Xylem, Phloem and Transpiration Flows in a Grape: Application of a Technique for Measuring the Volume of Attached Fruits to High Resolution Using Archimedes' Principle

The minute changes in volume of a grape berry which occur fromhour to hour were measured non-destructively in the field usingreadily available and cheap laboratory equipment and a modernelectronic balance. The method, applicable even to small (approximately10 g) fruits, is based on Archimedes' principle and gave a resolutionof about 1 part in 1 000 by measuring the buoyant upthrust experiencedby a berry when immersed in water. Volume data from control,pedicel-steamed, and detached berries were used to calculatethe magnitudes and directions of the fluid flows which tookplace through the stalk of the phloem and xylem streams andthrough the skin in the transpiration stream. In the latter stages of fruit development, after the onset ofripening, net volume growth more or less ceases in grapes althoughtheir rate of sugar import is at its strongest. Cessation ofvolume growth comes about because the strong inflow of sugarywater in the phloem is closely balanced in part by transpirationalwater loss through the skin and in part by the backflow of xylemwater to the parent vine. This xylem backflow appears to persistthroughout the diurnal cycle. The net backflow direction of the xylem stream, together withthe inability of the phloem stream to carry certain ions (notablycalcium), may explain how some mineral imbalance disorders arisein the later stages of fruit development. The intense manner in which fruiting sinks compete with vegetativesinks in Vitis finds its explanation in the breakdown of apoplast:symplast compartmentation in the berry which occurs around thetime of onset of ripening. The breakdown exposes the terminalsieve tubes of the berry to a highly negative water potentialenvironment, serving to increase both the speed and the concentrationof the translocation stream. Key words: Archimedes' principle, volume measurement, mineral nutrition, xylem, phloem, assimilate partitioning, fruit splitting     

Response of Developing Apple Fruits to Ethylene Treatment

Knee, M., Hatfield, S. G. S. and Bramlage, W. B. 1987. Responseof developing fruits to ethylene treatment.—J. exp. Bot.38: 972–979. Fruits of apple (Malus domestica Borkh. cv. Cox's Orange Pippin)were treated with various concentrations of ethylene usuallyfor 48 h to determine their response in relation to stage ofdevelopment. The main response recorded was the reduction byethylene of the delay in onset of rapid ethylene production(DEP) in individual fruits. Early in development low concentrationsof ethylene had little effect but DEP was progressively reducedby concentrations up to 10⁷ mm³ m^–3. As the fruit approachedthe natural onset of rapid ethylene synthesis concentrationsof 10² and 10³ mm³ m^–3 became increasingly effective.Increasing the duration of treatments with a fixed concentrationreduced DEP proportionately. Delay after harvest in applyinga 48 h treatment had little effect on the relation between DEPand concentration of ethylene applied. Although resistance todiffusion of gas in fruits increased during fruit developmentthis resistance was never large enough to affect the relationof concentration and response. Key words: ethylene, fruit ripening, Malus domestica     

The Supply of Water and Solutes by Phloem and Xylem to Growing Fruits of Yucca flaccida Haw

Analyses of successively collected fractions of phloem exudate of Yucca flaccida, and of Yucca fruits picked at various stages of growth, together with experiments on transpiration from fruits, have led to the following conclusions:

• 1 During fruit growth potassium, sodium, magnesium, phosphorus compounds, and nitrogenous substances are delivered to the fruit by both the xylem and the phloem. These solutes move also easily in radial direction between the xylem and phloem part of the vascular bundles. Actually they can be regarded as constituents of one stream of nutrients.
• 2 The overall efficiency of conversion of vascular-fluid dry matter into mature-fruit dry matter is approximately 61 %.
• 3 During its whole period of growth the fruit transpires an amount of water vapour of at least 6 times its own mature fresh weight.
• 4 Estimates could be made for the relative contributions of xylem and phloem in the delivery of fruit constituents. 18% of the water imported by the fruit during its growth had a phloem, 82 % a xylem origin; 89% is transpired, 11 % retained as a fruit constituent. At least 94 % of the dry matter, 69% of the potassium, 56% of the magnesium, 26% of the phosphorus, and 7% of the calcium of the average fruit have been delivered by the phloem. The translocation of nitrogenous substances occurs probably partly in a more indirect way with temporary storage in inflorescence parenchyma.

     

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.     

苹果果实韧皮部及其周围薄壁细胞的超微结构观察和功能分析

利用透射电镜技术,对发育过程中的苹果（Ｍａｌｕｓ ｄｏｍｅｓｔｉｃａ Ｂｏｒｋｈ）果实韧皮部及其周围薄壁细胞的超微结构进行了观察研究。结果表明,在主脉和细脉的筛分子（ＳＥ）和伴胞（ＣＣ）之间存在胞间连丝,胞间连丝在筛分子一侧是单通道,在伴胞一侧呈多分枝通道。在细脉中筛分子小,伴胞大,在主脉中则是筛分子大,伴胞小。伴胞内胞质和核质稠密,富含线粒体、内质网和高尔基体,液泡内往往呈现多膜包被的囊泡结构,     

Arsenic Speciation in Phloem and Xylem Exudates of Castor Bean

How arsenic (As) is transported in phloem remains unknown. To help answer this question, we quantified the chemical species of As in phloem and xylem exudates of castor bean (Ricinus communis) exposed to arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid [MMA(V)], or dimethylarsinic acid. In the As(V)- and As(III)-exposed plants, As(V) was the main species in xylem exudate (55%–83%) whereas As(III) predominated in phloem exudate (70%–94%). The ratio of As concentrations in phloem to xylem exudate varied from 0.7 to 3.9. Analyses of phloem exudate using high-resolution inductively coupled plasma-mass spectrometry and accurate mass electrospray mass spectrometry coupled to high-performance liquid chromatography identified high concentrations of reduced and oxidized glutathione and some oxidized phytochelatin, but no As(III)-thiol complexes. It is thought that As(III)-thiol complexes would not be stable in the alkaline conditions of phloem sap. Small concentrations of oxidized glutathione and oxidized phytochelatin were found in xylem exudate, where there was also no evidence of As(III)-thiol complexes. MMA(V) was partially reduced to MMA(III) in roots, but only MMA(V) was found in xylem and phloem exudate. Despite the smallest uptake among the four As species supplied to plants, dimethylarsinic acid was most efficiently transported in both xylem and phloem, and its phloem concentration was 3.2 times that in xylem. Our results show that free inorganic As, mainly As(III), was transported in the phloem of castor bean exposed to either As(V) or As(III), and that methylated As species were more mobile than inorganic As in the phloem.Arsenic (As) is an environmental and food chain contaminant that has attracted much attention in recent years. Soil contamination with As may lead to phytotoxicity and reduced crop yield (Panaullah et al., 2009). Food crops are also an important source of inorganic As, a class-one carcinogen, in human dietary intake, and there is a need to decrease the exposure to this toxin (European Food Safety Authority, 2009). Paddy rice (Oryza sativa) is particularly efficient in As accumulation, which poses a potential risk to the population based on a rice diet (Meharg et al., 2009; Zhao et al., 2010a). Other terrestrial food crops generally do not accumulate as much As as paddy rice; however, where soils are contaminated, relatively high concentrations of As in wheat (Triticum aestivum) grain have been reported (Williams et al., 2007; Zhao et al., 2010b). On the other hand, some fern species in the Pteridaceae family are able to tolerate and hyperaccumulate As in the aboveground part to >1,000 mg kg⁻¹ dry weight (e.g. Ma et al., 2001; Zhao et al., 2002); these plants offer the possibility for remediation of As-contaminated soil or water (Salido et al., 2003; Huang et al., 2004). A better understanding of As uptake and long-distance transport, metabolism, and detoxification is needed for developing strategies for mitigating As contamination, through either decreased As accumulation in food crops or enhanced As accumulation for phytoremediation.The pathways of As uptake by plant roots differ between different As species; arsenate [As(V)] enters plant cells via phosphate transporters, whereas arsenite [As(III)] is taken up via some aquaporins (for review, see Zhao et al., 2009). In rice, a silicic acid efflux protein also mediates As(III) efflux toward stele for xylem loading (Ma et al., 2008). Methylated As species, such as monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)], which may be present in the environment as products of microbial or algal methylation of inorganic As or from past uses of methylated As pesticides, are taken up by rice roots partly through the aquaporin NIP2;1 (for nodulin 26-like intrinsic protein; also named Lsi1; Li et al., 2009). Once inside plant cells, As(V) is reduced to As(III), possibly catalyzed by As(V) reductase(s) such as the plant homologs of the yeast (Saccharomyces cerevisiae) ACR2 (Bleeker et al., 2006; Dhankher et al., 2006; Ellis et al., 2006; Duan et al., 2007). As(III) has a high affinity to thiol (-SH) groups and is detoxified by complexation with thiol-rich phytochelatins (PCs; Pickering et al., 2000; Schmöger et al., 2000; Raab et al., 2005; Bluemlein et al., 2009; Liu et al., 2010). As(III)-PC complexation in roots was found to result in reduced mobility for efflux and for long-distance transport, possibly because the complexes are stored in the vacuoles (Liu et al., 2010). Excess As(III) causes cellular toxicity by binding to the vicinal thiol groups of enzymes, such as the plastidial lipoamide dehydrogenase, which has been shown to be a sensitive target of As toxicity (Chen et al., 2010). The As hyperaccumulating Pteris species differ from nonhyperaccumulating plants because of enhanced As(V) uptake (Wang et al., 2002; Poynton et al., 2004), little As(III)-thiol complexation (Zhao et al., 2003; Raab et al., 2004), and efficient xylem loading of As(III) (Su et al., 2008). Recently, an As(III) efflux transporter, PvACR3, has been found to play an important role in As(III) detoxification by transporting As(III) into vacuoles in Pteris vittata (Indriolo et al., 2010).With the exception of As hyperaccumulators, most plant species have a limited root-to-shoot translocation of As (Zhao et al., 2009). The chemical species of As in xylem exudate have been determined in a number of plant species. As(III) was found to be the predominant species (80%–100%) in the xylem sap of rice, tomato (Solanum lycopersicum), cucumber (Cucumis sativus), and P. vittata even when these plants were fed As(V) (Mihucz et al., 2005; Xu et al., 2007; Ma et al., 2008; Su et al., 2010), suggesting that As(V) is reduced in roots before being loaded into the xylem. In other plant species, such as Brassica juncea (Pickering et al., 2000), wheat, and barley (Hordeum vulgare; Su et al., 2010), As(V) accounted for larger proportions (40%–50%) of the total As in the xylem sap. Studies using HPLC-inductively coupled plasma (ICP)-mass spectrometry (MS) coupled with electrospray (ES)-MS showed no evidence of As(III)-thiol complexation in the xylem sap of sunflower (Helianthus annuus; Raab et al., 2005). When rice plants were exposed to MMA(V) or DMA(V), both As species were found in the xylem sap (Li et al., 2009). Generally, methylated As species are taken up by roots at slower rates than inorganic As, but they are more mobile during the xylem transport from roots to shoots (Marin et al., 1992; Raab et al., 2007; Li et al., 2009).It has been shown that phloem transport contributes substantially to As accumulation in rice grain (Carey et al., 2010). However, little is known about how As is transported in phloem (Zhao et al., 2009). There are no reports on the chemical species of As in phloem exudate. The speciation of As in phloem is important because it dictates how As is loaded in the source tissues and unloaded in the sink tissues, such as grain. Questions with regard to the oxidation state, methylation, and complexation of As in phloem sap remain to be answered. Unlike xylem sap, phloem sap is much more difficult to obtain in sufficient quantities for analysis. In this study, we investigated As speciation in phloem and xylem exudates of castor bean (Ricinus communis), which is widely used as a model plant to investigate phloem transport of solutes (e.g. Hall et al., 1971; Hall and Baker, 1972; Allen and Smith, 1986; Bromilow et al., 1987).     

Xylem and Phloem Derived Polyamines during Flowering in Two Diverse Rose Species

Polyamine contents in xylem (root) and phloem (leaf) exudates in two diverse species of rose, viz. Rosa damascena Mill and Rosa bourboniana Desport, were analyzed before, during, and after flowering in the main flowering season, that is, April–May. Only free putrescine (Put) was detected in the xylem and phloem exudates at these time points, and it was high during the peak flowering period. In phloem, Put content was significantly higher in R. bourboniana than in R. damascena at all three stages; whereas in the xylem exudate it was relatively higher in R. damascena at the peak flowering period. A spray of α-difluoromethylornithine (DFMO), an irreversible inhibitor of the putrescine biosynthetic inhibitor ornithine decarboxylase (ODC), markedly decreased the flowering. This effect was reversed by application of Put alone or in combination with DFMO. The significance of this finding is discussed in light of polyamine translocation during flowering. *IHBT Communication: 0354     

Carbon Dioxide Exchange of Developing Apple (Malus pumila Mill.) Fruits

The carbon dioxide exchange of developing apple fruits was monitoredduring development. The results of measurements on detachedfruits in the laboratory were consistent with those made onattached fruit in the field. Respiration rate at 20 °C inthe dark declined from 120 ng CO₂ g^–1 fr. wt. s_–1on 5 June (4 weeks after full bloom) to less than 3 ng g^–1fr. wt. s^–1 by late September. In the light, net CO₂ evolutionwas much decreased, but on no occasion did photosynthesis exceedrespiration and no net CO₂ uptake was detected. The Q₁₀ fordark respiration over the interval from 15 to 25 °C changedfrom 2.8 in early June to 1.6 in early August     

Xylem and Phloem Transport of Mineral Nutrients from Solanum tuberosum Roots

The xylem and phloem transport of mineral elements from stemnodal roots to the stem and stolon of growing potato (Solanumtuberosum L. cv. ‘Russet Burbank’) plants was investigated.Adventitious roots, originating from below-ground nodes of thestem of potato seedlings, were exposed to solutions of SrCI₂or MnSO₄. Relative elemental concentrations were measured inthe conductive tissues using energy dispersive X-ray analysis.After a 5 h daylight uptake period, Sr (a Ca-transport analogue)levels were elevated in the stem xylem tissue, but Sr did notincrease in the stem phloem, nor was it present in either ofthe conductive tissues of stolons located 1–2 nodes abovethe treated roots. In contrast, elevated levels of Cl, S, andMn were found in stolon xylem and phloem tissue during the sameperiod. The absence of Sr in the stolon after 5 h suggests thatno xylem flow into the stolon occurred during the uptake periodand, furthermore, phloem flow is responsible for the transportof the Cl, S, and Mn into the stolon. Elevated levels of thesemobile nutrients in the xylem of the stolon were attributedto xylem-to-phloem transfer in the stem or leaves, transportto the stolon in the phloem, and phloem-to-xylem transfer inthe stolon. During a 19 h uptake period, some Sr was observedin the phloem tissue of the stem, demonstrating slow exchangeof Sr with sieve elements or proximal phloem parenchyma andcompanion cells. Key words: Calcium, manganese, X-ray analysis     

Bruise and Pressure Injury in Apple Fruits

The degree of injury in a freshly bruised region can be quantitativelymeasured by means of the resistance ratio (low-frequency resistance/high-frequencyresistance), since this ratio decreases progressively and significantlywith increased severity of bruising. The decrease is attributedmainly to mechanical injury of cell membranes. In addition to the prompt decrease in resistance ratio in thebruised regions there is also a delayed decrease, attributedto delayed injury resulting from the toxic after-effects ofliberated sap. Slight but significant decreases in resistanceratio are found external to the bruised region and are attributedto the toxic effects of extruded sap.     

Surface Conductance and Water Balance of Developing Apple (Malus pumila Mill.) Fruits

Studies of the relative magnitude of the various componentsof the water balance of developing apple fruits are described.Water entering the fruit can be used in growth, evaporationor, subsequently, a reverse flow to other tissues. Estimatesof evaporation were obtained from the weight loss by detachedfruit hanging in their natural positions in the orchard. Evaporationrates could be estimated with reasonable precision using themaximum daily vapour pressure deficit x fruit surface conductanceto water vapour. A method is described for the measurement offruit surface conductance in the laboratory or the field. Althoughthe conductance declined markedly during the season from ca.1.5 x 10^–3 m s^–1 in early May to less than 1.0 x10^–4 m s^–1 in September, the rate of water lossper fruit only declined slightly over this period. There weresignificant varietal differences in surface conductance, withBramley apples having the lowest conductances of those studied,and Egremont Russet the highest. Fruit diameter gauges wereused to provide continuous records of diurnal fluctuations insize of attached fruit. These records were used to estimatediurnal growth and shrinkage. The calculated volume shrinkageeach day averaged 31% of net growth in early August and waslargely accounted for by evaporation, with relatively littlewater flowing out of the fruit to other tissues.     

A Microfluidic Pump/Valve Inspired by Xylem Embolism and Transpiration in Plants

In plants, transpiration draws the water upward from the roots to the leaves. However, this flow can be blocked by air bubbles in the xylem conduits, which is called xylem embolism. In this research, we present the design of a biomimetic microfluidic pump/valve based on water transpiration and xylem embolism. This micropump/valve is mainly composed of three parts: the first is a silicon sheet with an array of slit-like micropores to mimic the stomata in a plant leaf; the second is a piece of agarose gel to mimic the mesophyll cells in the sub-cavities of a stoma; the third is a micro-heater which is used to mimic the xylem embolism and its self-repairing. The solution in the microchannels of a microfluidic chip can be driven by the biomimetic “leaf” composed of the silicon sheet and the agarose gel. The halting and flowing of the solution is controlled by the micro-heater. Results have shown that a steady flow rate of 1.12 µl/min can be obtained by using this micropump/valve. The time interval between the turning on/off of the micro-heater and the halt (or flow) of the fluid is only 2∼3 s. This micropump/valve can be used as a “plug and play” fluid-driven unit. It has the potential to be used in many application fields.     

Endogenous Gibberellins in the Xylem Exudate from Apple Trees

In the xylem exudate extracted from the current-year stems of apple (Malus domestica Borkh.), gibberellins A₁₅, A₁₇, A₁₈, A₁₉, A₂₃, A₄₄, and A₅₃ were identified, and 16,17-dihydro-17-hydroxy GA₁₉ was presumed from full-scan mass spectra and Kovats retention indices.     

Turnip mosaic virus Moves Systemically through Both Phloem and Xylem as Membrane-Associated Complexes

Plant viruses move systemically in plants through the phloem. They move as virions or as ribonucleic protein complexes, although it is not clear what these complexes are made of. The approximately 10-kb RNA genome of Turnip mosaic virus (TuMV) encodes a membrane protein, known as 6K₂, that induces endomembrane rearrangements for the formation of viral replication factories. These factories take the form of vesicles that contain viral RNA (vRNA) and viral replication proteins. In this study, we report the presence of 6K₂-tagged vesicles containing vRNA and the vRNA-dependent RNA polymerase in phloem sieve elements and in xylem vessels. Transmission electron microscopy observations showed the presence in the xylem vessels of vRNA-containing vesicles that were associated with viral particles. Stem-girdling experiments, which leave xylem vessels intact but destroy the surrounding tissues, confirmed that TuMV could establish a systemic infection of the plant by going through xylem vessels. Phloem sieve elements and xylem vessels from Potato virus X-infected plants also contained lipid-associated nonencapsidated vRNA, indicating that the presence of membrane-associated ribonucleic protein complexes in the phloem and xylem may not be limited to TuMV. Collectively, these studies indicate that viral replication factories could end up in the phloem and the xylem.Plant viruses use the host preexisting transport routes to propagate infection to the whole plant. After replication in the initially infected cells, viruses move cell to cell through plasmodesmata (PD) and start a new round of replication in the newly infected cells. This cycle is repeated until viruses reach vascular tissues, where they enter into the conducting tubes for systemic movement. Several studies have indicated that plant viruses are passively transported along the source-to-sink flow of photoassimilates and thus are believed to move systemically through the phloem (for review, see Hipper et al., 2013).The conducting tube of the phloem is the sieve element. The mature sieve element is enucleated and relies on the associated companion cells for the maintenance of its physiological function (Fisher et al., 1992). The specialized PD connecting one sieve element with one companion cell is called the pore plasmodesmal unit (PPU). Different from the other PDs, PPUs are always branched on the companion cell side but have only one channel on the sieve element side (Oparka and Turgeon, 1999). It is believed that the loading and uploading of viral material during phloem transport are through PPUs. Even though the size exclusion limit of PPUs (Kempers and Bel, 1997) is larger than that of the other PDs (Wolf et al., 1989; Derrick et al., 1990), PPUs should not allow, in their native state, virions or viral ribonucleoprotein (vRNP) complexes to pass through. It is thus believed that specific interactions between virus and host factors are required to allow the viral entity to go through. For instance, the movement protein of Cucumber mosaic virus (CMV) is targeted to PPUs (Blackman et al., 1998), suggesting that this viral protein modifies the size exclusion limit of PPUs and helps viral entry into sieve elements.Most plant viruses are assumed to move systemically through the phloem as virions. This assumption is based on the observation that Coat Protein (CP) deletions debilitating virus assembly prevent systemic infection (Brault et al., 2003; Zhang et al., 2013; Hipper et al., 2014). Some investigations showed the actual presence of virions in sieve elements. This is the case for the icosahedral Tobacco ringspot virus (Halk and McGuire, 1973), Carrot red leaf virus (Murant and Roberts, 1979), Potato leaf roll virus (Shepardson et al., 1980), and Beet western yellows virus (Hoefert, 1984). In addition, virions also were observed in phloem sap, such as the icosahedral CMV (Requena et al., 2006) and the rigid rod-shaped Cucumber green mottle mosaic virus (Simón-Buela and García-Arenal, 1999). Some viruses also are believed to move as ribonucleic protein (RNP) complexes, since systemic movement was observed in CP mutants where virion assembly was hindered. For instance, Tobacco rattle virus, Potato mop-top virus, Brome mosaic virus, and Tomato bushy stunt virus can still move systemically when the CP gene has been deleted from the viral genome (Swanson et al., 2002; Savenkov et al., 2003; Gopinath and Kao, 2007; Manabayeva et al., 2013). For potyviruses, it is still not clear if long-distance transport involves exclusively viral particles or if vRNP complexes also are implicated (Dolja et al., 1994, 1995; Cronin et al., 1995; Schaad et al., 1997; Kasschau and Carrington, 2001; Rajamaki and Valkonen, 2002). But whether virions or vRNP complexes are involved in viral movement, the full nature of the viral entity being implicated has not been defined.Xylem also is used for systemic infection of viruses, but its importance in viral transport generally has been overlooked. Vessel elements are the building blocks of xylem vessels, which constitute the major part of the water-upward-transporting system in a plant. The side walls of mature vessel elements contain pits, which are areas lacking a secondary cell wall; the end walls of the mature vessel elements are removed, and the openings are called perforation plates (Roberts and McCann, 2000). CP or virions of some plant viruses of all different shapes have been detected in the xylem vessels and/or guttation fluid, suggesting that these viruses may move systemically through xylem vessels. For example, the CP of the icosahedral Tomato bushy stunt virus (Manabayeva et al., 2013) and Rice yellow mottle virus (Opalka et al., 1998), the CP of the rigid rod-shaped Soilborne wheat mosaic virus (Verchot et al., 2001) and Cucumber green mottle mosaic virus (Moreno et al., 2004), and the flexuous rod-shaped Potato virus X (PVX; Betti et al., 2012) were detected in xylem vessels. Colocalization of anti-Rice yellow mottle virus antibodies and a cell wall marker for cellulosic β-(1-4)-d-glucans over vessel pit membranes suggests that the pit membranes might be a pathway for virus migration between vessels (Opalka et al., 1998). Moreover, flexuous rod-shaped virions of Zucchini yellow mosaic virus were found in both xylem vessels of root tissue and the guttation fluid (French and Elder, 1999). Finally, icosahedral Brome mosaic virus (Ding et al., 2001) and rigid rod-shaped Tomato mosaic virus and Pepper mild mottle virus (French et al., 1993) virions were found in guttation fluid. Guttation fluid originates from xylem exudate, indicating that these plant viruses can move through xylem within the infected plant. The above studies, however, mainly relied on electron microscopy and infection assays and may have missed the presence of other viral components that might be involved in transport.Turnip mosaic virus (TuMV) is a positive-strand RNA virus belonging to the family Potyviridae, genus Potyvirus, which contains around 30% of the currently known plant viruses and causes serious diseases in numerous crops (Shukla et al., 1994). Potyviruses are nonenveloped, flexuous rod-shaped particles of 680 to 900 nm in length and 11 to 13 nm in diameter. The genomic approximately 10-kb RNA encodes a polyprotein, which is processed into at least 11 mature proteins. TuMV remodels cellular membranes into viral factories, which are intracellular compartments involved in viral replication and movement. These compartments take the form of vesicles of approximately 100 nm in diameter originating from the endoplasmic reticulum (Grangeon et al., 2012). These vesicles contain viral RNA (vRNA) and viral and host proteins involved in vRNA replication (Beauchemin et al., 2007; Beauchemin and Laliberté, 2007; Dufresne et al., 2008; Huang et al., 2010; Grangeon et al., 2012). The viral membrane 6K₂ protein is involved in the membrane alterations and vesicle production (Beauchemin et al., 2007). The membrane-bound replication complexes can move intracellularly and cell to cell (Grangeon et al., 2013) at a rate of one cell being infected every 3 h (Agbeci et al., 2013). Intercellular trafficking of the replication complex is likely mediated by the PD-localized potyviral proteins Cytoplasmic Inclusion (CI) and P3N-PIPO (for N-terminal Half of P3 fused to the Pretty Interesting Potyviridae ORF; Carrington et al., 1998; Wei et al., 2010; Vijayapalani et al., 2012) as well as CP (Dolja et al., 1994, 1995), Viral Protein genome-linked (VPg; Nicolas et al., 1997; Rajamaki and Valkonen, 1999, 2002), and Helper Component-Proteinase (HC-Pro; Cronin et al., 1995; Kasschau et al., 1997; Rojas et al., 1997; Kasschau and Carrington, 2001), which are involved in both cell-to-cell and vascular movement.It is expected that, ultimately, TuMV reaches the vascular tissues of the plant, but how and under what form it is released into the conducting tubes are not known. To further understand viral spread and systemic movement, we investigated the distribution of 6K₂-tagged TuMV factories in all of the leaf and stem tissues other than the epidermal cells. We found TuMV factories in all tissues. Interestingly, we observed 6K₂-tagged vesicles, containing vRNA and viral replication proteins, in both phloem sieve elements and xylem vessels. We confirmed that TuMV could move systemically through xylem by a so-called stem-girdling assay, which induces cell death of the phloem without affecting xylem integrity. Hence, our study indicates that membrane-associated TuMV replication complexes are involved in the systemic movement of the virus.     

苹果果实细胞质中依赖活体组织的ABA高亲和力特异结合蛋白

将苹果（Malus pumila L.cv.Starkrimon)果肉微粒体和细胞可溶组分在含有^3H-ABA的缓冲介质中分别温育,仅在细胞可溶组分中测到微弱的^3H-ABA结合活性。但是,如何将果肉组织圆片在^3H-ABA缓冲介质中直接温育,经制备亚细胞组分后直接测定,在细胞可溶组分中测到很高的^3H-ABA特异结合活性。果肉圆片用沸水预先热处理使细胞可溶组分中的^3H-ABA结合活性完全丧失,说明ABA结合依赖于组织的活体状态。药理实验证明了ABA结合位点的蛋白质性质,同时证明该蛋白的活性中心具有－SH和丝氨酸基因。ABA结合蛋白对ABA的结合具有可饱和性、可逆性和高亲和力。Scatchard作图证明存在2种ABA结合蛋白,一种具有较高的亲和力,其解离常数（Kd)为2.9mmol/L,另一种亲和力相对较低,其Kd值为71.4nmol/L。用ABA结构相似物进行的竞争实验证明了ABA结合蛋白对配体结合的立体特异性。分析了ABA结合蛋白与ABA结合的时间曲线、pH和温度依赖性。本研究检测到的依赖活体组织的ABA结合蛋白可能是果实发育过程中介导ABA信号的受体。     

Gibberellin-like Substances in the Transpiration Stream of Apple and Pear Trees

Gibberellin-like substances have been detected in sap exudingfrom decapitated apple and pear trees and also in the xylemsap sucked from stems of apple. The quantity of gibberellinin the sap appears sufficient to produce important effects onshoot development, and this result is discussed in relationto rootstock effects of fruit trees.     

Collection of Xylem Sap at Flow Rate Similar to in vivo Transpiration Flux

We have explored a method to collect xylem sap using a Scholanderpressure chamber for potted plants. Intact root system in potswhich fitted the pressure chamber was pressurised at a pneumaticpressure numerically equal to the absolute value of shoot waterpotential. The rate of xylem flow obtained from the stem stumpunder such pressure was found similar to the rate of transpirationbefore detopping. The rate of pressurised flow from detop-pedroots was linearly related to the pressure applied in both well-wateredand soil-dried plants. The osmotic concentration of the xylemsap was negatively related to the rate of volume flow, suggestingthe necessity to collect xylem sap at in vivo flow rate if originalsolute concentration is to be evaluated. The concentration ofABA in the xylem sap, however, did not show such a relationshipwith water flux. Both well-watered and soil-dried plants showedthe concentration of ABA in xylem sap largely stable with arange of volume flow rate, indicating a linear relationshipbetween the rate of ABA delivery through xylem and that of volumeflow. We also compared the concentrations of ABA in xylem sapsequentially collected from pressurised roots with that fromdetached shoots of the same plants. The concentration of ABAin the initial saps from shoots showed to be similar to thatfrom roots. However, a decrease in the concentration of ABAin the xylem sap collected from detached leaf or twig was observedwhen more volume of sap was collected, which might also be dependenton the plant species and the volume of xylem vessels concerned. (Received February 3, 1997; Accepted October 7, 1997)     

Transfer of Cesium from the Xylem to the Phloem in the Stem of Wheat

Steam-girdling experiments with detached wheat shoots showed that cesium was eliminated from the xylem sap and loaded into the phloem during acropetal transport. This transfer is important for the accumulation of cesium (especially also of the radiopollutants ¹³⁴Cs and ¹³⁷Cs) in maturing wheat grains.     

Phloem Unloading in Developing Leaves of Sugar Beet : II. Termination of Phloem Unloading

Phloem unloading in developing leaves of Beta vulgaris L. (`Klein E' multigerm) occurred from successively higher order branches of veins as leaves matured. Phloem unloading was studied in autoradiographs of leaf samples taken at various times during the arrival of a pulse of ¹⁴C-labeled photoassimilate. Extension of mass flow of sieve element contents into leaf vein branches was determined from the high level of radiolabel in veins soon after first arrival of the pulse. Rapid entry, indicative of mass flow through open sieve pores, occurred down to the fourth division of veins in young, importing leaves and to the fifth or terminal branch in importing regions near the zone of transition from sink to source. The rate of unloading decreased with leaf age, as evidenced by the increased time required for the vein-mesophyll demarcation to become obscured. The rate of import per unit leaf area, measured by steady state labeling with ¹⁴CO₂ also decreased as a leaf matured. The decline in import appeared to result from progressive changes that increased resistance to unloading of sieve elements and eventually terminated phloem unloading.     

Apical Dominance is not Due to a Lack of Functional Xylem and Phloem in Inhibited Buds

Mature sieve tubes were located in inhibited cotyledonary budsof soybean plants. They were connected to the pholem systemof the stem and were shown to be functional by observing theirability to transport a phloem-mobile tracor. The associatedxylem system was also shown to be functional by using a decolourizedbasic dye tracer. The inhibited buds in the axils of the primaryleaves also contained sieve tubes and xylem elements which wereconnected to their counterparts in the stom. It is concludedthat in soybeans the inhibition of bud growth due to apicaldominance cannot be caused by an incomplete or non-functionalvascular system.