Sequential leaf senescence and correlatively controlled increases in xylem flow resistance

In bean, Phaseolus vulgaris L. (Contender), the directly measured hydraulic resistance of the xylem pathway between roots and primary leaf pulvinal junctions increased rapidly and progressively from 21 to 28 days after planting. These increases in xylem resistance (+390%) were specifically located in the pulvinal junction of the primary leaf. Moreover, they occurred just prior to the onset of primary leaf yellowing. Developmental increases in xylem hydraulic flow resistance and stomatal resistance, as well as subsequent primary leaf yellowing, were completely prevented by detopping the shoots above the primary leaves at 21 days. Thus, the onset of these senescence-associated symptoms was correlatively controlled. In short-term investigations of the mechanisms involved, flow between petiole and cut tip of excised leaves was rapidly reduced by infiltration of 20 picomoles of soluble dextran into the xylem. Moreover, imbibition of approximately 120 picomoles of dextran by excised leaves increased stomatal resistances. A programmed secretion of hormonal concentrations of similar polysaccharides into specific xylem sections in vivo might provide a mechanism for regulating the partitioning of essential xylem supplies between leaves, thus inducing sequential leaf senescence.     

三种植物生长物质对大豆叶茎解剖结构的影响

在大田栽培条件下,大豆‘垦农4号’于开花始期叶面喷施植物生长物质2-N,N-二乙氨基乙基己酸酯（DTA）、氯化胆碱（CC）和SOD模拟物（SODM）,并比较不同植物生长物质影响大豆叶片、叶柄和茎的解剖结构。结果表明,喷施植物生长物质后30d,叶中栅栏组织厚度及栅海比均增加;喷施SODM、DTA的叶中主脉维管束横截面积和木质部导管数目增加,CC对主脉维管柬横截面积和木质部导管数目的影响不明显;喷施3种植物生长物质的叶柄表皮细胞厚度、叶柄维管束横截面积和导管数量增加,茎部薄壁组织、韧皮部和木质部厚度增加,茎的直径也增加。     

Induction to Resistance to Crown Rust in Oat

Induction of resistance to crown rust, caused by Puccinia coronata Cda. var, avenae Frazer et Led., occurred in seedlings of three genetic lines of oats (Avena sativa L.) inoculated sequentially with one of three inducer-challenger combinations of the pathogen. There was significant reduction in both the number of pustules per leaf and the weight of urediospores harvested in all three host genotypes tested. There were highly significant differences between individual spore harvests correlated with age of the pustules. Development of telia by the challenger was found to occur considerably later, and was less intense, when the challenger was inoculated alone in contrast to treatments in which the challenger followed pre-inoculation with the inducer. There was no correlation between the numbers of resistant and susceptible lesions within the range of inoculum density used in these experiments.     

The structure of xylem vessels in grapevine (Vitaceae) and a possible passive mechanism for the systemic spread of bacterial disease

Xylem-dwelling pathogens become systemic, suggesting that microorganisms move efficiently in the xylem. To better understand xylem pathways and how bacteria move within the xylem, vessel connectivity between stems and leaves of Vitis vinifera cv. Chardonnay and Muscadinia rotundifolia cv. Cowart was studied. Three methods were used: (1) the light-producing bacterium, Yersinia enterocolitica, (Ye) strain GY5232 was loaded into petioles and followed using X-ray film, (2) fluorescent beads were loaded and followed by microscopy, and (3) low-pressure air was pumped into leaves and extruded bubbles from cuts in submerged leaves were followed. Bacteria, beads, and air moved through long and branched xylem vessels from the petiole into the veins in leaves of both varieties. From the stem, bacteria and air traveled into primary and secondary veins of leaves one, two, and three nodes above the loading point of the bacteria or air. Particles and air could move unimpeded through single xylem vessels or multiple vessels (conduits) connected possibly through broken pit membranes from within the stem axis into leaf blades. Bacteria were also able to move long distances within minutes from stem to leaf passively without having to cross pit membranes. Such complex, open xylem conduits have not been well documented before; these findings will help elucidate mechanisms involved in the systemic spread of pathogens.     

Resistance of Pepper Lines to the Movement of Cucumber Mosaic Virus

The multiplication and the migration of cucumber mosaic virus (CMV) were studied in greenhouse conditions in one susceptible ‘Yolo wonder’ and two resistant ‘Milord’ and ‘Vania’ pepper varieties. DAS-ELISA tests have revealed that the virus is replicated in inoculated leaves of the resistant varieties as high as in the susceptible variety. In the susceptible variety ‘Yolo wonder’, CMV migrated from the leaf lamina to the petiole two days after inoculation and it became systemic three days later regardless the season. In ‘Milord’ the virus migrated from the leaf lamina to the petiole five days after inoculation and it became systemic during the winter 16 days after inoculation. Whereas plants of the same genotype were not infected systemically during the summer. In ‘Vania’, during the two seasons, CMV spread from the blade to the petiole five days after inoculation, but the virus was not detected beyond the inoculated leaf. These results show that ‘Milord’ and ‘Vania’ are resistant to CMV migration. Therefore, the resistance to CMV migration is affected by plant genotype and temperature. The study of effect of pepper plant phenology on infection has revealed that resistance to CMV migration is also affected by the development stage of the plants.     

Relationships among strains of bean common mosaic virus and blackeye cowpea mosaic virus - members of the potyvirus group

In host-range studies, bean common mosaic virus strains (BCMV-NL1, -NL3 and -NY 15) usually induced distinct systemic symptoms in susceptible bean cultivars and latent infection in several Vigna genotypes (except NY15 which gave mosaic symptoms in the latter), while blackeye cowpea mosaic virus (B1CMV-W) caused distinct systemic symptoms in several Vigna genotypes and only weak systemic symptoms in a few bean genotypes only. Biologically, B1CMV-W was closest to BCMV-NY15 and less close to -NL1. When using antisera to the three BCMV strains and five strains of B1CMV (including a strain originally considered cowpea aphid-borne mosaic virus CAMV-Mor) in SDS-immunodiffusion and ELISA, BCMV-NL1 and -NY15 were found to be closely related to each other and to BICMV-Fla, -NR and -W, and less closely to BICMV-Ind and -Mor. Serological relationships of BCMV-NL1 and -NY15 to BCMV- NL3 were more distant, which is in line with the biological distinction of NL3 in causing temperature-independent necrosis in bean cultivars with the necrosis gene I. PAGE analysis of coat proteins revealed that the three strains of BCMV and B1 CMV-W have similar but non-identical molecular masses. Although molecular hybridisation may further elucidate quantitative relationships between potyvir-uses, variation within and among the potyviruses may continue to pose problems in their classification and identification.     

Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Alteration of Response of Bean Leaves to Compatible and Incompatible Bacteria

Injection of Red Mexican bean leaves with Pseudomonas phascolicolaRace 2 (compatible, 18 h before P. mors-prunorum or P. phaseolicolaRace 1 (incompatible), or simultaneous inoculation with compatibleand incompatible bacteria (3:1) greatly delayed the appearanceof hypersensitive responses. When compatible bacteria were inoculated12 h or less before incompatible bacteria, or when the ratioof these bacteria was 1:1 or 0.3:1 in simultaneous inoculation,hypersensitive responses did develop. Earlier inoculation withincompatible bacteria delayed the appearance and severity ofdisease symptoms following later inoculation with compatiblebacteria. When selected areas of leaves were inoculated with compatiblebacteria, effects on hypersensitive responses were confinedto these areas when the whole leaf was inoculated later withincompatible bacteria. Inoculation through the upper leaf surfacewith incompatible bacteria did not affect susceptible responseswhen compatible bacteria were inoculated 24 h later throughthe lower surface. Treatment of leaves with heat-killed bacteria or live bacteriain numbers insufficient to cause hypersensitive responses didnot prevent development of these responses following later inoculationwith incompatible bacteria. Use of heat-killed bacteria didsuppress hypersensitive responses in tobacco leaves. Injectionof leaves with cycloheximide (20 p.p.m.) 30 min before inoculationwith incompatible bacteria suppressed leakage of electrolytesand browning of tissues associated with hypersensitive responses.Cycloheximide had little effect on leakage of electrolytes fromleaves inoculated with compatible bacteria or with the developmentof susceptible responses. Exposure of leaves to chloroform vapourdelayed hypersensitive responses by 24 h; treatment with peroxidasehad no effect.     

Xylem structure of four grape varieties and 12 alternative hosts to the xylem-limited bacterium Xylella fastidious

Background and Aims

The bacterium Xylella fastidiosa (Xf), responsible for Pierce''s disease (PD) of grapevine, colonizes the xylem conduits of vines, ultimately killing the plant. However, Vitis vinifera grapevine varieties differ in their susceptibility to Xf and numerous other plant species tolerate Xf populations without showing symptoms. The aim of this study was to examine the xylem structure of grapevines with different susceptibilities to Xf infection, as well as the xylem structure of non-grape plant species that support or limit movement of Xf to determine if anatomical differences might explain some of the differences in susceptibility to Xf.

Methods

Air and paint were introduced into leaves and stems to examine the connectivity between stem and leaves and the length distribution of their vessels. Leaf petiole and stem anatomies were studied to determine the basis for the free or restricted movement of Xf into the plant.

Key Results

There were no obvious differences in stem or petiole vascular anatomy among the grape varieties examined, nor among the other plant species that would explain differences in resistance to Xf. Among grape varieties, the more tolerant ‘Sylvaner’ had smaller stem vessel diameters and 20 % more parenchyma rays than the other three varieties. Alternative hosts supporting Xf movement had slightly longer open xylem conduits within leaves, and more connection between stem and leaves, when compared with alternative hosts that limit Xf movement.

Conclusions

Stem–leaf connectivity via open xylem conduits and vessel length is not responsible for differences in PD tolerance among grape varieties, or for limiting bacterial movement in the tolerant plant species. However, it was found that tolerant host plants had narrower vessels and more parenchyma rays, possibly restricting bacterial movement at the level of the vessels. The implications of xylem structure and connectivity for the means and regulation of bacterial movement are discussed.     

Autoradiographic and biochemical evidence for the systemic translocation of systemin in tomato plants

The movement of systemin, the 18-amino-acid polypeptide inducer of proteinase inhibitors in tomato (Lycopersicon esculentum L.) plants, was investigated in young tomato plants following the application of [¹⁴C]systemin to wounds on the surface of leaves. Wholeleaf autoradiographic analyses revealed that [¹⁴C]systemin was distributed throughout the wounded leaf within 30 min, and then during the next several hours was transported to the petiole, to the main stem, and to the upper leaves. The movement of [¹⁴C]systemin was similar to the movement of [¹⁴C]sucrose when applied to leaf wounds, except that sucrose was slightly more mobile than systemin. Analyses of the radioactivity in the petiole phloem exudates at intervals over a 5-h period following the application of [¹⁴C]systemin to a wound demonstrated that intact [¹⁴C]systemin was present in the phloem over the entire time, indicating that the polypeptide was either stable for long periods in the phloem or was being continually loaded into the phloem from the source leaf. The translocation pathway of systemin was also investigated at the cellular level, using light microscopy and autoradiography. Within 15 min after application of [³H]systemin to a wound on a terminal leaflet, it was found distributed throughout the wounded leaf and was primarily concentrated in the xylem and phloem tissues within the leaf veins. After 30 min, the radioactivity was found mainly associated with vascular strands of phloem tissue in the petiole and, at 90 min, label was found in the phloem of the main stem. Altogether, these and previous results support a role for systemin as a systemic wound signal in tomato plants.The authors acknowledge the Washington State University Electron Microscope Center and staff for their technical advice and collaboration. We also thank Greg Wichelns for growing our plants and Dr. Steven Doares for providing [³H]systemin. This research was supported in part by the Washington State College of Agriculture and Home Economics Project No. 1791 and National Science Foundation grants IBN 9117795 and IBN 9104542     

Preinoculation with Tobacco Necrosis Virus Enhances Peroxidase Activity and Lignification in Cucumber, as a Resistance Response to Sphaerotheca fuliginea

Abstract Cucumber plants were mechanically inoculated with TNV and challenge-inoculated after 7 days with Sphaerotheca fuliginea. The development of the fungus was followed by light microscopy and the modifications of the host leaf cells were studied by histochemical and cytochemical reactions. Conidial germination was similar in control and TNV-infected plants, and in the following 2 or 3 days S. fuliginea development was also similar. Thereafter in control plants S. fuliginea development progressed steadily and no modifications appeared in the leaf host cells, while in TNV-infected plants strongly autofluorescing papillae were formed, and peroxidase activity was detected in the walls of many epidermal cells of the challenge-inoculated, leaf. Lignification ensued, and fungal growth was strongly inhibited. Protection was obtained provided the number of necrotic lesions was at least 12 per cotyledon, and was elicited even if the TNV-infected leaf was removed 7 days after infection, before challenge inoculation. No protection was induced when the TNV-infected leaf was removed 3 days after infection.     

Age-related Resistance in Bell Pepper to Cucumber mosaic virus

We demonstrated the occurrence of mature plant resistance in Capsicum annuum‘Early Calwonder’ to Cucumber mosaic virus (CMV) under greenhouse conditions. When Early Calwonder plants were sown at 10 day intervals and transplanted to 10‐cm square pots, three distinct plant sizes were identified that were designated small, medium and large. Trials conducted during each season showed that CMV accumulated in inoculated leaves of all plants of each size category. All small plants (with the exception of the winter trial) developed a systemic infection that included accumulation of CMV in uninoculated leaves and severe systemic symptoms. Medium plants had a range of responses that included no systemic infection to detection of CMV in uninoculated leaves with the systemically infected plants being either symptomless or expressing only mild symptoms. None of the large plants contained detectable amounts of CMV in uninoculated leaves or developed symptoms. When plants were challenged by inoculation of leaves positioned at different locations along the stem or different numbers of leaves were inoculated, large plants continued to accumulate CMV in inoculated leaves but no systemic infection was observed. When systemic infection of large plants did occur, e.g. when CMV‐infected pepper was used as a source of inoculum, virus accumulation in uninoculated leaves was relatively low and plants remained symptomless. A time‐course study of CMV accumulation in inoculated leaves revealed no difference between small and large plants. Analyses to examine movement of CMV into the petiole of inoculated leaves and throughout the stem showed a range in the extent of infection. While all large plants contained CMV in inoculated leaves, some had no detectable amounts of virus beyond the leaf blade, whereas others contained virus throughout the length of the stem but with limited accumulation relative to controls.     

Phloem mobility of magnesium

Magnesium-28 was applied to specific leaves of bean (Phaseolus vulgaris) and barley (Hordeum vulgare) plants. After 24 hours, as much as 7% of the absorbed Mg was exported from the treated bean leaves and 11% was transported basipetally from the treated zone of the barley leaves. Transport of Mg did not occur past a heat-killed section of the treated leaf, thereby indicating that translocation took place via the phloem. Mg movement in the phloem was also evident in autoradiograms of bean stem segments in which the xylem was separated from the phloem by a thin sheet of plastic.     

Crown Gall on Castor Bean Leaves: II. FORMATION OF SECONDARY TUMOURS

Secondary tumours were formed on the cotyledonary leaf petiole,the hypocotyl, and first true leaf of castor bean seedlingsafter inoculating the blades of the cotyledonary leaves withAgrobacterium tumefaciens. Depending on the strain of bacteriaemployed, 0 to 80 per cent of the plants developed secondarytumours. The ability of different strains to initiate secondarytumours was not obviously correlated with their relative effectivenessin initiating primary tumours. Though all produced primary tumours,five out of ten auxotrophic strains failed to yield secondarytumours, whereas only one out of 14 prototrophic strains failedto do so. Both the number of plants developing secondary tumoursand the frequency with which these tumours occurred on differentparts of the plant were positively correlated with the concentrationof the primary inoculum. Tumours also developed on the cotyledonaryleaf petiole and on the hypocotyl after vacuum infiltrationof A. tumefaciens into the blade of cotyledonary leaves. Inmost instances (9 out of 11 plants) no tumours were formed onthe blade of the infiltrated leaf. Thus, tumour formation equivalentto secondary tumours can occur in the absence of a primary tumouror an overt primary wound. Excision of inoculated leaves showedthat the stimulus for secondary tumour formation moves fromthe blade to the hypocotyl within 24 h. Attempts to demonstratethe presence of a sub-cellular tumour-initiating agent in homogenatesof inoculated leaves were unsuccessful. A. tumefaciens, however,was found in the petiole of the cotyledonary leaf and in thehypocotyl within 24 h of inoculation. The migrating agent responsiblefor secondary tumour formation in castor beans is concludedto be intact bacteria.     

Distinction of strains of bean common mosaic virus and blackeye cowpea mosaic virus using antibodies to N- and C- or N-terminal peptide domains of coat proteins

Earlier attempts to discriminate serologically strains NL1, NL3 and NY15 of bean common mosaic virus (BCMV) and strain W of blackeye cowpea mosaic virus (B1CMV) had been unsuccessful. Antibodies directed towards N- and C-, or N-terminal peptide regions of the coat proteins of the above strains enabled the distinction between B1CMV-W, BCMV-NY15 and BCMV-NL3 in electroblot immunoassay and in ELISA. The distinction was better with antibodies directed towards N-termini than with those to N- and C-termini. Strain NL1 of BCMV cross-reacted with both B1CMV-W and BCMV-NY15, but not with BCMV-NL3. Taxonomic implications of these findings are discussed.     

STUDIES IN THE BACTERIAL DISEASES OF SUDAN CROPS

The atypical symptoms first described by Bryan (1932) of the angular leaf spot disease of cotton caused by Xanthomonas malvacearum (E. F. Sm.) Dowson were reproduced by inoculation into seeds, stem apices or buds. The lesions that developed on the veins of the newly produced leaves were elongated and water-soaked, becoming dark brown. The leaf tissue dependent upon infected veins became yellow, flaccid and withered. The development of these symptoms was enhanced when inoculations were made into opening buds or germinating seeds as compared with inoculations into closed buds or dormant seeds.
In other bacterial diseases caused by Xanthomonas spp., somewhat atypical symptoms could also be produced by bud inoculation into the appropriate host. Those produced by X. ricini (Yoshi & Takimato) Dowson on castor, closely resembled the vein lesions described above on cotton but resulted only from bud inoculations; inoculations into stem apices and seeds failed to produce them. In dolichos bean inoculated with X. phaseoli (Smith) Dowson, atypical symptoms were produced only by seed inoculations and were confined to the first simple leaves (prophylls).
The differences in the production of atypical symptoms on the three hosts are correlated with differences in host structure and with the degree of virulence of the pathogen. The leaf parasite X. ricini , for example, which cannot infect castor bean stems, does not produce atypical symptoms when inoculated below the stem apex.
From the data discussed below, the incidence of atypical symptoms is attributed to infection either of an actively growing tissue or of a telescoped structure which subsequently completes its development.
The atypical symptoms of the cotton disease are not caused by a special strain of X. malvacearum. Further, they are not a peculiarity of this disease but may also develop in other necrotic diseases under similar conditions.     

Scaling of xylem vessels and veins within the leaves of oak species

General models of plant vascular architecture, based on scaling of pipe diameters to remove the length dependence of hydraulic resistance within the xylem, have attracted strong interest. However, these models have neglected to consider the leaf, an important hydraulic component; they assume all leaves to have similar hydraulic properties, including similar pipe diameters in the petiole. We examine the scaling of the leaf xylem in 10 temperate oak species, an important hydraulic component. The mean hydraulic diameter of petiole xylem vessels varied by 30% among the 10 oak species. Conduit diameters narrowed from the petiole to the midrib to the secondary veins, consistent with resistance minimization, but the power function scaling exponent differed from that predicted for stems. Leaf size was an organizing trait within and across species. These findings indicate that leaf vasculature needs to be included in whole-plant scaling models, for these to accurately reflect and predict whole-plant transport and its implications for performance and ecology.     

The leaf development process and its significance for reducing self-shading of a tropical pioneer tree species

On a monoaxial erect stem of trees with continuous leafing, the older leaves would be quickly shaded by newer (upper) leaves if the trees did not have any compensating mechanisms to avoid self-shading. We hypothesized that the dynamic adjustment of leaf deployment, by regulating the patterns of leaf growth and by changing leaf orientation as leaves age, is a compensating mechanism. To verify this hypothesis, we analyzed leaf development and crown structure of a Far Eastern tropical pioneer tree species, Macaranga gigantea (Rub. f. et Toll.) M.A., which unfolds huge leaves directly on a monoaxial stem with a short leafing interval. Petioles required more than 90 days for full elongation and the petiole angle (the angle between the petiole axis and the vertical) increased over time. Thus, a series of leaves on a stem progressively increased in petiole length and petiole angle from the youngest to the oldest leaves. This is beneficial because it decreases the degree of self-shading within a crown. A simulation suggested that an average crown for the M. gigantea seedlings, which was constructed using empirically determined morphometric data cannot entirely eliminate self-shading within the crown. But an average crown had a lower degree of self-shading, with less dry mass allocation to the petiole than simulated crowns that were identical to the average crown in all but one respect: they had constant petiole lengths or petiole angles. We conclude that M. gigantea seedlings reduce self-shading by regulating elongation of the petiole and changes in the petiole angle with increasing leaf age.     

PETIOLE DEVELOPMENT AND XYLEM DIFFERENTIATION IN XANTHIUM REPRESENTED BY THE PLASTOCHRON INDEX

Petiole development and formation of xylem vessels have been investigated in Xanthium leaves from early ontogeny to maturity. Kinetics of growth was presented in terms of absolute and relative elemental rates of elongation. The process of vascularization was assessed by the number of differentiated xylem vessels. The leaf plastochron index (LPI) developed by Erickson and Michelini (1957) was used for designating the various stages of development. An exponential increase in petiole length was observed between the LPIs –3 and +4 indicating a constant relative rate of 0.20 or 20% increase per day. After cessation of lamina elongation at LPI 8, petiole elongation continued for an additional 5 day period, to LPI 9.5. Relative elemental rate analysis revealed that the basipetal pattern of elongation was maintained throughout the leaf development. At a specific plastochron age, the only growth was due to the petiole elongation. Leaves which ceased elongating had not completed their internal development, since the process of xylem formation continued for several plastochrons, or about 8 days. The highest rate of xylem formation was ten vessels per day at LPI 5. On the average, about five xylem vessels differentiated per day in the middle portion of a Xanthium petiole. Mature petioles contained an average of 218 xylem vessels. About 12 canals of schizogenous origin preceeded the development of the vascular tissue.     

Transport of 14C-Assimilates in Seedlings of Phaseolus vulgaris L. in Relation to Vascular Anatomy

Patterns of ¹⁴C-photosynthate translocation in bean (Phaseolusvulgaris L.) seedlings have been investigated in relation tovascular-bundle continuity between exporting and importing organsby use of radioassay and tissue-clearing techniques. Assimilatefrom the primary leaves reaches the first trifoliate leaf byan indirect route. There is no direct vascular connection betweenthe primary leaves and distal tissues. Bundles of the primaryleaf petiole connect with an anastomosis at the node, and allbundles which originate from this structure descend the stem.Assimilate from primary leaves moves down the stem, and is thentransferred to an ascending pathway, the bundles of which traversethe anastomosis at the second node. The lateral leaflets ofthe first trifoliate leaf are served differentially by primaryleaves with respect to assimilate supply. Differences are relatedto position, and may be accounted for by differences in vascularcontinuity.