the first counter resulted in anapproximately 40% increase in speed. Analysis of the variationin speed between the first and second 10-cm segments indicated(Fig. 8B) that there was a decline in speed in the second seg-ment, but approximately a doubling of the speed in the segmentimmediately downstream from the shaded area. One speed,shown in the figure as being in excess of 7 cm min-', was 11 1cm min-'.On the same leaf, a series of measurements were made in

which the feeding chamber was covered. There was no effect ofshading this small part of the leaf on the mean speed of translo-cation, measured over a length of 20 cm downstream from thefed region.

Factors Determining Pulse ShapeLight Level and the Shape of the Isotope Pulse in Phloem. A

further characteristic of the solution flowing in the sieve tubes isthe shape of the isotope pulse. The parameter used in theseexperiments to describe the shape is the width at half-maximumheight. An indication of the variation that can occur in the shapeof the pulse was previously given (Fig. 4, A, B, and C). Manyexperiments were terminated before the half-widths could bedetermined and the results in these cases are presented as beinggreater than 100 (or 120) min.The half-width of the pulse at any one position in the leaf was

relatively constant (that is ±5 min for a variation of ±10%)

when the environment was constant (Fig. 9A). The half-widthunder normal conditions and at high light was 45 10 min formost positions within the leaf. In most examples given here, thehalf-width of the pulse increased with distance down the leaf,although the extent of the increase was variable and in someexamples doubled along a 30-cm segment. In many experiments,the increase in half-width with distance remained constantthroughout a 12-hr period when measurements were made at 2-hr intervals (9A).Shading Effects on Pulse Shape. A 10-cm segment upstream

from the feeding chamber was shaded and it had little effect onthe shape of the pulse at any counter (Fig. 9A). The influence ofshading on the shape of the pulse in other treatments was morepronounced. In another experiment, 10 cm between the first twocounters was shaded but this only caused an increase in half-width 4 hr after shading.

In another experiment, the half-width was relatively constantat all positions along the leaf, and shading the leaf between thefeeding chamber and the first counter initially removed anydifferences (Fig. 9B). Subsequently, this treatment caused thehalf-width at the first counter to be greater than at the second orthird counters. After 4 hr of shading, the half-width of the pulseincreased at all counters. In contrast to the lack of changes inhalf-width, this treatment caused substantial variations in thespeed of translocation between the first two counters (Fig. 8B).

Shading the feeding chamber while the rest of the leaf waskept in high light resulted in a dramatic increase in half-widthfrom 55 to 100 min within 2 hr (Fig. 9C). Subsequently, all half-widths were greater than 120 min. These effects were relativelyindependent of the speed of translocation, as the speeds at 2-hrintervals throughout the day were consecutively 3.77, 4.21, 2.8,3.7, 3, and 3.6 cm min-' over a 20-cm length of the leaf. Theseresults provide evidence that the speed is independent of theshape of the pulse but that the half-width is substantially affectedby light level in the fed region. A reverse treatment was alsoapplied, i.e. where the whole leaf except the feeding chamberwas shaded. The results shown in Figure 9D indicate that this

AA

x

x AA

x

x0

0000~~~~~

0

I I I II

815Plant Physiol. Vol. 59, 1977



TROUGHTON, CURRIE, AND CHANG Plant Physiol. Vol. 59, 1977

8

7

^ 6

c

E 5

E

4

3

lU

2

0 2 4 6 8

TIME (hrs.)

B

10 12 14

l1

2 4 6 8

TIME (hrs.)10

FIG. 8. A: Measurements of the speed of translocation on the sameleaf on 3 consecutive days with different shading treatments. The plantwas under controlled environment conditions, a light level of 240 + 10 wm-2 and light period of 14 hr. On each day, the whole leaf was left in thelight for about 5 hr before shading was applied, and the speed over 20 cmafter 4 hr of light was the same, 4.2 + 0.1 cm min-' on each day. A 10-cm length of leaf was shaded each day: day 1 (0) immediately upstreamfrom the leaf chamber, day 2 (O) over the counters; and day 3 (x)downstream but adjacent to the leaf chamber.-.: time of application ofthe shade treatments. Shading the fed chamber gave results the same asfor shading immediately upstream from the feeding chamber. B: Data onspeed for the shading treatment between the fed region and the firstcounter in A was analyzed to give the variation in speed over two 10-cmsegments. (-p): time of shading; (0): segment adjacent to the shadedregion; (x): speed in the segment furthest downstream. There was muchvariation in the speed in the first region and the value indicated as > was11 cm min-'.

starch, and sucrose. The total carbohydrate level in the leavescontinued to increase, even after 40 hr of continuous light andirrespective of the previous treatment (Fig. 1 1A). This accumu-lation was as starch after the first 6 hr in the light. The level ofsucrose in both examples reached a maximum value within 12 hrin the light, although the time taken to reach this value wasconsiderably longer in a plant pretreated 48 hr in the dark. Thelevel of the reducing sugars was relatively constant throughoutthe light period.There was a significant interrelationship between the rate of

starch formation and sucrose concentration (Fig. 1 IA). Irre-spective of the dark pretreatment, the rate of starch accumula-tion showed a marked increase about the time that the sucroseconcentration reached a maximum value. This enhanced rate ofstarch formation did not continue for a long period and the rateof accumulation of starch eventually slowed down. The en-hanced rate of starch formation was apparently linked to themaximum sucrose concentration, which also influenced thespeed of translocation (Fig. 12, A and B).On transfer from 48 hr of continuous light to darkness, there

was a relatively rapid decline in the sucrose concentration of theleaves. The decline in starch was parallel to the decline insucrose but there was relatively little change in the level of thereducing sugars (Fig. 1 B).From previously reported data on the speed during prolonged

light or on transfer to darkness (21, 35), it was possible todevelop relationships between the speed of translocation and thecarbohydrate content of the leaves. These measurements werenot made on the same leaves, but on plants treated in a comple-mentary manner with respect to the environment (includingdaylength, light level, temperature, humidity, and pretreatment,and the stage of plant growth). The measurements of speed andcarbohydrate levels were obtained from plants transferred fromlight to darkness. The results indicated that the speed of translo-cation was high, even when the sucrose concentration was onlyN% of the dry weight. Over the range of several concentrations

from 1 to 8%, there was a relatively small increase in the speedof translocation.The level of sucrose was limited by the dark treatment and to

obtain higher concentrations of sucrose, it was necessary toinvestigate the relationship between the speed of translocationand sucrose concentration in the light. This is shown for twodifferent pretreatments in Figure 12, A and B. In both cases, theresults confirmed the previous conclusions, i.e. that the speed oftranslocation at low sucrose concentrations of about 1% wasrelatively high, and between 1 and about 8% there was a plateauregion over which the increase in sucrose concentration caused arelatively small increase in speed. It was most significant, how-ever, for both sets of data, that increases in sucrose concentra-tion above about 8% resulted in large increases in speed. Thisresponse effectively limited the level of sucrose that could beaccumulated in the leaf. It was significant also that this samelevel of sucrose affected the rate of starch accumulation (Fig.1 A). The combination of the effect of sucrose concentration onboth speed and starch formation suggests that there is a self-regulatory process in maize leaves which prevents the sucroseconcentration in the leaf from increasing much above 8%.

treatment caused a slow increase in half-width with time at allcounters.The light level on the whole plant had a significant effect on

the half-width of the pulse at the first counter. The half-width ofpulse as a function of light level is shown in Figure 10, for anexperiment previously reported (Fig. 5, A and B).

Variation in Carbohydrate Levels in Leaves in ProlongedLight and Dark. The behavior of carbohydrates in the leaveswere characterized by measuring the level of reducing sugars,

DISCUSSION

Carbon 11 Technique. These data provide several examples ofthe reliability of the carbon 11 technique for measuring thespeed of translocation and the physiological variability of speed.The standard deviation for the speed was between 0.1 and 0.3cm min-' for results in Table I, although many different plantswere used. The plants were grown under the same controlledenvironments. In another example (Fig. 5A), the speed was+0.1 cm min-' for the same leaf and at a constant light level,

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TRANSLOCATION IN ZEA MA YS LEAVES

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TIME (hrs.)

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a 0

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a x A6x

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10 12 14 0 2 4 6 8TIME (hrs.)

FIG. 9. Half-widths of the pulses as a function of position in the leaf and shading treatments. Arrows indicate the start of the shading treatments.A: Influence of shading a 10-cm length upstream from the feeding area on the half-width of the pulse at three counters, 10 (0), 20 (X), and 30 (A)cm downstream from the fed region. B: Half-width of the pulse at 10 (0), 20 (x), and 30 (A) cm from the fed chamber when the region between 0and 10 cm was shaded. C: Influence of shading the feeding chamber on the half-width of the pulse at the counter 10 (0), 20 (x), and 30 (A) cm

away. After 4 hr of shading, the half-widths were greater than 120 min at all positions. D: Whole leaf was shaded except the feeding chamber. Half-width of the pulse was measured at 10 (x), 20 (A) and 30 (0) cm.

throughout a day. The speed was also within ±0.1 cm min-' inthe dark at the start of 4 days of experiments and after a darkperiod of 12 hr. Further evidence for the repeatability of thedata is given in Figure 8. The speed was 4.2 ± 0.1 cm min-' on

the same leaf, on 3 consecutive days after 4 hr of high light eachday.A similar degree of reliability was evident in the half-width

measurements, although there was more physiological variabil-ity, as illustrated by the standard deviation of the measurementsat low light (Fig. 10).Speed of Translocation. Measurements of the speed of trans-

location made over the last 40 years have been summarized byCrafts and Crisp (4), and Canny (2). They include about 50estimates of the speed of translocation from numerous species,using a variety of techniques and under many environmentalconditions. In this paper, we report about 100 measurements ofspeed; the same technique and species are used and the influence

of one environmental parameter, light, is investigated. Our ex-

perience with the carbon 11 isotope during these experiments,and in those reported previously, has established that 'IC can beused in a routine manner to monitor several attributes of thetranslocation system in higher plants (21, 35, 36).Speed referred to in this paper was defined as an apparent

mean speed of translocation measured from the time delay inarrival of the midpoint at the front of the pulse over a knownpath length. As previously discussed (36), there are differencesin the estimate of speed made from different positions within thepulse, particularly between the front and the tail of the pulse. Inmany respects, it is only a mean speed; e.g. it reflects all Ccompounds and different compounds may move at differentspeeds, and it is a mean of variations in speed within an individ-ual pulse, between sieve tubes within the same bundle andbetween different vascular bundles. Furthermore, it only refersto the first pulse of material loaded into the phloem following

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817Plant Physiol. Vol. 59, 1977

w




feeding. There may also be losses of isotope along the path andthis in turn would influence the shape of the pulse and thereforethe speed.

In view of the variety of techniques previously used to esti-mate the speed, it is difficult to compare the previous andpresent results directly, although there is no reason to suggestthat they would be substantially different if the same methods foranalysis are used. Previous estimates have yielded a wide varietyof speeds, but the bulk of the results indicate values between0.025 and 2.5 cm min-'. Values up to 6 cm min-' have beenrecorded for cucumber and sugarcane with one unexplained

cE

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FIG. 10. Relationship between the half-width of the pulse and thelight level. Light level on the whole plant was varied. Half-width wasmeasured at the counter 10 cm downstream from the fed region and isthe mean of three values at each light level.

TIME (hrs)

exceptional value of 120 cm min-' (4). The speeds measured formaize in the experiments reported here were between 0.25 and11 cm min-', and therefore are within the general range ofprevious estimates. They illustrate the variability that can beexpected for any one species and make it difficult to define aspeed characteristic of maize.

Environment Effects on the Speed of Translocation. The vari-ability in the speed of translocation in maize was not random andwas due to environmental conditions.A schematic summary of all results is given in Figure 13.

There is a significant speed of translocation in the dark under anormal light/dark cycle, but with light the speed increases to anew equilibrium value, depending on the light level, with a half-time of about 30 min. Shading part or all of the leaf reduces thespeed with a half-time of about 1.5 hr. Previously, it was shownthat the half-time for the initial decay in speed in the dark was 5hr for a leaf previously treated in 48 hr of continuous light (21).In this paper, a half-time of about 1.5 hr was observed in a plantpretreated with 7 hr of high light. The speed in the dark declinedto about 1 cm min-' during a day length of 12 hr of darknessand, with prolonged darkness (48 hr), the speed can be as low as0.3 cm min-' (21).

Speeds up to 5 cm min-' were observed in parts of the leaf indarkness, which indicates that there is not likely to be a directeffect of light on the translocation process. This does not pre-clude some effect on the loading of photosynthate into sievetubes. Shading of the feeding chamber did not alter the speedmeasured 20 cm downstream, but it did cause a pronouncedbroadening of the pulse. There was only a 20% increase in half-width of the pulse during the first measurements after the lightswere turned off (Fig. 9C). This suggests that light was actingindirectly in determining the pulse shape, which reflects theloading phenomena. Light level influences the loading process asindicated by the reduction of the half-width from 1 15 ± 21 to 46± 2 min when the light level was increased from 25 to 170 wm-2.

Sucrose Concentration and the Speed of Translocation. Prod-ucts of the C cycle of photosynthesis are intimately involved intranslocation because sucrose and other compounds are translo-

24TIME (hrs.)

FIG. 1 1. A: Accumulation of carbohydrates in the light. Time course of changes in the reducing sugar, sucrose, starch, and total carbohydrates ontransfer from a normal dark period (12 hr) to continuous light. Concentrations are expressed as per cent dry weight of the leaf. B: Reduction incarbohydrate levels in a maize leaf on transfer from light to dark. Same plant as used in A.

I

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Plant Physiol. Vol. 59, 1977 TRANSLOCATION IN ZEA MAYS LEAVES

TotalA soluble X xd sugar -

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,i- x sugar * starch

30

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FIG. 12. Relationship between speed and the concentration of sucrose, total soluble sugars, and total soluble sugars plus starch. A: Measure-ments made in the light in a plant pretreated with a normal dark period (12 hr). B: Plant pretreated with 48 hr of darkness before measurements.

6

5

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LIGHT OFF

LIGHT LEVEL

20

50

10

2 4 6 8 0 2

TIME (hrs.)

AREA SHADED

10(a)

50

100

4 6 8

FIG. 13. Diagram of response of speed of translocation to light leveland darkness as established in experiments in this paper; 100% lightlevel was 240 w m-2. 10(a): Normal response if 10% of the area

upstream from the counters is shaded; 10(b): observed response when an

area immediately downstream from the chamber was shaded.

cated. Sucrose may also be involved in the mechanism of translo-cation because it is osmotically active and it can provide Cskeletons for an energy source. In considering a possible controlmechanism linking C fixation-translocation-utilization, it is at-tractive to postulate that it would revolve about sucrose (or someother specific sugar). This would allow adjustment between theC supply and the rate of transfer to sites of utilization.The preliminary measurements relating the gross sucrose lev-

els in the leaf to the speed of translocation established that a

biphasic response was involved. Multiphasic responses relatingthe rate of uptake to the concentration are familiar in ion uptakestudies (18) and have also been shown for sugar uptake as a

function of the external sugar solution in castor bean seedlings(19), tobacco cells (3), and in sugar beet leaves (27). Thesesimilar relationships may suggest common mechanisms but thecause of the multiphasic responses has yet to be established.

That mesophyll cells, veins, and sieve tubes are capable ofaccumulating sugar from an external source must be considereda basic feature of the operation of these cells. In photosyntheti-cally active cells, sucrose is generated inside the cell but outsidethe chloroplast. Leakage or even diffusion of sucrose from cellsmay occur and the presence of an active uptake process may besignificant in maintaining the sucrose within the plasmalemma.This active uptake of sucrose process presumably is significant inpreventing leakage of sucrose along thc sieve tubes during thetranslocation process. The small amounts of energy required forthis mechanism to operate may be provided by the companioncells.High speeds of translocation (>3 cm min-') were recorded in

maize only when the sucrose concentration exceeded 7%. Forsucrose concentrations between 1 and 7%, the speed for anormal maize leaf was 2 to 2.7 cm min-' at high light, althoughthis could be reduced to 1.5 to 2 cm min-' for a leaf exposed to48 hr of darkness before the light treatment. This suggests that a

prerequisite for high speeds may be a high sucrose concentrationin the leaves. This relationship may be significant in interpretingprevious measurements of speed where maximum speeds ofabout 2.5 cm min-' have been indicated.Using the C-11 technique with wheat, cotton, and tomato, the

speeds were low (<2 cm min-') and the sucrose levels were low(<5%). In rice, however, we have measured speeds of 3.35 cmmin-' (35) and the concentration of sucrose in the rice leaveswas about 8% (Chang, unpublished results).The sucrose concentrations reported here are for the whole

leaf tissue, whereas there are probably wide variations in sucrose

concentration between cells within the leaf. The gradients insucrose concentration within the leaf will be significant for themovement to, and loading of, sucrose into the sieve tubes (3, 27,29). The multiphasic relationship between sucrose concentrationand the speed of translocation may suggest that there is dualmechanism for sucrose accumulation into the sieve tubes.At low sucrose concentrations (up to 8%), there is active

uptake, resulting in a higher sucrose concentration in the sievetube region than in the bulk tissue. Above a sucrose concentra-

A

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CONCENTRATION (/. DW)14

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820 TROUGHTON, CUI

tion of 7 to 8%, the concentration may be higher in the bulktissue (this may be in the bundle sheath cells) than in the sievetube zone, resulting in loading due to sucrose concentrationgradients. The high sucrose concentrations in leaves may influ-ence metabolic activity through an osmotic or hydrostatic pres-sure effect, such as osmotically induced biochemical alterationsof enzyme activity or membrane permeability (1, 10, 12, 26, 30,38).

Pulse-Shape Analysis. The shape of the carbon 11 isotopepulse at any position within the leaf is a consequence of numer-ous factors: (a) spatial resolution of the counters; (b) timecourse of feeding "CO2; (c) metabolic steps between '1CO2 and"'C sucrose; (d) mixing of labeled and unlabeled sucrose mole-cules; (e) transport to the sieve tube region; (f) mechanism ofuptake into the sieve tubes; (g) loss of tracer from the solutionflowing in the sieve tubes; (h) the mechanism of translocation;(i) the speed distribution between different sieve tubes; and (j)the speed distribution within a sieve tube. For the techniquesused in this paper, the first three factors will not substantiallyalter the shape of the pulse and (g) and (j) have previously beensuggested to be unimportant (6, 36).

Events in the fed area (factors d, e and f) have been shown, inthis study, to be especially significant in determining the shape ofthe pulse. Results in which there was no change in shape of thepulse with distance along the leaf can be used to indicate thehalf-width of the loading pulse. For these results, it was shownthat the half-width of 2 min for l1CO2 fed to the leaf wastransformed to 45 min between the mesophyll cells and the sievetubes. The half-width of the pulse at the first counter rapidlybroadened from 45 to 120 min (and maybe 170 min, Fig. 10)when the fed region was shaded. Furthermore, the half-widthwas invariably related to the light level. This can be seen directlyby differentiating the efflux curves such as those shown in Figure3. The contrast in the shape of the efflux curves for those twoexamples illustrates the variation in loading processes in the fedregion.The shape of the pulse was not always constant with distance

along the leaf. This would be expected under nonequilibriumconditions after a perturbation was introduced, but it was alsoevident under apparently equilibrium conditions. A likely causewas that there was a variation in speed between sieve tubes butthe increase in half-width was not always linear with distance.For example, it was approximately linear in Figure 9, B, C, andD, but not in A. Interpretation of these data would be compli-cated by the photosynthetic activity occurring along the length ofthe leaf.These results suggest that the shape of the pulse was primarily

determined in the fed region of the leaf, although there may alsobe an effect due to variation in speeds between sieve tubes.These conclusions are consistent with those previously reportedfor maize (36) and soybean (6-9).The results provide evidence of a slow phloem-loading process

and a bulk flow mechanism in the sieve tubes. There was sub-stantial evidence for variations in both speed and half-width withdistance along the leaf. These would be expected in a systemsuch as the maize leaf where sucrose is being loaded along thelength of the system investigated. In the Munch scheme fortranslocation (23, 24), the driving force for bulk flow is apressure gradient from source to sink. The high speeds recordedin this study suggest that large pressure gradients are presentwith low hydraulic conductivity of the maize sieve tubes.Whether the pressure differentials required to sustain the highspeeds recorded in maize would occur over longer distances(such as in trees) remains an important aspect of translocationfor future investigation.

R

Acknowledgments-The authors appreciate the cooperation of T. A. Rafter and G. Mc-Callum of the Institute of Nuclear Sciences, D.S.I.R., in the use of the accelerator and associatedfacilities, and R. More for technical assistance in operating these facilities. We thank K. A. Card

'RIE, AND CHANG Plant Physiol. Vol. 59, 1977

of the Physics and Engineering Laboratory, D.S.I.R., for growing the maize plants. The purifiedamyloglucosidase was obtained from J. C. MacRae, Applied Biochemistry Division, D.S.I.R.

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15. HART-r CE, HP KORTSCHAK, AJ FORBES, GO BURR 1963 Translocation of 1C in sugarcane.Plant Physiol 38: 305-318

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17. KILBURN DM, PM TAYLOR 1969 Effect of sulfhydryl reagents on glucose determination bythe glucose oxidase method. Anal Biochem 27: 555-558

18. KINASK J, GG LATIES 1973 Multiphasic absorption of glucose and 3-0-methyl glucose byaged potato slices. Plant Physiol 51: 289-294

19. KRIEDEMANN P, H BEEVERS 1967 Sugar uptake and translocation in the castor beanseedling. I. Characteristics of transfer in intact and excised seedlings. Plant Physiol 42:161-173

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34. TROUGHTON JH 1971 The lack of carbon dioxide evolution in maize leaves in the light.Planta 100: 87-92

35. TROUGHTON JH, FH CHANG, BG CURRIE 1974 Estimates of a mean speed of translocationin leaves of Oryza sativa L. Plant Sci Lett 3: 49-54

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AND · TRANSLOCATION IN ZEA MAYS LEAVES parameter in translocation because it is required for...

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Transcript of AND · TRANSLOCATION IN ZEA MAYS LEAVES parameter in translocation because it is required for...