Effects of shoot bending of apple trees on accumulation and translocation of14C-labelled assimilates

The translocation of¹⁴C-labelled assimilates from a single leaf in bent and intact apple shoots was studied in varying stages of shoot development. In actively growing shoots¹⁴C-labelled assimilates translocated from the treated leaf and accumulated mainly in the shoot apex. In moderately growing apple shoots radioactive assimilates were translocated from the treated leaf in both directions towards and down the shoot. In apple shoots showing only slight growth activity the¹⁴C-labelled assimilates were transported from the treated leaf mainly to the base of the shoot, stem and roots. Bending shoots changed the pattern of distribution of radioactive assimilates. Bending actively and moderately growing shoots resulted in higher concentration of 14-carbon in the shoot apex than in controls. In slowly growing and non-growing apple shoots bending caused a higher accumulation of radioactive assimilates in the bent section than in an equal section of control shoots.     

Carbon distribution in grass (Festuca pratensis L.) during regrowth after cutting—utilization of stored and newly assimilated carbon

Distribution of net assimilated C in meadow fescue (Fectuca pratensi L.) was followed before and after cutting of the shoots. Plants were continuously labelled in a growth chamber with ¹⁴C-labelled CO₂ in the atmosphere from seedling to cutting and with ¹³C-labelled CO₂ in the atmosphere during regrowth after the cutting. Labelled C, both ¹⁴C and ¹³C, was determined at the end of the two growth periods in shoots, crowns, roots, soil and rhizosphere respiration. Distribution of net assimilated C followed almost the same pattern at the end of the two growth periods, i.e. at the end of the ¹⁴C- and the ¹³C-labelling periods. Shoots retained 71–73% of net assimilated C while 9% was detected in the roots and 11–14% was released from the roots, determined as labelled C in soil and as rhizosphere respiration. At the end of the 2nd growth period, after cutting and regrowth, 21% of the residual plant ¹⁴C at cutting (¹⁴C in crowns and roots) was found in the new shoot biomass. A minor part of the residual plant ¹⁴C, 12%, was lost from the plants. The decreases in ¹⁴C in crowns and roots during the regrowth period suggest that ¹⁴C in both crowns and roots was translocated to new shoot tissue. Approximately half of the total root C at the end of the regrowth period after cutting was ¹³C-labelled C and thus represents new root growth. Root death after cutting could not be determined in this experiment, since the decline in root ¹⁴C during the regrowth period may also be assigned to root respiration, root exudation and translocation to the shoots. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Effect of nitrates supplied with the transpiration flow on assimilate translocation

Solutions of nitrates (0.5% KNO₃, 0.2% NH₄NO₃) or urea (0.15%) were fed under the pressure of 10⁴ Pa to 50–60-cm-long detached shoots of common flax (Linum usitatissimum L.). One hour after the start of supplying the solutions, an assimilation clip chamber was fastened to the middle part of the shoot (¹⁴C source area), and ¹⁴CO₂ was blown through in the light for 2.5 min. The analysis of distribution of ¹⁴C among the labeled products of photosynthesis produced by source leaves showed that nitrates reduced the incorporation of the label into sucrose. At the same time, the ratio of labeled sucrose to labeled hexoses decreased, and the incorporation of the label into serine greatly increased. Urea did not produce such effects. The pattern of distribution of ¹⁴C within the plant 3 h after the assimilation of ¹⁴CO₂ points to the suppression of assimilate efflux from the leaves of plants fed with nitrates. In plants supplied with water or urea, 17–20% of labeled carbon was found below the ¹⁴C source area of the shoot, in nitrate type of treatment, only 3–5% was found there. In plants supplied with nitrates, the cortex tissue below the source leaf contained more ¹⁴C in proteins and less in low-molecular substances. In the wood tissue, such a correlation was not observed. When the shoot was supplied with water or urea, the content of ¹⁴C in sucrose in the source leaves in 3 h declined from 55–60% to 38–42%. When the shoot was fed with nitrates, the share of label in sucrose increased from 50 to 62–73%. Autoradiography of the source leaves showed that, in plants supplied with water or urea, the label was predominantly accumulated in large vascular bundles, and in nitrate type of treatment, it was accumulated outside large bundles. Electron microscopy showed that, in nitrate plants, the companion cells of phloem endings were very much vacuolated.     

14C-Studies on Apple Trees. II. Distribution of Photosynthates from Top and Base Leaves from Extension Shoots

The photosynthates from leaves from extension shoots may be used either for new growth of the shoot itself, or for growth in other parts of the tree. An attempt has been made to elucidate this problem by determining the content of ¹⁴C in long shoots to which ¹⁴CO₂ was applied either through fully developed leaves at the base, or through very young leaves at the apex. In the case of ¹⁴C application at the base, 80 per cent or more of the ¹⁴C initially taken up disappears from the shoot, and only a very minor part is translocated upwards in the shoot. When the leaves at the apex are exposed, on the other hand, 80 per cent of the ¹⁴C absorbed is retained in the leaves and shoot components treated as long as there is still considerable terminal growth taking place, although a small percentage is deposited in the lower parts of the shoot. At the same time, a much higher proportion is incorporated into methanol-insoluble components. As terminal growth decreases, a larger proportion of the ¹⁴C activity of the apex leaves also disappears from the shoot. The distribution of activity between the sorbitol, sucrose, glucose and fructose fractions was not significantly different in young and in fully developed leaves. The ¹⁴C labelling in the sugar fraction was highest for sorbitol, then sucrose, but decreases with time compared to glucose and fructose.     

Reversal of the Direction of Photosynthate Allocation in Urtica dioica L. Plants by Increasing Cytokinin Import into the Shoot

The natural cytokinin import from the root into the shoot of Urtica dioica plants was enhanced by supplying zeatin riboside (ZR) solutions of various concentrations to a portion less than 10 % of the root system after removal of their tips. After 6 h ZR pretreatment of the plants, ¹⁴CO₂ was supplied for 3 h to a mature (source) leaf or to an expanding leaf and the ¹⁴C-distribution in the whole plant was determined after a subsequent dark period of 14 h. ZR substantially increased ¹⁴C fixation by the expanding leaves and also enhanced export of carbon and transport to the shoot apex. The effect of the hormone treatment was, however, more pronounced when the ¹⁴CO₂ was supplied to a mature leaf. In the control plants these leaves exported carbon only to the roots: When the amount of the natural daily ZR input from the roots to the shoot was enhanced by 20%, the bulk of the ¹⁴C exported from a mature leaf moved to the shoot apex and only a minor portion of ¹⁴C was still detected in the root fraction. A several-fold increase of the natural daily ZR input into the shoot resulted in a flow of ¹⁴C only to the growing parts of the shoot. The results suggest control of the sink strength of the shoot apex by ZR in Urtica diocia.     

Independent translocation of 14C-labelled assimilates and of the floral stimulus in Lolium temulentum

Summary It is widely accepted that the floral stimulus produced in leaves is carried to the shoot apex passively in the phloem with the assimilate stream. Three kinds of evidence presented here suggest that the floral stimulus moves independently of the assimilates.Simultaneous determination of the velocities of translocation out of the seventh leaf blade, in comparable plants under the same conditions, yielded estimates of 1–2.4 cm/hr for the floral stimulus, and 77–105 cm/hr for ¹⁴C-labelled assimilates.The effect of the size of the seventh leaf on its ability to export assimilates or to initiate flowering was quite different. Leaves with only 14–26% of their final blade area emerged exported little assimilate, yet were highly active in inducing flowering.The effect of DCMU applications at a range of concentrations on the translocation of assimilates was quite different from their effect on the flowering response.     

Interrelationships of Cotyledonary Shoots in Pisum sativum

Unequal two shoot systems of tall and dwarf varieties of Pisum sativum L. ev, Improved Pilot and Meteor, respectively, were used in experiments where manipulative treatments involving the removal of one or other or both of the shoot apices were carried out. These were combined with GA treatments. Twenty four hours after treatment, ¹⁴CO₂ was applied to the lowest expanded leaf on the larger shoot, and the distribution of radioactivity was determined after a further 24 hours. Removal ot the apex of the larger shoot reduced the level of translocation from the treated leaf, the effect being enhanced by the additional removal of the other apex. This (Heel was more marked in the tall than in the dwarf variety. In untreated control plants of both varieties there was very little transfer ot labelled material from the larger (dominant) shoot to the smaller (weaker) shoot. This pattern was not affected by the removal of the apex from the weaker shoot. In Improved Pilot, removal of the apex from the stronger shoot led to considerable transfer of radioactive material to the weaker shoot, GA treatment having little effect. However, in Meteor, transfer of radioactivity from the stronger to the weaker shoot. after removal of the apex from the stronger shoot, only occurred after the application of GA. Removal of both shoot apices again resulted in low levels of transfer of labelled material to the weaker shoot in both varieties. These results are consistent with the hypothesis that there is competition between the two shoots for photosynthates produced by the leaves and that treatments reducing the competitive ability of one shoot, reduce the level of nutrients received by that shoot and can result in transfer of materials from it to the other shoot. Continuation of such a situation will result in increased inequality and perhaps ultimately in the death of the weaker shoot.     

The patterns of growth, assimilation of 14CO2 and distribution of 14C-assimilate within vegetative plants of Loliumperenne at low and high density

The patterns of growth, assimilation of ¹⁴CO₂ and distribution of ¹⁴C-labelled assimilate were followed for 12 wk from sowing in individual plants of Lolium perenne grown in miniswards at either low (500 plants m^-2) or high (5000 plants m^-2) density. At the latter density, plants were characterised by a 50% reduction in RGR, by the production of fewer tillers, especially second- and third-order tillers, and by a reduction in mean tiller weight. All the green and senescing leaves of each tiller assimilated ¹⁴CO₂ and the overall assimilatory capacity of a tiller was directly related to its dry weight. At both densities the plant consisted of a main shoot and established tillers with comparable assimilatory activities and a range of developing tillers that assimilated relatively small amounts of ¹⁴CO₂. As each successive primary tiller developed it was supplied with assimilate from the main shoot and the degree of support was inversely proportional to the dry weight of the tiller. At both densities it was concluded that the first primary tiller could be regarded as an independent assimilatory unit when it reached a dry weight of about 25 mg even though some import of main shoot assimilate continued until the tiller was double this weight. The supply of assimilate to the root system was greatly reduced at both densities compared with previous observations on plants grown singly.     

Distribution of labelled assimilates within a young apple tree after supplying14CO2to a leaf or shoot

Labelled carbon dioxide was supplied for 22 hrs to a leaf of the leader or to the lateral shoot in two-year-old apple seedlings. The distribution of radioactive assimilates within the plant following this treatment was investigated by using radioautography. The transport of labelled assimilates from the young leaf of the leader was very meagre and affected only parts of the stem and the leaves situated in the close vicinity of the treated leaf. The¹⁴C-labelled assimilates from the mature leaf of the leader were transported in a considerable amount to the apex and to the other leaves of the leader. They were also found in an appreciable amount in the stem and the roots, as well as in some lateral shoots. After supplying¹⁴CO₂ to the lateral shoot remarkable transport of labelled assimilates was observed. Radioactivity was detected in the tip and in the youngest leaves of the leader, as well as in the roots. Their path in the stem was studied by dissecting the plant and examining the cross section from each internode. This method revealed that the assimilates from the treated leaf or shoot were transported downward only on one side of the stem in a helical pattern. The lateral shoots situated on the radioactive side of the stem were also labelled, whereas those situated on the opposite (non-radioactive) side were not labelled.     

Below-ground carbon distribution in barley (Hordeum vulgare L.) with and without nitrogen fertilization

The distribution of net assimilated C in barley (Hordeum vulgare L.) grown at two N-levels was determined in a growth chamber. The N-fertilization involved 0 and 3.61 mol N g^-1 dry soil. After growth for seven weeks in an atmosphere with continuously ¹⁴C-labelled CO₂, ¹⁴C was determined in shoots, roots, rhizosphere respiration and soil. At the low N-level, 32% of the net assimilated ¹⁴C was translocated below ground, whereas at the high N-level 27% was translocated below ground. The release of C from roots (root respiration, microbial respiration originating from decomposition of ¹⁴C-labelled root material and ¹⁴C remaining in soil) was greater with no N-supply (19% of net assimilated ¹⁴C) than in the treatment with N-supply (15%). Thus, the effect of N-supply on both translocation of assimilated ¹⁴C below ground and the release of ¹⁴C from growing roots was relatively small.     

The response of short day plantChenopodium rubrum L. to abscisic acid and gibberellic acid treatment applied at two levels of photoperiodic induction

Abscisic acid (ABA) (5 x 10^-4M and 5 x 10^-5M) and gibberellic acid (1 x 10^-4M) was applied to the plumula ofChenopodium plants with partly (one dark period) or completely (three dark periods) fulfilled photoperiodic requirements for flowering. Morphological and cytoogical criteria were used to investigate the time-course of the differentiation of the treated shoot apices. Both substances were ineffective in increasing the mitotic activity of the shoot apex at the suboptimal level of induction. The degree of branching was temporarily stimulated by ABA and GA treatment under these conditions. Moreover, GA caused the elongation of the shoot apex. With the completely induced plants ABA hastened flowering and the rise in branching was observed in all the treatment 48 h following the application of growth substances.     

The Effect of Nitrogen Supply to Urtica dioica L. Plants on the Distribution of Assimilate Between Shoot and Roots

In this study the influence of nitrogen nutrition on the patterns of carbon distribution was investigated with Urtica dioica. The nettles were grown in sand culture at 3 levels of NO^?₃, namely 3 (low), 15 (medium) and 22 (high) mM. These levels encompassed a range within which nitrogen did not affect total biomass production. The ratio of root: shoot biomass of the low nitrogen plants was, however, significantly higher than that of the nettles grown at medium and high N supply. Carbon allocation from one leaf of each pair of leaves was examined after a ¹⁴CO₂-pulse and a subsequent ¹⁴C distribution period of one night. Only the youngest two leaf pairs did not export assimilates. Carbon (¹⁴C) export to the shoot apex and to the roots, as measured at the individual nodes responded to the nitrogen status: At medium and high nitrogen supply the 3rd, 4th and 5th leaf pairs exported to the shoot apex, while lower leaves exported to the root. At low nitrogen supply only the 3rd leaf exported towards the shoot apex. The results illustrate the plastic response of carbon distribution patterns to the nitrogen supply, even when net photosynthesis, carbon export from the source leaves and biomass production were not affected by the nitrogen supply to the plant.     

Cell length,light and14C-labelled indol-3yl-acetic acid transport inPisum satisum L. andPhaseolus vulgaris L

The putative auxin-transporting cells of the intact herbaceous dicotyledon are the young, differentiating vascular elements. The length of these cells was found to be considerably greater in dwarf (Meteor) than in tall (Alderman) varieties ofPisum sativum L., and to be greater in etiolated than in light-grown plants ofP. sativum cv Meteor andPhaseolus vulgaris L. cv Mexican Black. Under given light conditions during transport these large differences in cell length did not influence the shapes of the transport profiles or the velocity of transport of¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud. However, in both etiolated and light-grown bean and dwarf pea plants the velocity of transport in darkness was ca. 25% lower than that in light. Under the same conditions of transport velocities in bean were about twice those observed in the dwarf pea. Exposure to light during transport increased the rate of export of¹⁴C from the labelled shoot apex in green dwarf pea plants but not in etiolated plants. The light conditions to which the plants were exposed during growth and transport had little effect on the rates of uptake of IAA from the applied solutions. The results indicate that the velocity of auxin transport is independent of the frequency of cell-to-cell interfaces along the transport pathway and it is suggested that in intact plants auxin transport is entirely symplastic.     

Translocation of14C-abscisic acid from roots into the aboveground part of pea (pisum sativum L.) seedlings

The translocation of¹⁴C-ABA from roots into other parts of the plant was followed in intact and decapitated pea seedlings. In intact plants ABA from roots was translocated above all into the apical part of epicotyl. In decapitated plants the regulative ability of intact apex can be partly simulated by exogenous IAA. The growth of lateral buds occurring after decapitation was associated with an intensive flow of¹⁴C-ABA from roots into released lateral buds as late as 72 h after decapitation,i.e. in the stage of intensive elongation growth of buds.     

The transport of substances out of developing fruits in relation to the induction of apical senescence in Pisum sativum line G2

Apical sencscence in G2 peas occurs only in long days in the presence of fruit. The effect of fruits could be caused by the export of a senescence hormone from the fruits to the shoot tip. Export of radiolabeled material from developing fruits of G2 peas grown in long days was therefore examined following injection of the pods with [¹⁴C]-sucrose, [¹⁴C]-acetate, or [¹⁴C]-mevalonate or after allowing the pods to photosynthesize in ¹⁴CO₂ for 48 h. In all cases a small amount (<1%) of radioactivity was exported, primarily to the younger fruits on the same side of the plant and the to the shoot apex. After feeding ¹⁴CO₂ to the fruit, the radiolabeled material partitioned into acidic ethyl acctate and possessed a carboxyl group. While this radioactivity had chromatographic properties similar to abscisic acid (ABA) in a number of solvent systems, it was not identical to either ABA, phascic acid or dihydrophaseic acid. The nature of the labeled material found in the apex was different in short days, in which senescence does not occur, or when the leaves were the source of the radioactive compounds. The labeled material in the apex was similar after feeding ¹⁴CO₂, [¹⁴C]-acctatc, or [¹⁴C]-sucrose, but different if the fruits were injected with[¹⁴C]-mevalonate.Identification of the chemical nature of the labeled material in the apex was not possible due to the small amount present. Parallel purification of an extract from treated fruits led to the identification of N-benzoylaspartate and N-phenylacetyl-aspartate. The radiolabeled substance from the apex was run with these two chemically synthesized compounds on several gas chromatogtaphic columns, and was also recrystallized together several times. The label and the pure material did not have identical retention times; neither did they co-purify so that, while similar, the material exported to the apex is not the above compounds.     

Seasonal differences in the pattern of assimilate movement in branches of Coffea arabica L

At Ruiru, Kenya, ¹⁴CO₂ was fed to single leaves at different distances from the apex on branches of mature fruiting and non-fruiting trees of Coffea arabica on six occasions from the Short Rains (November) 1966 to the Long Rains (April-May) 1967. The location of ¹⁴C-labelled assimilates in the treated branches was determined 26 h later by autoradiography. The direction of movement of labelled assimilates indicated large seasonal differences in the relative sink strengths of the shoot tip, fruit and trunk-root systems. On vegetative trees the sink strength of trunk-roots was much smaller, as compared with the shoots, at the beginning of the Long Rains than at the end of the previous Short Rains or in the intervening dry season. Assimilate use by growing fruits did not alter the pattern of distribution of assimilates to the other sinks at the end of the Short Rains, but it did restrict assimilate movement to both shoot tips and trunk-roots at the_ beginning of the Long Rains. In the dry season, virtually all assimilates were utilized by growing fruits when these were present. Vegetative secondary shoots provided assimilates to growing fruits and trunk-roots at the end of the Short Rains and in the dry season. Some practical implications are noted.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

SOURCE-SINK RELATIONSHIPS IN THE BLADE OF ALARIA ESCULENTA (LAMINARIALES,PHAEOPHYCEAE)1

The translocation of ¹⁴C-labelled photoassimilate was studied in blades of the kelp, Alaria esculenta (L.) Grev., which had been surgically modified to produce source and sink regions of various sizes. Thirty cm above the blade base a 2.5 cm dia. section of the blade received a 1 h pulse of H¹⁴CO₃^?. Efflux of ¹⁴C-labelled photoassimilate from this “source disc” was monitored over the next 8 days using a Geiger-Müller detector probe. Accumulation of ¹⁴C-labelled solutes by a “sink disc” of similar diameter, 20 cm below the source disc, was also monitored. Rate of ^l4C efflux from the source disc was governed by two factors: (1) total sink size and (2) feed-back from competing sources. In the latter case, source export was depressed if the portion of the blade, just distal to the source disc, was present. While the initial ¹⁴C influx rate into the sink disc was greater when competing sinks were present, the total accumulation of ¹⁴C-photoassimilates was 2–3 times higher in the sink disc when competing sources were not present. Basipetal translocation velocity (1.3–1.7 cm h^?1) was unaffected by competing sources and sinks.     

Conductance of needle and twig axis phloem of damaged and intact Norway spruce (Picea abies (L.) Karst.) as investigated by application of14C in situ

Summary Phloem conductance of¹⁴C-labelled assimilates was investigated in natural stands of Norway spruce showing substantial damage from needle yellowing and needle loss disease. Terminal current-year shoots of a branch were allowed to fix¹⁴CO₂ (300–600 ppm in air) and carbon dioxide net uptake was monitored with a gas analyser. The difference between¹⁴C-uptake and the amount of radiocarbon determined in the photosynthesizing needles was interpreted to reflect assimilate export from the needles to the axis of the tree. Compared with an undamaged control tree,¹⁴C-export from the assimilating needles was not impaired in the yellowing tree and only slightly reduced in the tree showing needle loss. Incorporation of¹⁴C into starch increased significantly during autumn particularly in the tree showing needle loss. Import of radiocarbon from the¹⁴C-labelled phloem sap in twig axes and needles older than 1 year was used as a measure of phloem conductivity of older sections of a branch which showed considerable damage. Carbon uptake by these older plant parts was more pronounced than in undamaged twigs. In the case of older needles enhancement of¹⁴C-incorporation suggested an increased sink strength, while the same phenomenon in the twig axes was interpreted as a consequence of partially impaired conductivity of individual sieve elements resulting in an inhomogeneous velocity of phloem transport. The hypothesis is put forward that curtailed viability of the sieve cells is responsible for a delay of transport, which is compensated for by an augmented production of phloem elements from the cambium.     

Comparative Basipetal Transport of 6-Benzylaminopurine-8-14C, Gibberellin A3-3H, IAA-2-14C, and Sucrose-14C in the Root of Intact Citrus aurantium Seedlings

The transport and distribution of IAA-2-¹⁴C, gibberellin A₃-³H, 6-benzylaminopurine-8-¹⁴C and sucrose-¹⁴C (U) were studied in whole seedlings of Citrus aurantium L. after “replacing” the root tip with the solution of radiochemicals. All four substances were transported basipetally in the root and were distributed to the stem and leaves. The pattern of distribution of the label from 6-benzylaminopurine was similar to that of sucrose, while a considerably larger amount of gibberellin A₃ was transported to basal regions of the root, away from the tip, and into the shoot. Contrary to these three substances, the basipetal transport of IAA in the root was very low, and the majority of the label was retained in the terminal sections of the root. It is suggested that the different efficiencies at which various hormones move in the transpiration stream in the root may be an important factor in the attainment of a certain balance of hormones in the shoot.