Effects of Pruning of Young and Old Shoots on Ribulose Bisphosphate Carboxylase and Other Constituents in Leaves of the Mulberry Tree (Morus alba L.)

Changes in the contents of ribulose 1,5-bisphosphate carboxylase(RuBPCase) and other cellular constituents were measured inthe leaves remaining on mulberry shoots after removal of thetop one-third of shoots (old) which had developed since thespring and shoots (young) which developed following shoot harveston 1 July. Top pruning of the young summer shoots increasedthe contents of chlorophyll, total soluble protein, RuBPCase,RNA and starch in the leaves. In the old shoots that had developedsince the spring, top-pruning in the late summer produced noappreciable change except in the content of RNA. The contentof RuBPCase of the young summer shoots doubled as a result oftop-pruning; this was the most conspicuous change in the cellularconstituents examined. The increase in RuBPCase protein accountedsatisfactorily for the increase in total soluble protein. Key words: RuBP carboxylase, shoot top-pruning, mulberry (Morus alba)     

Changes in Ribulose 1,5-Bisphosphate Carboxylase Concentration due to External Nitrogen Supply in Mulberry Leaves (Morus alba L)

Effects of external nitrogen supply on quantity and activityof ribulose 1,5-bisphosphate carboxylase (RuBPCase) in mulberryleaves (Morus alba L. cv. Shin-ichinose) were examined. PhotosyntheticCO₂ fixation and the contents of chlorophyll and phosphoruswere increased by the increased N supply. RuBPCase-protein concentrationand its proportion to total soluble-protein or to total-N rosecorrespondingly as the N supply increased. Specific enzyme activityexpressed on a RuBPCase protein basis was not affected by theN supply. Morus alba L. (mulberry), N nutrition, RuBPCase protein content, photosynthesis     

Effect of Top Pruning on Stem Carbohydrates Reserves and Foliar 14CO2 Assimilation in Mulberry Trees during Autumn

Removal of shoot tops from mulberry plants in autumn hastenedthe accumulation of starch in the stems even though photosynthesisin the remaining uppermost leaves was supressed. The proportionof the amount of ¹⁴CO₂ assimilate taken into the veins and thecontents of foliar carbohydrates and protein were not affectedby pruning. (Received December 22, 1983; Accepted March 21, 1984)     

Plant Regeneration Through Meristem Culture from Vegetative Buds of Mulberry (Morus bombycis Koidz.) Stored in Liquid Nitrogen

Intact vegetative buds of mulberry (Morus bombycis) attachedto shoot segments were prefrozen, stored in liquid nitrogen,thawed, and the meristems excised for culture on Murashige andSkoog's medium supplemented with 1 mg 1^–1 BA to regenerateplants. Either prefreezing at –10 °C or –20°C along with rapid thawing at 37 °C or prefreezingat –20 °C or –30 °C along with slow thawingat 0 °C was a suitable condition for high percentages ofsurvival and shoot regeneration. Potted mulberry plants couldbe finally obtained from the cryopreserved material. The systemusing intact buds as the material for cryopreservation is quitesimple when compared to conventional systems using isolatedmeristems together with cryoprotectants. Morus bombycis Koidz., mulberry, cryopreservation, meristem culture, plant regeneration     

Regulation of Ribulose Bisphosphate Carboxylase Activity in Intact Wheat Leaves by Light, CO2, and Temperature

The activity of the enzyme ribulose bisphosphate carboxylase(RuBPCase) was estimated after rapidly extracting it from intactwheat leaves pretreated under different light and CO₂ levels.No HCO₃^– was added to the extraction buffer since it isshown to inhibit RuBPCase. The activity increased as light intensityor CO₂ concentration during pretreatment was increased. Enzymeactivity increased as temperature during pretreatment was decreased.Light activation did not affect the affinity of RuBPCase forCO₂. A K_m of 30 µM CO₂ under air level O₂ was determined.CO₂, light and temperature are three main limiting factors ofphotosynthesis. It seems that the activity of RuBPCase is regulatedby these factors according to the requirements for CO₂ fixation.     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

Variation in Concentration of Ribulose-1, 5-bisphosphate Carboxylase in Leaves of Barley, Wheat and Hybrids of Crested Wheatgrasses

Amounts of the enzyme ribulose-1,5-bisphosphate carboxylasewere estimated in seedling leaves of barley (Hordewn vulgareL.) and flag leaves of wheat (Triticum aestitum L.) by radialimmuno diffusion. A fourfold variation among barley varietiesfor amount of RuBPCase at the seedling stage was observed (c.3.5–15mg g^–1 fr. wt). Range in variation for amountof flag leaf RuBPCase among wheat varieties was 6-09-9.39 mgRuBPCase g^–1 fr. wt. F₁ hybrids from interspecific andintergeneric crosses of crested wheatgrasses (Agropyron andElymus spp.) and their amphidiploid analogues were comparedfor amount of RuBPCase in the most recent fully expanded leavesharvested before seed set. Amount of enzyme varied from 3.4to 77.6 mg g^–1 fr. wt among the hybrids. No effect chromosomenumber on enzyme concentration was observed among 13 hybridsand their amphidiploid counterparts. Key words: RuBPCase, wheatgrasses     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Changes in Contents of Adenine Nucleotides and Development of Ribulose Bisphosphate Carboxylase Activity During Shooting of Mulberry Saplings

At the onset of budding in mulberry saplings (Morus alba L.,cv. Shin-ichinose), the ATP, ADP and carbohydrate contents beganto decline rapidly. This decline continued until RuBPCase activitybegan during the development of the leaves. The concentrationsof these constituents and the value for the adenylate energycharge, though partially restored, were lower than the initialvalues even eight weeks after planting. (Received March 7, 1983; Accepted May 25, 1983)     

A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger,J. 1985. A CO₂ concentrating system in leaves of higher C₃-plantspredicted by a model based on RuBP carboxylase/oxygenase kineticsand ¹⁴CO₂/¹²CO₂ exchange.–J. exp. Bot. 36: 1542–1550. A model is presented which compares the ratio of the two activitiesof the enzyme nbulose bisphosphate carboxylase/oxygenase asdetermined in vitro with the ratio of photosynthesis to photorespirationin leaves as determined from differential ¹⁴CO₂/¹²CO₂ uptakeor from CO₂ compensation concentration. Discrepancies betweenmeasurements made in vitro and in vivo are attributed to theeffect of a CO₂ concentrating system in the leaf cells. Interferencefrom dark respiration is discussed. A CO₂ concentrating systemis postulated which is efficient mainly at low temperature andlow CO₂ concentration. Key words: —Photosynthesis, photorespiration, ribulose bisphosphate carboxylase/oxygenase     

The Export and Distribution of 14C-labelled Assimilates from each Leaf on the Shoot of Lolium temulentum during Reproductive and Vegetative Growth

In both reproductive and vegetative plants of Lolium temulentumL., the export of ¹⁴C-labelled assimilates from each healthyleaf on the main shoot to terminal meristem, stem, tillers,and roots was measured each time a new leaf was expanded, fora period of 5 to 6 weeks. Some labelled assimilates moved fromeach leaf on the main shoot to every meristem in the same shoot,as well as to the tops and roots of adjacent organically attachedtillers. The terminal meristem of the reproductive shoot, which includedthe developing inflorescence, received 70–80 per centof the carbon assimilated by the emerged portion of the growingleaf, 15–25 per cent of the carbon assimilated by thetwo youngest expanded leaves, and 5–10 per cent of thatfrom each of the older leaves. A similar pattern of carbon supplyto the terminal meristem was found in vegetative shoots, exceptthat older leaves on young vegetative shoots supplied even lessof their carbon to the terminal meristem. The general conclusionis that developing leaves at the tip of the shoot receive aboutthe same proportion of carbon from each leaf as does a developinginflorescence. Young expanded leaves provided most labelled assimilates forstem growth; during both reproductive and vegetative growth,expanded leaves increased their export of labelled carbon tostem, and exported less of their ¹⁴C to roots and sometimesto tillers. In these reproductive and vegetative shoots, grown in a constantexternal environment, the major changes in the pattern of distributionof labelled assimilates appeared to be the result of increasedmeristematic activity in stem internodes; the development ofan inflorescence had no obvious direct effect on the carboneconomy of shoots.     

Intercellular Transport and Photosynthetic Differentiation in Chara corallina

Intercellular transport of ¹⁴C-labelled photoassimilates, bothin isolated upper shoots and in isolated internode-branchletcomplexes of Chara corallina, was measured. The isolated uppershoots were composed of a primary apex, two mature internodes,and three branchlet whorls. A 10 min loading of the isolatedupper shoot with H¹⁴CO₃^– resulted in a greater accumulationof ¹⁴C in the apical complex and branchlets than in the internodes,while a subsequent 50 min chase with unlabelled solution inthe light resulted in a greater accumulation of ¹⁴C in internodesthan in other parts of the shoot. In the isolated internode-branchlet complex, when the apex wasnot detached, the amount of ¹⁴C transported from branchletsto internodes was about fives times that transported from internodesto branchlets. Removal of the apex resulted in a decrease intransport from branchlets to internodes and an increase in transportin the opposite direction. In an attempt to explain the mechanism of the polar transportof photosynthetically fixed carbon between branchlets and internodes,photosynthetic activities of both types of cells were investigated.Detached branchlets have higher photosynthetic ¹⁴C-fixationactivities than those of internodes. Chlorophyll contents, measuredin terms of surface area, in internodes and branchlets werealmost identical. The ribulose-l,5-bisphosphate carboxylase(RuBPCase) activity of branchlets was 1.6 times that of internodes,and the rate of ferricyanide-dependent evolution of oxygen inbranchlets was 1.4 times that in internodes. Key words: Chara, internode, branchlet, polar transport, photosynthesis     

Photoinhibition under Atmospheric O2, the Activation State of RuBP Carboxylase and the Content of Photosynthetic Intermediates in Soybean and Wheat

Ward, D. A. and Drake, B. G. 1987. Photoinhibition under atmosphericO₂, the activation state of RuBP carboxylase and the contentof photosynthetic intermediates in soybean and wheat.—J.exp. Bot. 38: 1937–1948. Associations between photosynthesis, the activation state ofRuBP carboxylase and the contents of photosynthetic intermediateswere compared in soybean and wheat leaves before and after exposureto photoinhibitory treatments in the presence of atmosphericO₂. Exposing attached leaves to a supra-saturating irradiance(3 800 µmol quanta m^{– 2} s^–1) for 2 h in CO_2-freeair decreased carboxylation efficiency and the light-saturatedphotosynthetic rate in air by approximately 50%. Exposure tothe photoinhibitory treatment for periods in excess of 2 h didnot cause a further decrease of photosynthesis in soybean. Althoughphotosynthesis was reduced, the initial and total (fully-activated)activities of ribulose 1,5-bisphosphate carboxylase (RuBPCase)in leaf extracts were unaltered in each species by the photoinhibitorytreatment. This was true for leaves sampled under both air andat a rate-limiting intercellular CO₂ partial pressure (C_i) of75 µPa Pa^–1. The contents of ribulose l,5-bisphosphate(RuBP) and 3-phosphoglyceric acid (3-PGA) were reduced by thephotoinhibitory treatment in soybean leaves sampled in air andat a rate-limiting C_i, although the RuBP/3-PGA ratio was unaffected.The relative reduction of RuBP content in soybean leaves atrate-limiting C₁ was similar to the corresponding reductionof carboxylation efficiency. For wheat,the relative reductionof RuBP content at rate-limiting C_i (–19%) caused by thephotoinhibitory treatment was considerably less than the correspondingdecrease of carboxylation efficiency (–49%).The RuBP/3-PGAratio of wheat was also increased significantly by the photoinhibitorytreatment The significance of these observations to the regulationof CO₂-limited photosynthesis in leaves experiencing photoinhibitionunder atmospheric oxygen is discussed. Consideration is alsogiven to the previous contention that contemporary measurementsof initial activity in crude extracts may provide a spuriousindication of the amount of the enzyme-CO₂-Mg_{2 +} form of RuBPcarboxylase present in the leaf. Key words: Carboxylation efficiency, RuBP carboxylase, photoinhibition, RuBP, 3-PGA     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.     

Isolation of Native Pyrenoid Core Matrix Having Ribulose 1,5-bisphosphate Caxrboxylase Activity from the Green Alga Bryopsis maxima

The native pyrenoid core matrix of the green alga Bryopsis maximawas isolated by diethyl ether treatment and sucrose densitygradient centrifugation using 1.8 M phosphate buffer. The purityof the pyrenoids was examined by microscopy, polyacrylamidegel electrophoresis and marker materials. The purified pyrenoidscontained the large subunit and the small subunit of ribulose1,5-bisphosphate carboxylase (RuBPCase) and more than 10 minorpolypeptides. They also showed RuBPCase activity when solubilizedon being transferred to a low-concentration buffer. The specificactivity was 0.62 µmol CO₂ fixed (mg protein)^–1min^–1. This isolation method is suitable for obtainingintact pyrenoids not covered by starch sheaths or membraneswithout the need for chloroplast fixation. (Received July 27, 1987; Accepted October 20, 1987)     

Dark Respiration of Several Varieties of Winter Wheat Given Different Amounts of Nitrogen Fertilizer

Carbon dioxide production in the dark by ears and by the restof the shoot of winter wheat grown in the field was measuredin 2 years during grain growth. The respiration rate per g d.wt of the ears was increased by nitrogen fertilizer. Ears ofthe semi-dwarf varieties Maris Fundin and Hobbit respired moreslowly than ears of Maris Huntsman and Cappelle-Desprez. Respirationrates of the rest of the shoot were unaffected by nitrogen orvariety. The amount of carbohydrate required to provide the CO₂ respiredduring the whole period of grain growth varied from 163 to 443g m^–2, or 42 to 76 per cent of the dry weight of the grain.More than half the CO₂ lost was respired by the ear. The additionof 180 kg N ha^–1, which increased grain yield by 78 percent in 1975, almost trebled the amount of CO₂ lost by the ears.The semi-dwarf varieties lost less CO₂ from ears and shootsthan did the taller ones, and had larger yields of grain. Respiration was also estimated from the difference between the¹⁴C contents of shoots sampled immediately after a 30 s exposureto ¹⁴CO₂ and at maturity. When 14C was supplied 10 days afteranthesis, the loss by maturity amounted to 16–28 per centof that initially absorbed by flag leaves and 40 per cent ofthat absorbed by the leaf below the flag leaf. Most of the lossoccurred in the first day. The loss of 14C by maturity was significantlyincreased by nitrogen fertilizer in 1975. Triticum aestivum L., wheat, respiration, nitrogen supply, fertilizer treatment     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Effect of Gibberellic Acid on the Distribution of Products of Photosynthesis in Sunflower

Single leaves on growing sunflower plants were allowed to assimilate¹⁴CO₂. Gibberellic acid was applied to the same leaf or to theterminal bud or the roots, and the distribution of assimilated¹⁴C was determined at intervals of 1–96 h. Gibberellicacid had no significant effect on initial distribution of ¹⁴Cduring the period of rapid export from the leaf, but enhancedre-export from the roots after translocation from the leaf hadvirtually ceased. Most of the ¹⁴C exported from the roots accumulatedin the shoot tip. The site of application of the hormone wasof relatively minor importance. Wherever it was applied themajor effect was enhancement of movement from the roots to theshoot tip. Application to the terminal bud was most effectivein this respect. There was no evidence that gibberellic aciddirectly affected the transport system, but the data supportthe hypothesis that it increases the strength of the sink inthe shoot tip. Helianthus annuus L., sunflower, assimilate transport, gibberellic acid, phloem transport