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.     

Distribution of leaf assimilates in the stem of Picea abies L.

Summary Autoradiographic and microautoradiographic studies of 2-year-old Picea abies plants show that in summer leaf assimilates from the second-year shoot are translocated basipetally. Leaf assimilates are first transported to the stem via leaf trace phloem, then to the base of the stem in the sieve cells of the latest increment of secondary phloem. On the way down leaf assimilates move radially from sieve cells into cells of the phloem parenchyma, the vascular cambium, the rays, the inner periderm and certain cells of pith and cortex, including the epithelial cells surrounding the resin ducts. Other cells of pith and cortex remain nearly free of label, despite the long translocation time (20 h). With the exception of the vascular cambial cells, the stem cells that gain leaf assimilates by radial distribution coincide with those that contain chlorophyll and starch.     

RADIAL GROWTH IN THE CYCADALES

The development of radial growth which leads to the pachycaulous form was investigated in eight of the 10 genera of the Cycadales; i.e., Ceratozamia, Cycas, Dioon, Encephalartos, Macrozamia, Microcycas, Stangeria, and Zamia. In all taxa, development of radial growth is essentially the same: a primary thickening meristem is differentiated in the stelar region of the cotyledonary node of the seedling at germination and produces derivatives mainly centrifugally. This primary thickening meristem (PTM) then differentiates acropetally and becomes continuous with the peripheral zone of the shoot apex. At first the PTM is a vertical cylinder, but as the seedling continues to grow into an adult plant, the PTM shows a more horizontal orientation (like an open umbrella) and produces the broad cortex. Secondary growth is by a vascular cambium which produces secondary xylem to the inside and secondary phloem to the outside. The broad pith originates from derivatives of the rib meristem of the massive shoot apex. The seedling and young plant is composed of a shortened shoot (i.e., no internodes) produced by the PTM and rib meristem, and a large fleshy primary root which results from a diffuse growth pattern. Individual cells in both the pith and cortex of the root divide. Their derivatives divide at right angles to the original division plane. Thus, quartets and even octets of cells are recognizable and can be traced to individual parent cells.     

Evidence supporting a non-phloem source of water for export of solutes in the xylem of soybean root nodules

The vascular anatomy of soybean nodules [Glycine max (L.) Merr.] suggests that export of solutes in the xylem should be dependent on influx of water in the phloem. However, after severing of stem xylem and phloem by shoot decapitation, export of ureides from nodules continued at an approximately linear rate for 5h. This result was obtained with decapitated roots remaining in the sand medium, but when roots were disturbed by removal from the rooting medium prior to shoot decapitation, export of ureides from nodules was greatly reduced. Stem exudate could not be collected from disturbed roots, indicating that flow in the root xylem had ceased. Thus, ureide export from nodules appeared to be dependent on a continuation of flow in the root xylem. When seedlings were fed a mixture of ³H₂O and ¹⁴C-inulin for periods of 14–21 min, nodules had higher ³H/¹⁴C ratios than roots from which they were detached. The combined results are not consistent with the proposal that export of nitrogenous compounds from nodules is dependent on import of water via the phloem. The results do support the view that a portion of the water required for xylem export from soybean nodules is supplied via a symplastic route from root cortex to nodule cortex to the nodule vascular apoplast.     

Sugar concentrations along and across the Ricinus communis L. hypocotyl measured by single cell sampling analysis

Single cell sap sampling and analysis were used to measure the longitudinal and radial distribution of sucrose, glucose and fructose in the apical cell division zone and in the basal, elongated zone of the Ricinus hypocotyl. Sucrose and hexose increased in concentration from the apex to the base of the seedling axis. In the cell division zone low hexose and sucrose concentrations prevailed in cortex and pith, with a slightly higher hexose concentration in pith cells. The sucrose concentrations in sieve tubes and in phloem were much higher than in the cortex and pith cells. In the basal zone of the hypocotyl high levels of sucrose in phloem, cortex and pith were found, therefore radial, diffusional sucrose flow away from the phloem was considered unlikely. It is proposed that radial flow of growth-water to the hypocotyl periphery together with the down-regulation of a sucrose transporter at the phloem leads to a preferential sucrose flow to the expanding cortex. The pith cells, which do not experience flow of growth-water, are probably insufficiently supplied with sucrose from the phloem resulting eventually in cell death as the plant grows. Shortage of sucrose supply, experimentally achieved by removal of the endosperm, led to sucrose hydrolysis in the pith. The sucrose levels in the other tissues decreased less. It appears that the hydrolysis to hexose was initiated to maintain the osmotic value in the pith cell sap. It is speculated that high hexose levels in the cells are indicative of insufficient sucrose supply via the phloem and that the pith cells are confronted with that situation during early seedling development.     

Das Phloem der Haustorien vonCuscuta

Summary Haustoria ofCuscuta odorata R. & P. andC. grandiflora H.B.K. show continuous traces of sieve elements, connecting the phloem of the host with that of theCuscuta shoot. The continuity of this haustorial phloem is discernible by callose fluorescence after staining with aniline blue. The fine structural criteria for sieve tubes are analyzed electronmicroscopically, with special respect to sieve pores, P-protein, and a distinct wall-standing smooth surfaced ER. Within the central part of the haustorium sieve tubes are elongated, while the elements abutting the phloem of theCuscuta shoot are nearly isodiametric in shape. Both elements are associated with rather large companion cells, derived from an unequal division.

     

THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. III. THE INTERNODE

Thomson, Betty F., and Pauline Monz Miller. (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum. III. The internode. Amer. Jour. Bot. 50(3): 219–227. Illus. 1963.—Seedlings of Pisum sativum were grown under constant conditions and exposed daily to red or white fluorescent light or kept in total darkness. Counts and measurements of internodal cells in both transverse and longitudinal directions show that light does not alter the sequence or pattern of tissue differentiation, including the sequence of xylem maturation within the vascular bundle. Light does accelerate the rate of a constant course of differentiation. Light advances the time of division and enlargement of cortex, xylem, phloem, and pith cells in the longitudinal direction but reduces both the final number and the final length attained in all cases. It is concluded that light accelerates all phases of shoot growth and differentiation and that cell division and elongation in the later phases of internodal growth are reduced by light because of accelerated cell maturation.     

白鲜根的发育解剖学研究

应用半薄切片、常规石蜡切片并结合离析法,对药用植物白鲜(Dictamnus dasycarpus Turcz.)根的发生发育过程进行了研究。结果表明:白鲜根的发生发育过程包括4个阶段,即原分生组织阶段、初生分生组织阶段、初生结构阶段以及次生结构阶段。原分生组织位于根冠内侧及初生分生组织之间,衍生细胞分化为初生分生组织。初生分生组织由原表皮、基本分生组织以及中柱原组成。原表皮分化为表皮,基本分生组织分化为皮层,中柱原分化为维管柱,共同组成根的初生结构;在初生结构中,部分表皮细胞外壁向外延伸形成根毛,皮层中分布有油细胞,内皮层有凯氏带,初生木质部为二原型或偶见三原型,外始式;根初生结构有髓或无。次生结构来源于原形成层起源的维管形成层的活动以及中柱鞘起源的木栓形成层的活动;白鲜次生韧皮部宽广,其中多年生根中可占根横切面积的85%,另外除基本组成分子外,还分布有油细胞;周皮发达,木栓层厚;初生皮层、次生木质部和次生韧皮部薄壁细胞中常充满丰富的淀粉粒。     

THE PHLOEM OF ETAPTERIS LECLERCQII AND BOTRYOPTERIS TRIDENTATA

The phloem of Etapteris leclercqii and Botryopteris tridentata petioles is described from Lower Pennsylvanian coal balls. Petioles of B. tridentata are characterized in transverse section by an omega-shaped xylem trace, a phloem zone which extends from 2-10 cells in width, and 2-parted cortex. Etapteris leclercqii petioles exhibit a 4–9 cell-wide phloem zone surrounding the central clepsydroid xylem mass, and a 3-parted cortex. In both taxa a 1–2 cell layer parenchyma sheath separates the xylem from the extra-xylary tissues. The phloem of both species consists of sieve elements that average about 20 μm in diam by 200 μm in length in Botryopteris, and 100 μm in length in Etapteris, with horizontal-slightly oblique end walls. In transmitted light, the radial walls of the sieve elements form an irregular reticulate pattern enclosing elliptical lighter areas. With the scanning electron microscope, these areas appear as horizontal-slightly oblique furrows on the cell wall, with many small indentations lining the furrows. These indentations, because of their regular occurrence and size (from a few fractions of a micron up to 1.0 μm in diam), are interpreted as sieve pores, and the elliptical areas that enclose them as sieve areas. The phloem of E. leclercqii and B. tridentata is compared with that described for other fossil genera and with that of extant ferns.     

紫菀根的结构与主要药用成分积累研究

为揭示紫菀根的结构、主要药用成分积累部位和含量,用石蜡切片法研究不同发育阶段根的结构、组织化学法定位三萜皂苷和黄酮类成分的积累部位、HPLC法测定根中紫菀酮、槲皮素和山奈酚的含量。结果表明,紫菀根的初生结构包括表皮、皮层和维管柱。次生结构包括外皮层、皮层和维管组织,其中分泌道位于皮层内侧,数量与韧皮部束一致,随着根的增粗,中央分化出髓部。三萜皂苷成分在韧皮部和皮层内侧积累较多;黄酮类成分积累于皮层和髓部。紫菀根下部的紫菀酮含量高于上部,槲皮素和山萘酚仅为上部的1/3。因此,建议加工时保留下部细根,实现资源综合利用。     

SUCCESSIVE CAMBIA IN THE STEM OF PHYTOLACCA DIOICA

Phytolacca dioica L., an evergreen tree of the Phytolaccaceae, is one of the species of Phytolacca which shows anomalous secondary thickening in its stem. This mode of thickening has been regarded as successive cambial activity or alternatively, in some more recent interpretations, as thickening by unidirectional activity of a cambial zone. The stem thickening of P. dioica is of the former type. The cambium produces fascicular strands, showing centrifugal differentiation of xylem and centripetal differentiation of phloem on opposite sides of the cambial layer, and rays are produced between the fascicular areas. In both xylem and phloem the younger elements are closer to the cambium than the older elements. Succeeding cambia arise periodically by periclinal divisions in a layer of parenchyma cells two or three cells beyond the outermost intact phloem derived from the current cambium. Each cambium forms a few parenchyma cells on both sides before it forms derivatives which mature into lignified xylem elements or conductive elements of the phloem. The parenchyma thus formed toward the outside later becomes the site of the origin of the succeeding cambium. Only one or two layers of this phloem parenchyma go on to form the new cambium; the remaining cells accumulate between the outermost phloem and the cortex. P. weberbaueri shows stem structure similar to P. dioica. P. meziana, a shrub, shows normal stem structure.     

Primary Phloem Development in the Shoot Apex of Rhizophora mangle L. (Rhizophoraceae)

The primary phloem in the shoot apex of the mangrove Rhizophora mangle L. is largely confined to the comparatively condensed area between the first three leaf pairs. The main extension zone, surrounded by the stipular sheath of the third leaf pair, contains vascular bundles arranged in a procambial ring and characterized by a well-developed primary phloem and a less advanced xylem. The phloem consists of a great number of sieve elements, an equal number of associated companion cells, and a few phloem-parenchyma cells. The differentiation of the sieve-element protoplast (with e.g., chromatolytic nuclear degeneration, loss of the vacuole and most organelles) proceeds largely according to a well-known pattern. Their P-type plastids, however, form their protein crystals rather late and therefore cannot be used as an early cell marker. Lateral sieve-element walls are distinct from other wall parts and walls of other cells by their heavy nacreous thickenings, the formation of which is shown to be strictly correlated with the occurrence and orderly arrangement of cortical microtubules.     

The histology of lesion development in raspberry canes infected by Elsinoe veneta

A histological survey was made of the canes of several raspberry genotypes inoculated in the field and glasshouse with Elsinoe veneta. Acervuli bearing conidia developed on all genotypes and hyphae penetrated the cortex, polyderm and phloem inducing extensive cell divisions, utilisation of starch reserves and deposition of suberin and lignin-like materials in these tissues. The results indicated that the polyderm was invaded via the passage cells at an early stage in its development. Phloem fibre cells surrounded by lignified middle lamellae were rarely infected, but since the fibre bundles did not form a complete sheath around the vascular tissues, hyphae spread between the bundles to invade the phloem in all genotypes. The deposition of suberin and lignin-like materials in host cells in the cortex, phelloid cells of the polyderm and phloem partially restricted the spread of mycelium.     

Laticifers in Camptotheca acuminata Decne: distribution and structure

Summary. In this paper, a system of laticifers in Camptotheca acuminata Decne (Nyssaceae) is described. Laticifers were already present in the leaf primordia of the shoot apex. In the mature leaves, laticifers were found in the midrib and in the larger veins, both in the parenchymatic region delimited by vascular bundles and in the cortex just external to the phloem. In the stem, laticifers were present in both the primary and secondary body, running parallel to the longitudinal axis. They were located in the pith and in the cortex proximal to the phloem. No laticifers were found in the roots. The histochemical analyses indicated that the main compounds accumulated in laticifers were phenols. Neutral lipids and fatty acids were also present. Ultrastructural observations showed osmiophilic globules both in the vacuoles and in the peripheral regions of the cytoplasm of the laticifer cells. Plastids were present, although altered, with some parallel membranes and lacking starch grains. The discovery in C. acuminata of a laticifer system, which had never been described for the order Cornales, could be of taxonomic value, also considering that this order has traditionally represented one of the most problematic groups of flowering plants. Correspondence and reprints: Dipartimento di Biologia Vegetale, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy.     

Transport Exchange of Carbon,Nitrogen and Water in the Context of Whole Plant Growth and Functioning — Case History of a Nodulated Annual Legume

The economy of carbon, nitrogen and water during growth of nodulated, nitrogen-fixing plants of white lupin (Lupinus albus L.) was studied by measuring C, N and H₂O content of plant parts, concentrations of C and N in bleeding sap of xylem and phloem, transpirational losses of whole shoots and shoot parts, and daily exchanges of CO₂ between shoot and root parts and the surrounding atmosphere. Relationships were studied between water use and dry matter accumulation of shoot and fruits, and between net photosynthesis rate and leaf area, transpiration rate and nitrogen fixation. Conversion efficiencies were computed for utilization of net photosynthate for nitrogen fixation and for production of dry matter and protein in seeds. Partitioning of the plant's intake of C, N and H₂O was described in terms of growth, transpiration, and respiration of plant parts. An empirically-based model was developed to describe transport exchanges in xylem and phloem for a 10-day interval of growth. The model depicted quantitatively the mixtures of xylem and phloem streams which matched precisely the recorded amounts of C, N and H₂O assimilated, absorbed or consumed by the various parts of the plant. The model provided information on phloem translocation of carbon and nitrogen to roots from shoots, the cycling of carbon and nitrogen through leaves, the relationship between transpiration and nitrogen partitioning to shoot organs through the xylem, the relative amount of the plant's water budget committed to phloem translocation, and the significance of xylem to phloem transfer of nitrogen in stems as a means of supplying nitrogen to apical regions of the shoot.     

云杉矮槲寄生内寄生系统的解剖学研究

采用石蜡切片和显微观察的方法,以被云杉矮槲寄生侵染的青海云杉枝条为材料,从解剖学水平观察云杉矮槲寄生的内寄生系统。结果表明:云杉矮槲寄生的内寄生系统由分布在寄主皮层内的皮层根和镶嵌在寄主木质部中的吸根组成。皮层根可以沿着寄主枝条,通过挤压寄主皮层细胞的方式扩展,并穿透寄主表皮层形成云杉矮槲寄生的寄生芽;在靠近或接触寄主次生韧皮部时形成吸根,吸根顶端细胞定向穿透寄主韧皮部和形成层,侵染木质部,并沿木射线方向生长。当云杉矮槲寄生侵入寄主以及皮层根向外生长时,寄主表皮层发生加厚现象。随着云杉矮槲寄生侵染部位的寄主皮层细胞数增多,引起枝条膨大。研究认为,云杉矮槲寄生可以进行系统侵染,反映了云杉矮槲寄生与其寄主在组织水平上的相互作用。     

Apple phloem cells contain some mRNAs transported over long distances

Apple (Malus × domestica Borkh.), like many fruit trees, cannot be propagated clonally from seed and is instead propagated by the grafting onto rootstocks. Rootstocks affect the growth of scions, but it is not known why. The circulation of some mRNAs throughout the phloem has recently been shown. To clarify whether RNAs are transported long distance through the graft union of apple trees, we analyzed cDNAs derived from shoot phloem cells by laser capture microdissection. We detected several mRNAs that have already been reported as phloem-transported RNAs in other plants. One of them, MpSLR/IAA14, was probed to transport a long distance through the graft union in grafted apple plant. These results suggest that a phloem RNA transport system may be involved in the effects of rootstocks on scion growth and cropping.     

Rapid shoot‐to‐root signalling regulates root hydraulic conductance via aquaporins

We investigated how root hydraulic conductance (normalized to root dry weight, L_o) is regulated by the shoot. Shoot topping (about 30% reduction in leaf area) reduced L_o of grapevine (Vitis vinifera L.), soybean (Glycine max L.) and maize (Zea mays L.) by 50 to 60%. More detailed investigations with soybean and grapevine showed that the reduction in L_o was not correlated with the reduction in leaf area, and shading or cutting single leaves had a similar effect. Percentage reduction in L_o was largest when initial L_o was high in soybean. Inhibition of L_o by weak acid (low pH) was smaller after shoot damage or leaf shading. The half time of reduction in L_o was approximately 5 min after total shoot decapitation. These characteristics indicate involvement of aquaporins. We excluded phloem‐borne signals and auxin‐mediated signals. Xylem‐mediated hydraulic signals are possible since turgor rapidly decreased within root cortex cells after shoot topping. There was a significant reduction in the expression of several aquaporins in the plasma membrane intrinsic protein (PIP) family of both grapevine and soybean. In soybean, there was a five‐ to 10‐fold reduction in GmPIP1;6 expression over 0.5–1 h which was sustained over the period of reduced L_o.