Comparative Phloem mobility of nickel in nonsenescent plants

⁶³Ni was applied to nonsenescent source leaves and found to be transported to sink tissues in pea (Pisum sativum L.) and geranium plants (Pelargonium zonale L.). The comparative mobilities (percent tracer transported out of source leaf ÷% ⁸⁶Rb transported) for ⁶³Ni in peas was 2.12 and in geranium 0.25. The value for the phloem mobile ⁸⁶Rb was 1.00. By contrast, the comparative mobility of ⁴⁵Ca, which is relatively immobile in the phloem, was low (0.05 in peas, 0.00 in geranium). Interruption of the phloem pathway between source and sink leaves by steam girdling almost completely inhibited ⁶³Ni accumulation in the sink leaves of both species. We conclude that Ni is transported from nonsenescent source leaves to sink tissues via the phloem of leguminous and nonleguminous plants.     

Phloem transport in Picea abies (L.) Karst. in mid-winter

Summary Translocation of ¹⁴C assimilates was studied on four different transport systems of Picea abies branches after induced activation in January. ¹⁴CO₂ assimilation of terminal shoots for 48 h at 25° C resulted in phloem loading and basipetal transport of ¹⁴C photosynthate into the following, older shoot generations. ¹⁴C import was enhanced, when these older shoot generations were kept in the dark. Microautoradiographs of the labelled terminal shoots showed that ¹⁴C assimilates were exported from needles via sieve elements of the leaf traces and loaded into the latest increment of the axial secondary phloem. No ¹⁴C label appeared in the obliterated sieve cells or in the tracheids. In addition, ¹⁴C photosynthate accumulated densely in the chlorophyllous cells of the cortex and in cells of the resin ducts, indicating certain sink activity. In the darkened 2-year-old shoot, imported ¹⁴C photosynthate was concentrated in the functional secondary phloem, while some ¹⁴C label was unloaded into the latest xylem increment. When 6-year-old shoots were exposed to ¹⁴CO₂ for 48 h in the light, ¹⁴C assimilates accumulated in the phloem of the leaf trace and in the latest increment of the axial secondary phloem. However, a substantial amount of radioactivity was unloaded into ray cells and phloem parenchyma cells. Thus, the presence of functioning phloem in needles and twigs of P. abies during winter allows long-distance translocation and radial distribution of assimilates according to existing source-sink relations.     

Ionic Interactions of Petiole and Lamina During the Life of a Leaf of Castor Bean (Ricinus communis L.) Under Moderately Saline Conditions

Ion (K⁺, Na⁺, Mg²⁺, Ca²⁺ and Cl) flows and partitioning in thepetiole and lamina of leaf 6 of castor bean {Ricinus communisL.) plants growing in the presence of a mean of 71 mol m^–3NaCl were described by an empirical modelling technique. Thiscombined data on changes in ion contents of petiole and lamina,ion: carbon molar ratios of phloem bleeding sap and pressure-inducedxylem exudates of the leaf with previously described informationon the economies of C and N in identical leaf material. Datawere expressed as daily exchanges of ions in xylem and phloem,or depicted as models of ion balance and transport activityof petiole and lamina during four consecutive phases of leaflife. The early import phase was characterized by high intakeof K and Mg through phloem, and of Ca mainly through xylem,but only limited intake of Na and Cl. The next phase up to fullleaf expansion showed similar relative differences in xylemintake between ions and the onset of rapid phloem export fromthe lamina of K and Mg, some export of Na and Cl but scarcelyany of Ca. The next mature phase, marked by maximal photosynthesisand transpiration by the leaf, showed high xylem intake of allions in xylem. This was more than matched by phloem export ofMg and K, but by only fractional re-export of Na and Cl andagain very limited cycling through the leaf of Ca. The finalpre-senescence phase exhibited similar behaviour, but with generallygreater contribution to phloem transport from mobilization ofion reserves of the lamina. The petiole retained particularlylarge amounts of Na and Cl in its early growth, thereby protectingthe lamina from excessive entry of salt, but these petiolarpools, together with those or other nutrient ions, were laterpartially mobilized to the lamina via the xylem stream. Datawere discussed in relation to the relatively high salt toleranceexhibited by the species. Key words: Ricinus communis, xylem and phloem transport, ion balance, K+ economy, Na+ exclusion, NaCl-stress, salt tolerance, leaf development     

The biosynthesis and translocation of 14C-IAA in Ricinus communis

The biosynthesis of ¹⁴C-IAA from ¹⁴C-tryptophan applied to abraded leaves of Ricinus communis and its subsequent export through the phloem were studied. Phloem sap was collected at intervals from incisions made in the stem below the IAA fed leaf. Any upward movement of label through the phloem or downward movement of phloem mobile compounds from leaves above the treated one were restricted by bark-ringing the plants.TLC and HPLC analyses of the collected sap indicate that some conversion of ¹⁴C-tryptophan to ¹⁴C-IAA had occurred. Subsequent GC-MS analysis of the HPLC purified samples of phloem sap revealed high levels of endogenous IAA transported from the fed leaf. The high ratio of unlabelled/labelled IAA in the phloem sap makes unequivocal confirmation by GC-MS of the predicted biosynthesis of ¹⁴C-IAA impossible. It is postulated that IAA is synthesised from tryptophan in mature leaves and exported to developing sink tissues with the flow of photoassimilates in the phloem.     

盐胁迫对苹果器官中钙镁铁锌含量的影响

以盆栽2年生富士苹果树(砧木为平邑甜茶M.hupehensisReld)为试材,研究了盐胁迫对苹果矿质营养平衡的影响.结果表明,在盐胁迫下,苹果各器官不同时期的单位干样中Ca、Mg、Fe和Zn含量的平均值没有明显变化,但各元素与Na的比值明显下降,特别是在高盐(3‰NaCl)胁迫下下降更为明显,从而破坏了树体内元素平衡.在无盐和盐胁迫下,苹果各器官中Ca含量的顺序为主干韧皮部＞叶片、新梢＞根＞主干木质部;Mg含量为新梢、根＞主干木质部、主干韧皮部、叶片;Fe含量为根＞叶片＞主干韧皮部、新梢＞主干木质部;Zn含量为新梢＞叶片＞根、主干韧皮部＞主干木质部.与对照相比,器官中各元素含量在胁迫期间表现出不同程度的波动性.     

Cytochemical localization of ATPase activity in phloem tissues ofRicinus communis

Summary Standard lead precipitation procedures have been used to examine the localization of ATPase activity in phloem tissues ofRicinus communis. Reaction product was localized on the plasma membrane of the companion cells associated with sieve elements and of parenchyma cells in phloem tissues from the leaf, petiole, stem and root. ATPase activity was also present on the plasma membrane and dispersed P-protein of sieve elements in petiole, stem and root tissue, but was absent from the plasma membrane of these cells in the leaf minor veins. Substitution of-glycerophosphate for ATP produced no change in the localization of reaction product in leaf tissue. These findings are discussed in relation to current theories on the mechanism of sugar transport and phloem loading.     

Litter fall and nutrient dynamics in a Nigerian rain forest seven years after a ground fire

Abstract. Seasonal litter fall and mineral element content (N, P, Ca, Mg, K) of regrowth forest communities at the base and on the slope of an inselberg in Ile-Ife, Nigeria, were studied 7 yr after a ground fire ravaged the forest. Litter fall (tha^?1 yr^?1) was 4.6 (total), 4.2 (leaf), 0.3 (small wood < 2.5 cm diameter) and 0.1 (reproductive parts: fruits and flowers) in the base community and 6.4 (total), 5.4 (leaf), 0.9 (small wood) and 0.1 (reproductive parts) in the slope community. There was significant monthly variation in litter fall in the two communities with lowest amount of litter recorded during the wettest months of the year (May - August) and the highest amount during the dry season. Significant monthly variation (P<0.05) in Ca, Mg and K concentration in leaf litter and for Mg (P < 0.01) in fruit litter occurred, with the lowest concentration recorded during the wettest months (May-August). In leaf and wood litter the order of mineral element concentration was Ca>N>K> Mg > P while in fruit litter it was N > K > Ca > Mg > P. Quantities of mineral element (kg ha^-1 yr¹) returned to the soil via litterfall were N: 66; P: 4; Ca: 97; Mg: 15; K: 45 in base forest, and N: 112; P: 5; Ca: 142; Mg: 20; K: 66 in slope forest. Through leaf litter >88.5% of these elements was returned into the two communities, through wood > 4.0% and through reproductive organs > 0.3%. The order of quantities of these elements returned in leaf and wood litter was Ca > N > K > Mg > P, in fruit litter N ～ K > Ca > Mg > P. Significant monthly variation in the amounts of the various elements returned were recorded in leaf litter, but not in wood and fruit litter. The lowest amount of various elements was returned during the wettest months (May-August) which coincided with the period of the lowest element concentration and litter fall.     

Semiquantitative estimates of Al and other cations in the leaf tissues of some Al-accumulating species using electron probe microanalysis

Electron probe microanalysis of transverse sections was used to provide semiquantitative estimates of Al and other cations in the leaf tissues of some Al-accumulating plants of the cerrado vegetation of central Brazil. Concentrations of Al were generally higher than those of other cations in the phloem elements of these plants growing on dystrophic as well as fertile acid soils. When one of the Al-accumulating species,Vochysia thyrsoidea, was grown in a calcareous soil, the concentration of Al in the phloem elements of leaves was lower than that of Ca and K though the cotyledons showed higher concentrations of Al than those of other cations.     

The Effect of 2,4-Dinitrophenol on Translocation in the Phloem

The effect of 2,4-dinitrophenol (DNP) on sucrose-¹⁴C transport in Soya seedlings has been analysed. The aim was to distinguish between an effect of the inhibitor on sugar movement within the phloem sieve tubes themselves, and on the prior steps of uptake and secretion of sugar into the conducting cells. DNP drastically inhibited sucrose-¹⁴C transport if it was applied to the ¹⁴C-treated leaf immediately before, or during, ¹⁴C supply. Transport was also strongly inhibited if DNP was applied along the translocation path while the ¹⁴C-treated leaflet was still in position on the plant. When, bowever, DNP was applied through the cut petioles of the primary leaves after removal of the ¹⁴C-treated terminal leaflet of the first trifoliate leaf, no inbibition was observed. On the contrary, transport appeared to have been promoted: significantly more ¹⁴C disappeared from the upper regions of DNP-treated plants as compared with controls, while in the lower plant parts more ¹⁴C accumulated. Different rates of synthesis of sucrose-¹⁴C into non-alcohol-soluble compounds could not account for this result. A similar stimulatory effect was observed when DNP was applied to the cut petiole of a primary leaf opposite that treated with ¹⁴C. Several indications were obtained that ¹⁴C which has reached the lower parts of the plant may circulate upwards again through the phloem within about 15 minutes. When sucrose-¹⁴C was introduced into the roots via the xylem, both DNP treatment and prior steam girdling resulted in the apparent accumulation of ¹⁴C in the lower plant parts. the results would be compatible with DNP inhibition of upwards movement in the phloem. DNP might also have affected sugar uptake processes in cells neighbouring the translocation path. It is concluded that the inhibitory effect of DNP on downwards phloem transport reported by earlier workers was probably due to an effect on uptake and/or secretion into the sieve tubes, not to an effect on the conducting cells themselves. Modern theories for phloem transport are discussed in the light of these findings.     

Phloem Unloading in Developing Leaves of Sugar Beet : I. Evidence for Pathway through the Symplast

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (`Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [¹⁴C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of ¹⁴C-label into sink leaves during steady state labeling of a source leaf with ¹⁴CO₂. Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, l-[¹⁴C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, l-[¹⁴C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.     

Differential distribution of proteins expressed in companion cells in the sieve element-companion cell complex of rice plants

Sieve tubes are comprised of sieve elements, enucleated cells that are incapable of RNA and protein synthesis. The proteins in sieve elements are supplied from the neighboring companion cells through plasmodesmata. In rice plants, it was unclear whether or not all proteins produced in companion cells had the same distribution pattern in the sieve element-companion cell complex. In this study, the distribution pattern of four proteins, beta-glucuronidase (GUS), green fluorescent protein (GFP), thioredoxin h (TRXh) and glutathione S-transferase (GST) were analyzed. The foreign proteins GUS and GFP were expressed in transgenic rice plants under the control of the TRXh gene promoter (PTRXh), a companion cell-specific promoter. Analysis of leaf cross-sections of PTRXh-GUS and PTRXh-GFP plants indicated high accumulation of GUS and GFP, respectively, in companion cells rather than in sieve elements. GUS and GFP were also detected in phloem sap collected from leaf sheaths of the transgenic rice plants, suggesting these proteins could enter sieve elements. Relative amounts of GFP and endogenous phloem proteins, TRXh and GST, in phloem sap and total leaf extracts were compared. Compared to TRXh and GST, GFP content was higher in total leaf extracts, but lower in phloem sap, suggesting that GFP accumulated mainly in companion cells rather than in sieve elements. On the other hand, TRXh and GST appeared to accumulate in sieve elements rather than in companion cells. These results indicate the evidence for differential distribution of proteins between sieve elements and companion cells in rice plants.     

Xylem and Phloem transport and the functional economy of carbon and nitrogen of a legume leaf

Exchanges of CO₂ and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to one-third area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. ¹⁴C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9 to 12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two% of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation, and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 milligrams C per milligram N; phase 2, 24.7 milligrams C per milligram N; phase 3, 91.9 milligrams C per milligram N; and phase 4, 47.7 milligrams C per milligram N.     

Leaf development and phloem transport in Cucurbita pepo: Carbon economy

Summary Net photosynthesis, dark respiration and growth for leaf 5 of Cucurbita pepo L. plants grown under controlled conditions were measured and the data used for an assessment of the changes in carbon balance during growth of the leaf through expansion to maturity. The blade is first capable of net CO₂ fixation when ca. 8% expanded but the initial rapid growth during this period is sustained almost entirely through imported nutrients. When the growth rate starts to decline rapidly the net photosynthetic capacity of the blade begins to increase. This increase is accompanied by an expansion of the intercellular spaces and by decreasing dark respiration measured at night and in dark periods during the day. The blade becomes completely independent of phloem imported nutrients and begins to export excess photosynthate when the phase of rapid decrease in relative growth rate is almost complete at about 45% expansion. Maximum net photosynthesis of ca. 11 mg CO₂ h^-1 dm^-2 is achieved at 70% expansion. The first detectable synthesis of the transport sugars stachyose and raffinose in the blade coincides with the beginning of intralaminar phloem transport from the tip to the base of the leaf. The synthesis of sucrose, the other major transport sugar, is detectable at all stages of leaf development.     

Investigating the in vivo calcium transport path to developing potato tuber using 45Ca: a new concept in potato tuber calcium nutrition

Calcium is believed to be transported with water in the xylem. Consistent with this proposal, low‐transpiring organs such as potato Solanum tuberosum tubers are known to suffer from calcium deficiency. Although roots on tubers and stolons have been shown to supply water to tubers, there is no direct evidence for the calcium transport pathway to tubers. Both a xylem and a phloem transport pathway have been suggested. We investigated in vivo calcium transport to developing potato, cv. Dark Red Norland and cv. Russet Burbank, tubers using ⁴⁵Ca in a controlled environment facility. Whole plant split pot experiments allowed the placement of ⁴⁵Ca either in the main (basal) root or the tuber and stolon areas of the pot. The results showed that ⁴⁵Ca was transported to the shoot with the transpiration stream from both areas but was not re‐translocated to tubers or the main (basal) root system even 57 days after ⁴⁵Ca application. Radioactivity could only be detected in the tuber when ⁴⁵Ca was fed to the stolon and tuber area. When ⁴⁵Ca was fed to specific tubers, radioactivity was detected in the aerial shoot; however, no activity was detected in other tubers or the main (basal) roots. In another set of experiments, roots on a stolon near a tuber were precisely fed ⁴⁵Ca and Safranin O. The radioactive signal exactly overlapped the water transport pathway in the tuber marked with Safranin O dye, suggesting that water and calcium can be simultaneously transported from stolon roots to the tuber. No transport of ⁴⁵Ca across the tuber periderm was detected 8 days after ⁴⁵Ca was applied to the tuber periderm. This indicated that no significant transport of calcium occurs from the soil across the periderm. Our results provide evidence that: (1) calcium is not re‐translocated via the phloem from the aerial shoot tubers and main (basal) roots; (2) the main root system does not supply calcium to the tuber; (3) calcium is not transported across the periderm to the interior tuber tissue; (4) calcium is transported to the tuber via the xylem along with water, and the roots on the stolon associated with the tuber supply water and calcium to the developing tuber; and (5) transpirational demand is a significant determinant of calcium distribution within the plant.     

Sink Effect of Cytokinin in Detached Leaves Is Not Caused by Structural Rearrangements of Phloem

The sink effect of cytokinin is manifested as a decrease in source capacity and the induction of sink activity in the phytohormone-treated region of a mature excised leaf. In order to find out whether this effect was due to the direct action of cytokinin on the phloem structure, two types of phloem terminals were examined. In pumpkin (Cucurbita pepo L.) leaves, the phloem terminals are open; i.e., they are linked to mesophyll by numerous symplastic connections, which are located in narrow areas called plasmodesmal pit fields. In broad bean (Vicia faba L.) leaves, the phloem terminals belong to the closed type and have no symplastic links with mesophyll. The electron microscopic study of terminal phloem did not reveal any structural changes in the companion cells, which could account for the suppression of assimilate export. The treatment of leaves with cytokinin neither disturbed the structure of plasmodesmal pit fields in pumpkin leaves nor eliminated the wall protuberances (the ingrowths promoting phloem loading) in bean leaves. No evidence was obtained that the cytokinin-induced import of assimilates in mature leaves is caused by the recovery of meristematic activity, i.e., by either formation of new phloem terminals having immature sieve elements capable of unloading or by the development of new sieve elements within the existing veins. Cytokinin did not induce de novo formation of phloem elements. Structural characteristics of the leaf phloem, such as the number of branching orders in the venation pattern, the number of vein endings per areole, the number of areoles per leaf, the area of one areole, and the number of sieve elements per bundle remained unaltered. It is concluded that the sink effect of cytokinin in excised leaves cannot be determined by alteration of the phloem structure.     

Inhibition of Loading of C Assimilates by p-Chloromercuribenzenesulfonic Acid : Localization of the Apoplastic Pathway in Vicia faba

The apoplast of mature leaves excised from broadbean (Vicia faba L.) plants was infiltrated with 2 millimolar p-chloromercuribenzenesulfonic acid (PCMBS) via the transpiration stream, and the ability of the tissues to take up sugars was tested. An infiltration time of 75 minutes was sufficient to obtain a maximal (75%) inhibition of exogenous [¹⁴C]sucrose (1 millimolar) uptake. This infiltration affected neither CO₂ assimilation nor the transmembrane potential difference of leaf cells but strongly inhibited phloem loading of endogenous [¹⁴C] assimilates. The study of the symplastic relations between the different cell types of the mature leaf showed that the density of the plasmodesmata is generally very low in comparison with other species investigated so far, particularly when considering the mesophyll/bundle sheath and the bundle sheath/phloem cells connections, as well as the connections of the transfer cell-sieve tube complex with the surrounding cells. These three successive barriers therefore strongly limit the possibilities of symplastic transit of the assimilates to the conducting cells. The comparison of the densities of plasmodesmata in an importing and an exporting leaf suggests that the maturation of the leaf is characterized by a marked symplastic isolation of the phloem, and, within the phloem itself, by the isolation of the conducting complex. As a consequence, these physiological and cytological data demonstrate the apoplastic nature of loading in the mature leaf of Vicia faba, this species undoubtedly presenting a typical model for apoplastic loading.     

Virus-like particles in phloem tissue of chervil (Anthriscus cerefolium) infected with carrot red leaf virus

Isometric virus-like particles c. 22–25 nm in diameter were found in ultrathin sections of chervil leaves infected with carrot red leaf virus (CRLV). The particles were confined to the phloem and occurred in less than 5% of the cells in the vascular bundles. They were commonest in companion cells, occurred frequently in sieve elements and were also found in phloem parenchyma cells. The observations support other evidence that CRLV should be classified in the luteovirus group.     

The Effects of Fusicoccin,Abscisic Acid,and Benzyladenine on the Transport and Partitioning of Substances as Related to Cytokinin Sink Activity

We tested the possible cytokinin effect on the functioning of the active transport system involved in the assimilate loading into the phloem as a cause for the cytokinin sink and retention effect. This effect is manifested in the deceleration of substance export from and the stimulation of substance import to the sites of local phytohormone application to the mature detached leaf from untreated leaf areas. To affect the membrane mechanisms of the substance transport, we used leaf treatment with the phytotoxin fusicoccin, an enhancer of plasmalemmal H⁺-ATPase and a potential stimulator of assimilates export, and with the phytohormone ABA affecting transport, metabolism, and plant growth. However, fusicoccin did not enhance ¹⁴C-sucrose export from the leaf blade and did not interfere with the cytokinin-induced export deceleration. ABA reduced substantially ¹⁴C export from the leaf but eliminated the cytokinin effect on this process. Similar results were obtained for broad bean (Vicia faba L.) leaves with apoplastic phloem loading, involving H⁺-ATPase activity, and pumpkin (Cucurbita pepo L.) leaves with symplastic phloem loading, that is, occurring without sucrose transmembrane translocation and without H⁺-ATPase involvement. The conclusion is that the cytokinin-induced development of sink zones in source leaves is not related to the membrane mechanisms of the substance transport in the mesophyll–phloem system. The data obtained support the idea that the cause for the cytokinin sink and retention effect is the enhancement of elongation growth and total activation of metabolism in the mesophyll cells of the detached leaf.     

A microautoradiographic study of Ca45 and S35 distribution in the intact bean root

Summary Microautoradiographic techniques were used to determine the distribution of Ca⁴⁵ and S³⁵ in regions of the bean root where anatomical features may influence the processes of ion uptake and translocation. Root tissue from intact plants was prepared by methods that preserve both soluble and insoluble Ca and S. Ca⁴⁵ distribution was determined after 1 hour and 15 min, of uptake, after 2 efflux periods, and after replacement by non-tracer Ca.S³⁵ distribution was determined after 1 hour and 15 min of uptake.The quantity of Ca⁴⁵ that entered the root was greater than the quantity of S³⁵. Ca⁴⁵ concentration within the root increased with linear distance from the 8-mm level behind the tip. The pathways of Ca and S across the cortex appeared to be different since Ca⁴⁵ was particularly associated with cell walls and S³⁵ was distributed more evenly through the cells. There was no evidence that the endodermis was a diffusion barrier for Ca; the small parenchyma cells associated with conducting elements acquired a high concentration of Ca⁴⁵ and thus appear to be implicated in absorption and perhaps in transfer to the xylem. The evidence suggests that the endodermis may have been a barrier for S, but if so, certain parenchyma cells inside the stele, especially at xylem poles, were equally involved. The region from 30 to 80 mm from the tip appeared to participate in Ca uptake and transfer to the xylem; because of tissue immaturity the 8-mm region, which contained the least Ca⁴⁵, was thought not to translocate to the shoot. Deposition of Ca⁴⁵ in oxalate crystals represented almost complete immobilization. Calcium oxalate metabolism was most active in the 30-mm region of secondary roots and in their small branches. S³⁵-labelled nuclei occurred in the cortex 2.5 to 3 mm behind the root tip.     

LEAF OF SPINACIA OLERACEA (SPINACH): ULTRASTRUCTURE,AND PLASMODESMATAL DISTRIBUTION AND FREQUENCY,IN RELATION TO SIEVE-TUBE LOADING

Minor veins and contiguous tissues of the Spinacia oleracea leaf were analyzed by electron microscopy to determine the characteristics of the component cells and the structure, distribution, and frequency of plasmodesmata between the various cell types of the leaf. Mesophyll and bundle-sheath cells contain components typical of photosynthetic cells although the latter cell type contains smaller chloroplasts and fewer mitochondria and microbodies than the mesophyll cells. In addition, the mesophyll cells contain numerous invaginations of the plasmalemma bordering the chloroplasts and evaginations of the outer membrane of the opposing chloroplast envelope. In places, these membranes appear continuous with each other. The minor veins consist of tracheary elements, xylem parenchyma cells, sieve-tube members, companion and phloem parenchyma cells, and other cells simply designated vascular parenchyma cells. The companion and phloem parenchyma cells are typically larger than the sieve-tube members with the companion cells containing a much denser cytoplasm that the phloem parenchyma. Cytoplasmic connections occur along all possible routes from the mesophyll to the sieve-tube members and consist of either simple or branched plasmodesmata between parenchymatic elements or pore-plasmodesmata between the sieve-tube members and parenchyma cells. The highest frequency of plasmodesmata occurs between the sieve-tube members and companion cells, although the value is essentially the same as between the various parenchymatic elements of the phloem. Compared to several previously studied species, the frequency of plasmodesmata between cell types of the spinach leaf is low. These results are discussed in relation to apoplastic vs. symplastic solute transport and sieve-tube loading in this species.