Distribution of imported 14C in developing leaves of eastern cottonwood according to phyllotaxy

Summary Individual leaves of eastern cottonwood (Populus deltoides Bartr.), representing an ontogenetic series from leaf plastochron index (LPI) 3.0 to 8.0, were fed ¹⁴CO₂ and harvested after 2–24 h. Importing leaves from LPI-1.0 through 8.0 on each plant were sectioned into 9 parts, and each part was quantitatively assayed for ¹⁴C activity. The highest level of ¹⁴C import was by leaves from LPI 1.0 to 3.0, irrespective of source-leaf age. ¹⁴C was translocated preferentially to either the right or left lamina-half depending on the position of the importing leaf in the phyllotactic sequence and its stage of development. For example, import was high when the importing leaf and the source leaf had two vascular bundles in common, moderately high with one bundle in common, and low with no bundles in common. The distribution of ¹⁴C within young importing leaves was highest in the lamina tip and decreased toward the base. With increasing leaf age, incorporation declined in the lamina tip and increased in the base.It may be concluded that each cottonwood leaf progresses through a continuum of importing and exporting stages as its lamina expands. The photosynthate imported by a given leaf is compartmentalized, with different exporting leaves supplying photosynthate to rather restricted regions of the lamina. Such localization within the importing leaf depends on its vascular connections with each of the exporting leaves, and these are predictable from a knowledge of the phyllotaxy.Plant Physiologists.     

14CO2 Fixation, Translocation, and Carbon Metabolism in Rapidly Expanding Leaves ofPopulus deltoides

To examine ¹⁴CO₂ fixation, potential translocation, and carbonflow among leaf chemical fractions of young developing leaves,the shoot tip of 24-leaf cottonwood (Populus deltoides Bartr.ex. Marsh) plants were cut off under water, placed in artificialxylem sap, and treated with ¹⁴CO₂ in continuous and pulse-chaseexperiments. Additional leaves on whole plants were spot treatedon the lamina tip to follow export from the tip only. The analysedleaves ranged in age from leaf plastochron index(LPI) –5to 3, the spot treated leaves from LPI 2 to 5. After 30 minfixation, the specific activity in the lamina tip increasedlinearly with leaf age from LPI –5 to 1 (0.5 to 4.5 kBqmg^–1). Specific activity in the lower lamina increasedslowly with leaf age and did not reach 500 kBq mg^–1 untilLPI –1. Total ¹⁴CO₂ fixed in the lower lamina exceededthat fixed in the tip by LPI –2 because of the large amountof tissue present in the lower lamina. Although the lamina tipfixed high levels of ¹⁴CO₂, pulse-chase studies coupled withautoradiography indicated no vein loading or translocation fromthe tip until about LPI 4 or 5. The ¹⁴C fixed in both tip andlower lamina was incorporated at the site of fixation and wasnot distributed to younger tissue or translocated from the lamina.Although the percentage distribution (¹⁴C in each chemical fractioncompared with the total in all fractions) of ¹⁴C among certainchemical fractions, e.g. sugars, amino acids and proteins, indicatedthat the mesophyll of the tip was more mature than the lowerlamina, physiologically both leaf sectors were immature basedon the expected ¹⁴C distribution in mature tissue. Informationfrom this and other studies indicates that the extreme tip ofa developing cottonwood leaf first begins to export photosynthateabout LPI 4 or 5 on a 24-leaf plant. The first photosynthatetranslocated may be incorporated into the vascular tissues andmesophyll directly below the tip. However, as the tip continuesto mature photosynthate is translocated past the immature lowerlamina into the petiole and out of the leaf. Populus deltoides Bartr. ex. Marsh, eastern cottonwood, translocation, leaf development, ¹⁴C fixation, carbon metabolism     

ACROPETAL LEAF DIFFERENTIATION IN QUERCUS RUBRA (FAGACEAE)

Northern red oak (Quercus rubra L.) leaves were shown to mature progressively from base to tip of the lamina based on studies of growth rates, anatomical differentiation, and ¹⁴C-transport. Lamina expansion in both length and width ceased in the basal quarter of the leaf before the apical quarter. Cell expansion and tissue differentiation were more advanced at the base than at the tip of leaves at 10%–20% of full expansion. Physiological data supported the morphological and anatomical data. Sink activity was examined by following the distribution of ¹⁴C imported into sink leaves with direct vascular connections to the source leaf to assure uniform assimilate supply to the sink leaves. Leaves approximately 50% of full expansion imported five to seven times more ^l4C-assimilates into the tip than into the base of the leaf, consistent with continued sink activity in the leaf tip after import by the leaf base has ceased. Transport of ¹⁴C from portions of the leaf, indicating source activity, occurred first in the basal portion of the lamina. The base functioned as a source at approximately 40% of full expansion; the tip, at approximately 60%. Thus, northern red oak displays an acropetal pattern of leaf expansion and differentiation, unlike the more typical pattern of basipetal leaf development defined in many other dicotyledonous genera with simple leaves.     

14C fixation,metabolic labeling patterns,and translocation profiles during leaf development in Populus deltoides

The incorporation of photosynthetically fixed ¹⁴CO₂ and the distribution of ¹⁴C among the main chemical constituents of laminae and petioles were examined in cottonwood (Populus deltoides Bartr. ex Marsh.) leaves ranging in age from Leaf Plastochron Index (LPI) 3 (about one-quarter to one-third expanded) to LPI 30 (beginning of senescence). In addition, carbon flow among chemical fractions and translocation from leaves of LPI 7 and 14 were examined periodically up to 24 h after labeling. Specific activity of ¹⁴C (on dry-weight basis) increased in developing laminae to full leaf expansion, decreased in the mature leaves to LPI 16, then remained constant to LPI 30. In developing leaves (LPI 3-5), after 2 h, most of the ¹⁴C was found in protein, pigments, lipids, and other structural and metabolic components necessary for cell development; only 28% was in the sugar fraction of the lamina. In fully expanded leaves (LPI 6-8), after 2 h, the sugar fraction contained 50–60% and about 90% of fixed ¹⁴C in the lamina and the petiole, respectively. In a pulsechase kinetic series with recently mature leaves, 60% of the ¹⁴C was found in the sugar fraction after 15 min of ¹⁴CO₂ fixation. Over the 24-h translocation period, ¹⁴C decreased in sugars to 23% and increased in the combined residue fraction (protein, starch, and structural carbohydrates) to about 60% of the total activity left in the lamina. Within 24 h after labeling, the turnover of ¹⁴C-organic acids,-sugar, and-amino acids (either metabolzed or translocated from the leaf) was 30, 70 and 80%, respectively, of that initially incorporated into these fractions by a leaf at LPI 7 (turnover was 55% of ¹⁴C-organic acids, 80% of ¹⁴C-sugar, and 95% of ¹⁴C-amino acids at LPI 14). Anatomical maturity in cottonwood leaves is closely correlated with physiological maturity and with production of translocatable sugar.Abbreviations LPI leaf plastochron index - PI plastochron index Research Plant Physiologist and Chief Plant Physiologist, respectively     

Translocation pathways in the petioles and stem between source and sink leaves of Populus deltoides Bartr. ex Marsh.

Microautoradiography was used to follow the translocation pathways of ¹⁴C-labeled photosynthate from mature source leaves, through the stem, to immature sink leaves three nodes above. Translocation occurred in specific bundles of the midveins and petioles of both the source and sink leaves and in the interjacent internodes. When each of six major veins in the lamina of an exporting leaf was independently spot-fed ¹⁴CO₂, label was exported through specific bundles in the petiole associated with that vein. When the whole lamina of a mature source leaf was fed ¹⁴CO₂, export occurred through all bundles of the lamina, but acropetal export in the stem was confined to bundles serving certain immature sink leaves. Cross-transfer occurred within the stem via phloem bridges. Leaves approaching maturity translocated photosynthate bidirectionally in adjacent subsidiary bundles of the petiole. That is, petiolar bundles serving the lamina apex were exporting unlabeled photosynthate while those serving the lamina base were simultaneously importing labeled photosynthate. The petioles and midveins of maturing leaves were strong sinks for photosynthate, which was diverted from the export front to differentiating structural tissues. The data support the idea of bidirectional transport in adjacent bundles of the petiole and possibly in adjacent sieve tubes within an individual bundle.Abbreviations C central leaf trace - L left leaf trace - LPI leaf plastochron index - R right leaf trace     

Incorporation of C-photosynthate into major chemical fractions of source and sink leaves of cottonwood

The incorporation and distribution of photosynthetically fixed ¹⁴CO₂ was followed for 48 hours in a recently matured source leaf (LPI 7) and in young expanding source and sink leaves (LPI 4) of cottonwood (Populus deltoides Bartr.). The major chemical constituents of leaf laminae and petioles were separated by sequential solvent extractions and enzyme hydrolyses. Two hours after labeling, about 80% of the ¹⁴C was found in water-alcohol-soluble constituents in the mature source lamina as compared to about 45% in those of the young expanding leaf. In both mature and expanding source leaves the water-alcohol-soluble constituents decreased while the CHCl₃-soluble and -insoluble compounds increased with time. After 48 hours, 7 and 37% of the total ¹⁴C was recovered from structural carbohydrates and from protein + CHCl₃-soluble fractions, respectively, in the mature source leaf; and 4 and 65%, respectively, in the young source leaf. When the distribution of ¹⁴C among major chemical fractions was calculated on per cent dpm/mg basis, the data showed that a young sink leaf incorporated over twice as much ¹⁴C into structural carbohydrates as a young source leaf (11% versus 4%). However, when calculated on an absolute dpm/mg basis, activity in this fraction of the young source leaf exceeded that in the sink leaf by a ratio of about 11:1 (9528 versus 845 dpm/mg). Thus, most of the material for synthesis of structural carbohydrates was derived from in situ photosynthate.     

Translocation and incorporation of 14C into the petiole from different regions within developing cottonwood leaves

Summary The ability of a developing cottonwood (Populus deltoides Bartr.) leaf to export ¹⁴C-labeled assimilates begins at the lamina tip and progresses basipetally with increasing LPI. This progression indicates that portions of leaves function quasi-independently in their ability to export ¹⁴C-photosynthate. Although most of the exported radioactivity was recovered in the petiole as water-80% alcohol-soluble compounds, there was also substantial incorporation into the chloroform and insoluble fractions. This observation indicates that assimilates translocated from the lamina are used in structural development of the petiole. Freeze substitution and epoxy embedding were used to prepare microautoradiographs for localization of water-soluble compounds. Radioactivity was found in all cell types within specific subsidiary bundles of the petiole. However, radioactive assimilates appeared to move from the translocation pathway in the phloem toward active sinks in the walls of the expanding metaxylem cells. Translocation in the mature xylem vessels was not observed.     

Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides

The transport and metabolism of xylem-borne amino compounds and sucrose were investigated in rapidly growing shoots of cottonwood (Populus deltoides Bartr. ex Marsh.). ¹⁴C-labeled glutamine, threonine, alanine, glutamic acid, aspartic acid, and sucrose were applied to the base of severed stems for transport in xylem. Distribution and metabolism of the compounds were followed with autoradiography, microautoradiography, and radioassay. Three utilization patterns were observed: (a) little alanine and sucrose was transported to the laminae of either mature leaves or developing leaves. These compounds were taken up from xylem free-space and utilized in adjacent tissue; (b) threonine also did not move into mature leaves but was translocated to developing leaves or utilized in the stem; (c) glutamic acid and aspartic acid were transported directly into the laminae of mature leaves via the xylem. Relatively less ¹⁴C was retained in stems compared to the other compounds.

Metabolism of the test compounds also differed considerably. ¹⁴C from amino acids moved primarily into organic acids and protein. The ¹⁴C from sucrose was widely distributed among the chemical fractions, with a high percentage found in structural carbohydrates. Clearly, cottonwood stems contain efficient uptake and transfer systems that differentiate among various compounds moving from root to shoot in xylem.

     

Fixation and distribution of 14C in Populus deltoides during dormancy induction

Photosynthetically fixed ¹⁴C was analyzed in various chemical fractions from leaves and stems of cottonwood (Populus deltoides Bartr. ex. Marsh.) during dormancy induction. Dormancy was induced by 8-h photoperiods and 20/14°C temperature regimes. Within 4 weeks under short days, terminal buds were set and leaf expansion and stem elongation had stopped. ¹⁴C₂ was fed to a leaf at Leaf Plastochron Index 7 for 30 min. Either after this 30 min feeding period or after a 48-h translocation period the plants were sampled, freeze-dried, extracted and analyzed for¹⁴C. ¹⁴C-fixation decreased during dormancy induction from 60% to 17% of the 3.7 MBq ¹⁴C applied at 0 week and 8 weeks, respectively. Percentage distribution of ¹⁴C in chemical fractions of source leaves reflected leaf age and translocation inhibition. In rapidly growing plants, considerable ¹⁴C was incorporated into leaf protein while most of the soluble¹⁴C-sugars were either metabolized or translocated out of the leaf. After terminal bud set, the percentage of ¹⁴C in the protein and residue fractions decreased rapidly and that in the sugar fraction increased. Percent distribution in stems closely reflected changing metabolic pathways of carbon flow as influenced by dormancy induction. For example, the ¹⁴C in structural carbohydrates decreased in 5 weeks under short days from 65 to less than 10% of the ¹⁴C recovered in the chemical fractions, thus indicating cambium inhibition. At the same time the percentage of ¹⁴C in starch and sugar increased indicating storage. Short term (after 30 min) incorporation of ¹⁴C into the protein and starch fractions of leaves changed relatively little throughout the 8-week induction period. In contrast the turnover rates of these fractions (¹⁴C present after 48 h) increased considerably after active growth of the whole plant stopped.     

14C Translocation pathways in honeylocust and green ash: Woody plants with complex leaf forms

Long-distance transport in plants requires precise knowledge of vascular pathways, and these pathways differ among species. This study examines the ¹⁴C translocation pathways in honeylocust (Gleditsia triacanthos L.) and green ash (Fraxinus pennsylvanica Marsh.), species with compound leaves, and compares them with those of cottonwood (Populus deltoides Bartr. ex Marsh.), a species with simple leaves. The stem vasculature of honeylocust conforms to a 2/5 helical phyllotaxy and that of green ash to a decussate phyllotaxy. The plastochron is relatively long in both species – 2.5+ days in honeylocust and 4.5+ days in green ash. Consequently, the transition from upward to downward translocation from mature source leaves is abrupt and occurs close to the apex. Export of ¹⁴C from localized treatment positions within a leaf was found to vary both quantitatively and spatially. To determine export patterns, ¹⁴CO₂ was administered to either individual leaflets of once-pinnate or pinnae of bipinnate leaves of honeylocust, and to either individual veins of simple or leaflets of compound leaves of green ash. Transections of either the petiole or rachis base were then examined for ¹⁴C by micro-autoradiography. In all cases, as treatment positions advanced acropetally in the leaves, the bundles translocating ¹⁴C were situated more dorsally in the basal petiole and rachis vasculatures. ¹⁴C was confined to the right side of the vasculature when structures on the right side of a leaf were treated. Compound leaves of both species mature acropetally. Thus, mature basal pinnae of honeylocust and basal leaflets of green ash translocate acropetally to younger leaf parts that are still rapidly expanding. All translocation pathways, both in the stem and leaf, conformed with vascular organization previously determined by anatomical analyses.     

Glutamine Transfer from Xylem to Phloem and Translocation to Developing Leaves of Populus deltoides

The distribution of ¹⁴C from xylem-borne [¹⁴C]glutamine, the major nitrogen compound moving in xylem sap of cottonwood (Populus deltoides Bartr. ex Marsh), was followed in rapidly growing shoots with a combination of autoradiographic, microautoradiographic, and radioassay techniques. Autoradiography and ¹⁴C analyses of tissues showed that xylem-borne glutamine did not move with the transpiration stream into mature leaves. Instead, most of it was transferred from xylem to phloem in the upper stem and then translocated to young developing tissues. Microautoradiography showed that metaxylem parenchyma, secondary xylem parenchyma, and rays were the major areas of uptake from xylem vessels in the stem. Accumulation in phloem (high ¹⁴C concentrations in sieve tubes) took place in internodes subtending recently mature leaves. Little ¹⁴C from xylem-borne glutamine was found in phloem of mature leaves, which indicates restricted retransport of glutamine that did enter the leaf. In the primary tissues of the upper stem, most ¹⁴C was found in the phloem. Cottonwood stems have an efficient uptake and transfer system that enhances glutamine movement to developing tissues of the upper stem.     

Leaf development and phloem transport in Cucurbita pepo: Transition from import to export

Summary The capacity of a growing leaf blade of Cucurbita pepo L. to import ¹⁴C-labelled photoassimilate is lost in a basipetal direction. Import into the lamina tip stops when the blade is 10% expanded. Development of the leaf progresses linearly with time and the lamina base stops importing when the blade is 45% expanded. Export capacity also develops basipetally and follows immediately the loss of import capacity, at least in the lamina base. The small amount of material initially exported from the leaf tip is redistributed to the still-importing leaf base, delaying export from the lamina until the blade is 35% expanded. Loss of import capacity by the petiole is both basipetal and dorsoventral. The proximal, adaxial portion of the petiole is the last region to cease importing ¹⁴C. Leaves of Beta vulgaris L. and Nicotiana tabacum L. also lose import capacity in a basipetal direction.     

Modification of Pattern of Photosynthate Movement within and between Shoots of Vitis vinifera L

During the prebloom and bloom stages, no movement of labeled photosynthates occurred from a shoot of Vitis vinifera L. fed with ¹⁴CO₂, to an adjacent shoot on the same spur. Movement of labeled assimilates into the unfed shoot was induced when this shoot was sprayed with 2.89 × 10⁻³m gibberellic acid during the prebloom stage. During the bloom stage darkening or defoliation of the unfed shoot resulted in the import of labeled photosynthates from the adjacent fed shoot. Similarly, movement of ¹⁴C into an untreated shoot was induced by removing the terminal 7.5 centimeters and deblossoming the fed shoot. During the berry set stage, translocation of labeled photosynthates from a newly exporting leaf was upwards to the shoot tip, but the direction of movement was reversed by removal of the shoot tip or by darkening or removal of the leaves below the fed leaf. Translocation of photosynthates was predominantly basipetal from a fed leaf near the base of the shoot during the berry set stage, but upward movement was induced by darkening or defoliation of the upper part of the shoot.     

Translocation of carbon in powdery mildewed barley

This paper compares translocation in healthy and powdery mildew (Erysiphe graminis f. sp. hordei, race CR3) infected barley (Hordeum vulgare, variety Manchuria). The sink-like properties of the powdery mildew infection were used to determine what effect imposing a sink in the midst of normal source tissue (mature primary leaf) had on the translocation process. The pattern of translocation was determined by monitoring the movement of ¹⁴C which was photosynthetically incorporated from ¹⁴C either by the primary or second leaf. In the healthy primary leaf of barley, ¹⁴C fixed in the tip section of the blade was preferentially translocated to the root, whereas ¹⁴C fixed in the basal section was primarily translocated to the shoot. When a sporulating powdery mildew infection was present in the mid-section of the primary leaf, ¹⁴C fixed in that section or in the acropetal healthy tip section readily accumulated in the infection area. Labeled carbon fixed in the healthy basal section was translocated into the other parts of the plant with only a small fraction moving acropetally into the infected mid-section. The ¹⁴C fixed by the second leaf was translocated to the root and younger shoot with very little entering the primary leaf. The presence of the mildew infection did not alter this pattern.     

Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions

Summary We examined the capacity of the galling aphid, Pemphigus betae, to manipulate the sink-source translocation patterns of its host, narrowleaf cottonwood (Populus angustifolia). A series of ¹⁴C-labeling experiments and a biomass allocation experiment showed that P. betae galls functioned as physiologic sinks, drawing in resources from surrounding plant sources. Early gall development was dependent on aphid sinks increasing allocation from storage reserves of the stem, and later development of the progeny within the gall was dependent on resources from the galled leaf blade and from neighboring leaves. Regardless of gall position within a leaf, aphids intercepted ¹⁴C exported from the galled leaf (a non-mobilized source). However, only aphid galls at the most basal site of the leaf were strong sinks for ¹⁴C fixed in neighboring leaves (a mobilized source). Drawing resources from neighboring leaves represents active herbivore manipulation of normal host transport patterns. Neighboring leaves supplied 29% of the ¹⁴C accumulating in aphids in basal galls, while only supplying 7% to aphids in distal galls. This additional resource available to aphids in basal galls can account for the 65% increase in progeny produced in basal galls compared to galls located more distally on the leaf and limited to the galled leaf as a food resource. Developing furits also act as skins and compete with aphid-induced sinks for food supply. Aphid success in producing galls was increased 31% when surrounding female catkins were removed.     

ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

Effect of Oxygen Concentration on C-Photoassimilate Transport from Leaves of Salvia splendens L

Partitioning and transport of recently fixed photosynthate was examined following ¹⁴CO₂ pulse-labeling of intact, attached leaves of Salvia splendens L. maintained in an atmosphere of 300 microliters per liter CO₂ and 20, 210, or 500 milliliters per liter O₂. Under conditions of increasing O₂ (210, 500 milliliters per liter), a smaller percentage of the recently fixed ¹⁴C in the leaf was allocated to starch, whereas a greater percentage of the fixed ¹⁴C appeared in amino acids, particularly serine. The increase in ¹⁴C in amino acids was reflected in material exported from source leaves. A higher percentage of ¹⁴C in serine, glycine, and glutamate was recovered in petiole extracts when source leaves were maintained under elevated O₂ levels. Although pool sizes of these amino acids were increased in both the leaves and petioles with increasing photorespiratory activity, no significant changes in either ¹⁴C distribution or concentration of transport sugars (i.e. stachyose, sucrose, verbascose) were observed. The data indicate that, in addition to being recycled intracellularly into Calvin cycle intermediates, amino acids produced during photorespiration may also serve as transport metabolites, allowing the mobilization of both carbon and nitrogen from the leaf under conditions of limited photosynthesis.     

Translocation of Photosynthate in Curly Top Virus-infected Tomatoes

Photosynthate translocation in single leaflets of healthy and curly top virus-infected tomatoes was investigated using ¹⁴C as a marker. The amount of radioactivity found in plant parts not exposed to ¹⁴CO₂ was substantially lower in diseased than in healthy plants. The time lag for the appearance of ¹⁴C in the petiole was considerably longer in the infected plants than in the healthy. The kinetics of disappearance of ¹⁴C from the lamina during the 24-hour period following labeling showed a strong retention of recent assimilates within the diseased leaf, not accompanied by increased immobilization into insoluble forms. Sucrose was the predominant compound participating in photosynthate transport in both healthy and diseased leaves. The amount of ¹⁴CO₂ fixed was approximately 40% lower in curly top virus-infected leaves than in healthy leaves.     

ANATOMICAL CHANGES DURING LEAF ONTOGENY IN POPULUS DELTOIDES

The leaf plastochron index (LPI) was used to interpret the anatomical changes during leaf ontogeny in the developing leaf zone of young cottonwood trees and to relate leaf anatomical structure to physiological function. The lamina tip matured precociously with respect to both structure and function. Below the lamina tip the intercellular spaces, stomates, and minor veins matured basipetally, while the major veins developed acropetally. Ontogenetically, maturation progressed from LPI –1.0, which was anatomically immature except for its lamina tip, to the first fully expanded leaf at LPI 6.0, which was anatomically mature. Physiological maturity also occurred at LPI 6.0, thus signifying a transition with respect to both structure and function. By evaluating the anatomical observations in conjunction with physiological data collected at comparable LPI's in other experiments, it could be demonstrated that anatomical development was a limiting factor in photosynthesis and translocation of assimilates.     

Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.