Concentrations In Phloem Research Articles

A sucrose-specific receptor in Bemisia tabaci and its putative role in phloem feeding

In insects, specialized feeding on the phloem sap (containing mainly the sugar sucrose) has evolved only in some hemipteran lineages. This feeding behavior requires an ability to locate feeding sites buried deeply within the plant tissue. To determine the molecular mechanism involved, we hypothesized that the phloem-feeding whitefly Bemisia tabaci relies on gustatory receptor (GR)-mediated sugar sensing. We first conducted choice assays, which indicated that B.tabaci adults consistently choose diets containing higher sucrose concentrations. Next, we identified four GR genes in the B.tabaci genome. One of them, BtabGR1, displayed significant sucrose specificity when expressed in Xenopus oocytes. Silencing of BtabGR1 significantly interfered with the ability of B.tabaci adults to discriminate between non-phloem and phloem concentrations of sucrose. These findings suggest that in phloem feeders, sugar sensing by sugar receptors might allow tracking an increasing gradient of sucrose concentrations in the leaf, leading eventually to the location of the feeding site.

A New Perspective on the Role of Glutamine Synthetase in Nitrogen Remobilization in Wheat (Triticum aestivum L.).

Glutamine synthetase (GS), a key enzyme in plant nitrogen metabolism, is closely related to nitrogen remobilization. However, how GS isoforms participate in nitrogen remobilization remains unclear. Here, the spatiotemporal expression of the TaGS gene family after anthesis was investigated, and the results showed that TaGS1;1 was mainly encoded by TaGS1;1-6A, while the other isozymes were mainly encoded by TaGS localized on the A and D subgenomes. TaGS1;2-4A/4D had the highest expression level, especially in rachis and peduncle. Furthermore, immunofluorescence showed TaGS1;2 was located in the phloem of rachis and peduncle. GUS (β-glucuronidase) staining confirmed that ProTaGS1;2-4A/4D::GUS activity was mainly present in the vascular system of leaves, roots, and petal of Arabidopsis. Ureides, an important transport form of nitrogen, were mainly synthesized in flag leaves and transported to grains through the phloem of peduncle and rachis during grain filling. TaAAH, which encodes the enzyme that degrades ureides to release NH4+, had a higher expression in rachis and peduncle and was synchronized with the increase in NH4+ concentration in phloem, indicating that NH4+ in phloem is from ureide degradation. Taking the above into account, TaGS1;2, which is highly expressed in the phloem of peduncle and rachis, may participate in N remobilization by assimilating NH4+ released from ureide degradation.

Lignin concentrations in phloem and outer bark are not associated with resistance to mountain pine beetle among high elevation pines.

A key component in understanding plant-insect interactions is the nature of host defenses. Research on defense traits among Pinus species has focused on specialized metabolites and axial resin ducts, but the role of lignin in defense within diverse systems is unclear. We investigated lignin levels in the outer bark and phloem of P. longaeva, P. balfouriana, and P. flexilis; tree species growing at high elevations in the western United States known to differ in susceptibility to mountain pine beetle (Dendroctonus ponderosae; MPB). Pinus longaeva and P. balfouriana are attacked by MPB less frequently than P. flexilis, and MPB brood production in P. longaeva is limited. Because greater lignification of feeding tissues has been shown to provide defense against bark beetles in related genera, such as Picea, we hypothesized that P. longaeva and P. balfouriana would have greater lignin concentrations than P. flexilis. Contrary to expectations, we found that the more MPB-susceptible P. flexilis had greater phloem lignin levels than the less susceptible P. longaeva and P. balfouriana. No differences in outer bark lignin levels among the species were found. We conclude that lignification in Pinus phloem and outer bark is likely not adaptive as a physical defense against MPB.

Why is phloem sap nitrate kept low?

Since the first description of phloem sap composition nearly 60 years ago, it is generally assumed that phloem sap does not contain nitrate and that there is little or no backflow of nitrate from shoots to roots. While it is true that nitrate can occasionally be absent from phloem sap, there is now substantial evidence that phloem can carry nitrate and furthermore, transporters involved in nitrate redistribution to shoot sink organs and roots have been found. This raises the question of why nitrate may or may not be present in phloem sap, why its concentration is generally kept low, and whether plant shoot-root nutrient cycling also involves nitrate. We propose here that phloem sap nitrate is not only an essential component of plant nutritional signaling but also contributes to physical properties of phloem sap and as such, its concentration must be controlled to ensure proper coordination of plant development and nutrient transport. This article is protected by copyright. All rights reserved.

Shading mediates the response of mycorrhizal maize (Zea mays L.) seedlings under varying levels of phosphorus

Arbuscular mycorrhizal fungi (AMF) form a beneficial symbiosis with most plant roots for the provision of phosphorus in exchange for plant carbohydrates, and the carbon-phosphorus relationship between AMF and host plant plays a key role in understanding the AM symbiosis. In agroecosystems, shading significantly affects the crop growth, however, it remains unclear how shading mediates the mycorrhizal response to phosphorus (P) supply. For this, a greenhouse experiment was conducted, and maize seedlings were subjected to shading and non-shading by means of leaves coverage under three P fertilization treatments, i.e., P15: 15 mg kg−1, P40: 40 mg kg−1, and P100: 100 mg kg−1. We used two AMF inocula for maize inoculation, i.e., Rhizophagus intraradices (R. intraradices) and Funneliformis mosseae (F. mosseae). We found that shading significantly decreased the soluble sugar concentration in phloem of mycorrhizal maize but not in the non-mycorrhizal ones. Root biomass and the root-shoot ratio of mycorrhizal maize decreased significantly under the shading treatment at P15, P40, and P100. The gene expression of ZmPht1;6 significantly decreased as the P supply level increased. Generally, shading and AM fungal species had no significant effect on the gene expression of ZmPht1;6. Mycorrhizal P response (MPR) decreased gradually as the P supply level increased. Sugar concentration in the phloem saps of inoculated maize was negatively correlated with the expression of ZmPht1;6 genes. Shading significantly increased the gene expression level of ZmPht1;6 inoculated with R. intraradices but had no effects with F. mosseae inoculation. However, the shoot and root MPR inoculated with F. mosseae were significantly higher than that inoculated with R. intraradices. Our findings demonstrate that shading mediates the carbon allocation of mycorrhizal maize seedlings, and the response is associated with P uptake systems, suggesting that tight linkages between aboveground carbon supply and P resource could play a role in maintaining mycorrhizal functioning in the agroecosystems.

Mutations in Two Aphid-Regulated β-1,3-Glucanase Genes by CRISPR/Cas9 Do Not Increase Barley Resistance to Rhopalosiphum padi L.

Callose deposition is induced in plants by various stress factors such as when plants are attacked by herbivores and pathogens. In the case of aphids, callose plugging of aphid-damaged phloem sieve tubes is expected to reduce aphid access to the phloem sap, while aphid-induced upregulation of callose-degrading β-1,3-glucanase genes in the host plant might counteract this negative effect on aphid performance. We have tested this hypothesis with barley mutants in which one or both of two β-1,3-glucanase genes (1636 and 1639) have been mutated by CRISPR/Cas9 technique in cv. Golden Promise. These two genes were previously found to be upregulated by the cereal pest Rhopalosiphum padi L. in susceptible barley genotypes. Four 1636/1639 double mutant, three 1636 single mutant and two 1639 single mutant lines were tested for aphid resistance along with control lines. All mutant lines had single base insertions, causing frame shifts and premature stop codons. Three of the four double mutant lines showed significantly reduced β-1,3-glucanase activity, and bacterial flagellin-induction resulted in significantly more callose formation in the leaves of double mutant compared to control and single mutant lines. However, we found no effect of these modified plant traits on barley resistance to R. padi. Both genes were confirmed to be upregulated by R. padi in Golden Promise. The gene 1637 is another β-1,3-glucanase gene known to be upregulated by R. padi in barley and was here found to be higher expressed in a double mutant line when compared with a control line. If this can compensate for the general reduction of β-1,3-glucanase activity in the double mutants is difficult to discern since phloem concentrations of these proteins are unknown.

Model-assisted analysis of the peach pedicel-fruit system suggests regulation of sugar uptake and a water-saving strategy.

We develop a model based on the biophysical representation of water and sugar flows between the pedicel, fruit xylem and phloem, and the fruit apoplast and symplast in order to identify diurnal patterns of transport in the pedicel-fruit system of peach. The model predicts that during the night water is mainly imported to the fruit through the xylem, and that fruit phloem-xylem transfer of water allows sugar concentrations in the phloem to be higher in the fruit than in the pedicel. This results in relatively high sugar transport to the fruit apoplast, leading to relatively high sugar uptake by the fruit symplast despite low sugar concentrations in the pedicel. At midday, the model predicts a xylem backflow of water driven by a lower pressure potential in the xylem than in the fruit apoplast. In addition, fruit xylem-to-phloem transfer of water decreases the fruit phloem sugar concentration, resulting in moderate sugar uptake by the fruit symplast, despite the high sugar concentration in the pedicel. Globally, the predicted fruit xylem-phloem water transfers buffer the sugar concentrations in the fruit phloem and apoplast, leading to a diurnally regulated uptake of sugar. A possible fruit xylem-to-apoplast recirculation of water through the fruit phloem reduces water lost by xylem backflow at midday.

Contrasting nickel and zinc hyperaccumulation in subspecies of Dichapetalum gelonioides from Southeast Asia

Hyperaccumulator plants have the unique ability to concentrate specific elements in their shoot in concentrations that can be thousands of times greater than in normal plants. Whereas all known zinc hyperaccumulator plants are facultative hyperaccumulators with only populations on metalliferous soils hyperaccumulating zinc (except for Arabidopsis halleri and Noccaea species that hyperaccumulate zinc irrespective of the substrate), the present study discovered that Dichapetalum gelonioides is the only (zinc) hyperaccumulator known to occur exclusively on ‘normal’ soils, while hyperaccumulating zinc. We recorded remarkable foliar zinc concentrations (10 730 µg g−1, dry weight) in Dichapetalum gelonioides subsp. sumatranum growing on ‘normal’ soils with total soil zinc concentrations of only 20 µg g−1. The discovery of zinc hyperaccumulation in this tropical woody plant, especially the extreme zinc concentrations in phloem and phloem-fed tissues (reaching up to 8465 µg g−1), has possible implications for advancing zinc biofortification in Southeast Asia. Furthermore, we report exceptionally high foliar nickel concentrations in D. subsp. tuberculatum (30 260 µg g−1) and >10 wt% nickel in the ash, which can be exploited for agromining. The unusual nickel and zinc accumulation behaviour suggest that Dichapetalum-species may be an attractive model to study hyperaccumulation and hypertolerance of these elements in tropical hyperaccumulator plants.

Carbon transport revisited: a novel approach for solving quasi-stationary carbon transport in a system with Michaelis-Menten sources and sinks

The Carbon Transport-Resistance Allocation Model (C-TRAM) developed for L-system plant models is based on an analogy between the equations for the osmoticpressure generated phloem-sap flow and Ohm's law. This analogy has proved to be rather confusing because some of the model variables, for example source/sink 'electromotive force', do not have any physiological interpretation. Also, the fact that the transport equations are formulated in terms of phloem-sap flow Jp, while organ growth rates are usually expressed in mass carbohydrate flux Js, does not improve model clarity and consistency. The current paper presents a direct way of solving transport equations based on the Munch hypothesis, consistent with boundary conditions in the form of Michaelis-Menten source/sink fluxes, and accounting for the effects of carbohydrate concentration on phloem resistance. Starting from a system of equations for steady-state coupled phloem/xylem flow (Hall and Minchin, 2013), for a constant water potential in the xylem, an equation for the solute mass flow Js in a conduit element can be obtained in a closed form. The boundary problem in the context of L-system plant models was solved using analytical transformations and computational methods similar to the folding/unfolding algorithm of C-TRAM but not based on the electric circuit analogy. The performance of the transport model was tested using a simple system where exact analytical solutions were available (Hall and Minchin, 2013). To our knowledge, this is the first L-system model that combines a rigorous treatment of the carbohydrate transport in a growing system with Michaelis-Menten source/sink functions. The model will be useful in cases where the actual value of the carbohydrate concentration in phloem is important, e.g., sugar signalling. Currently the model is being extended to take into account phloem/xylem coupling.

Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip.

Vascular plants rely on differences in osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as Münch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem sugar concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems, counterintuitively, to occur by means of passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the non-linear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, the phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of Münch transport, where the phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies.

ABA flow modelling in Ricinus communis exposed to salt stress and variable nutrition.

In a series of experiments with Ricinus communis, abscisic acid (ABA) concentrations in tissues and transport saps, its de novo biosynthesis, long-distance transport, and metabolism (degradation) were affected by nutritional conditions, nitrogen (N) source, and nutrient limitation, or salt stress. In the present study these data were statistically re-evaluated, and new correlations presented that underpin the importance of this universal phytohormone. The biggest differences in ABA concentration were observed in xylem sap. N source had the strongest effect; however, nutrient limitation (particularly phosphorus limitation) and salt also had significant effects. ABA was found in greater concentration in phloem sap compared with xylem sap; however, the effect of treatment on ABA concentration in phloem was lower. In the leaves, ABA concentration was most variable compared with the other tissues. This variation was only affected by the N source. In roots, ABA was significantly decreased by nutrient limitation. Of the compartments in which ABA was quantified, xylem sap ABA concentration was most significantly correlated with leaf stomatal conductance and leaf growth. Additionally, ABA concentration in xylem was significantly correlated to that in phloem, indicating a 6-fold concentration increase from xylem to phloem. The ABA flow model showed that biosynthesis of ABA in roots affected the xylem flow of ABA. Moreover, ABA concentration in xylem affected the degradation of the phytohormone in shoots and also its export from shoots via phloem. The role of phloem transport is discussed since it stimulates ABA metabolism in roots.

Exceptionally High Ni Concentration in Phloem of Roots of Nickel-hyperaccumulating Berkheya zeyheri subsp. rehmannii var. rogersiana

Exceptionally High Ni Concentration in Phloem of Roots of Nickel-hyperaccumulating Berkheya zeyheri subsp. rehmannii var. rogersiana

Evaluating the Community Land Model in a pine stand with shading manipulations and <sup>13</sup>CO<sub>2</sub> labeling

Abstract. Carbon allocation and flow through ecosystems regulates land surface–atmosphere CO2 exchange and thus is a key, albeit uncertain, component of mechanistic models. The Partitioning in Trees and Soil (PiTS) experiment–model project tracked carbon allocation through a young Pinus taeda stand following pulse labeling with 13CO2 and two levels of shading. The field component of this project provided process-oriented data that were used to evaluate terrestrial biosphere model simulations of rapid shifts in carbon allocation and hydrological dynamics under varying environmental conditions. Here we tested the performance of the Community Land Model version 4 (CLM4) in capturing short-term carbon and water dynamics in relation to manipulative shading treatments and the timing and magnitude of carbon fluxes through various compartments of the ecosystem. When calibrated with pretreatment observations, CLM4 was capable of closely simulating stand-level biomass, transpiration, leaf-level photosynthesis, and pre-labeling 13C values. Over the 3-week treatment period, CLM4 generally reproduced the impacts of shading on soil moisture changes, relative change in stem carbon, and soil CO2 efflux rate. Transpiration under moderate shading was also simulated well by the model, but even with optimization we were not able to simulate the high levels of transpiration observed in the heavy shading treatment, suggesting that the Ball–Berry conductance model is inadequate for these conditions. The calibrated version of CLM4 gave reasonable estimates of label concentration in phloem and in soil surface CO2 after 3 weeks of shade treatment, but it lacks the mechanisms needed to track the labeling pulse through plant tissues on shorter timescales. We developed a conceptual model for photosynthate transport based on the experimental observations, and we discussed conditions under which the hypothesized mechanisms could have an important influence on model behavior in larger-scale applications. Implications for future experimental studies are described, some of which are already being implemented in follow-on studies.

Rapid Induction of Multiple Terpenoid Groups by Ponderosa Pine in Response to Bark Beetle-Associated Fungi.

Ponderosa pine (Pinus ponderosa) is a major and widely distributed component of conifer biomes in western North America and provides substantial ecological and economic benefits. This tree is exposed to several tree-killing bark beetle-microbial complexes, including the mountain pine beetle (Dendroctonus ponderosae) and the phytopathogenic fungus Grosmannia clavigera that it vectors, which are among the most important. Induced responses play a crucial role in conifer defenses, yet these have not been reported in ponderosa pine. We compared concentrations of terpenes and a phenylpropanoid, two phytochemical classes with strong effects against bark beetles and their symbionts, in constitutive phloem tissue and in tissue following mechanical wounding or simulated D. ponderosae attack (mechanical wounding plus inoculation with G. clavigera). We also tested whether potential induced responses were localized or systemic. Ponderosa pines showed pronounced induced defenses to inoculation, increasing their total phloem concentrations of monoterpenes 22.3-fold, sesquiterpenes 56.7-fold, and diterpenes 34.8-fold within 17 days. In contrast, responses to mechanical wounding alone were only 5.2, 11.3, and 7.7-fold, respectively. Likewise, the phenylpropanoid estragole (4-allyanisole) rose to 19.1-fold constitutive levels after simulated attack but only 4.4-fold after mechanical wounding. Overall, we found no evidence of systemic induction after 17 days, which spans most of this herbivore's narrow peak attack period, as significant quantitative and compositional changes within and between terpenoid groups were localized to the wound site. Implications to the less frequent exploitation of ponderosa than lodgepole pine by D. ponderosae, and potential advantages of rapid localized over long-term systemic responses in this system, are discussed.

Separating water-potential induced swelling and shrinking from measured radial stem variations reveals a cambial growth and osmotic concentration signal.

The quantification of cambial growth over short time periods has been hampered by problems to discern between growth and the swelling and shrinking of a tree stem. This paper presents a model, which separates cambial growth and reversible water-potential induced diurnal changes from simultaneously measured whole stem and xylem radial variations, from field-measured Scots pine trees in Finland. The modelled growth, which includes osmotic concentration changes, was compared with (direct) dendrometer measurements and microcore samples. In addition, the relationship of modelled growth and dendrometer measurements to environmental factors was analysed. The results showed that the water-potential induced changes of tree radius were successfully separated from stem growth. Daily growth predicted by the model exhibited a high correlation with the modelled daily changes of osmotic concentration in phloem, and a temperature dependency in early summer. Late-summer growth saw higher dependency on water availability and temperature. Evaluation of the model against dendrometer measurements showed that the latter masked a true environmental signal in stem growth due to water-potential induced changes. The model provides better understanding of radial growth physiology and offers potential to examine growth dynamics and changes due to osmotic concentration, and how the environment affects growth.

Lethal yellowing disease of the coconut palms (cocos nucifera l.): An overview of the crises

Phytoplasmas are minute cell, wall-less prokaryotes with a diameter less than 1 micrometer ranging from 200 to 800 µm. They have cytoplasma, ribosome and strands of nucleus materials. They arose from gram-positive Clostridium-like bacterial ancestor of the lactobacillus lineage, which appears to have suffered extreme genome reductions when compared with their gram-positive relatives. The inability to culture phytoplasmas in any axenic media and the low concentration in phloem of infected plants are obstacles for efficient diagnosis. Polymerase chain reaction (PCR) amplification is the most important tool for detecting phytoplasmas in plant and insect hosts. Real-time PCR method has so many advantages over the conventional PCR. Recent reports across coconut growing regions tell increase in the incidence of lethal yellowing disease (LYD). Palms with greater than 25% leaf discoloration due to LYD should be removed. Fields replanted with the tolerant varieties must be monitored using molecular technique. The overview crises of more than 100 years after the incidence of phytoplasmas and more than 50 years of research efforts to control LYD of coconut palms world wide are presented. Key word: Polymerase chain reaction and lethal yellowing disease.

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.

Improvement of the enhanced phloem exudation technique to estimate phloem concentration and turgor pressure in tomato

A critical appraisal of the EDTA-enhanced exudation technique was performed and improvements for this technique were proposed. Two buffer solutions were used (HEPES and EDTA) at a concentration of 20 mM. HEPES is a buffering agent widely used in cell culture. Exudation from cut-ends of petioles and peduncles of tomato was allowed to occur over a period of 7 h. Variations of the buffer solution volume, carbohydrates and potassium concentrations, as well as osmotic pressure, were measured in the exudates. The use of HEPES buffer solution alone is proof of the large uptake of the buffer solution by the plant through the xylem. The effect of this uptake on the classical estimation of the carbohydrate flow and concentration of phloem is presented. An improvement of the technique is proposed in order to correctly estimate: (i) the phloem flow; (ii) its composition (carbohydrates and others solutes); and (iii) the turgor pressure of the phloem. The improved EDTA-enhanced exudation technique was applied to investigate the effect of water stress on the driving variables involved in the carbohydrate allocation within the plant. Phloem composition varied with the date and was affected by water stress as well as turgor pressure, which decreased. Furthermore, differences arose in the turgor pressure at the petiole and peduncle sites. Results showed that the exudation technique could be a useful tool to estimate local phloem composition and turgor pressure and to infer resource allocation within the plant.

Quantitative relationships between boron and mannitol concentrations in phloem exudates of Olea europaea leaves under contrasting boron supply conditions

Boron remobilization (BR) occurs in plants that form and export phloem–mobile borate–polyol complexes. Previous studies demonstrate that a quantitative relationship exists between polyol translocation and BR. Here we investigate if mannitol translocation is quantitatively related to BR in olive plants thus allowing acclimation to boron limiting conditions. Plants were cultivated under different boron supply ranging from adequate (23 μM) to insufficient or zero supply (0.5 μM or 0 μM). Measurement of boron in the leaf phloem sap exudates (BP) of olive leaves of low or zero boron supply treatments showed that, whereas boron was remobilized, its absolute amounts in the phloem were lower compared to the control. However, BR from source leaves at 0.5 μM or 0 μM was maintained at relatively high levels in regard to the amounts of boron available in the cells of these leaves, indicating a strategy of the source leaves to remobilize boron by depleting their cell sap boron pool. Concurrently, in the above treatments, leaf phloem mannitol (MP) was up to two-fold higher, resulting in a up to five-fold higher ratio of mannitol to boron in the phloem (MP/BP), compared to the control. Furthermore, both MP and MP/BP were negatively correlated with cell sap boron concentration of the leaves indicating the trend for BR under boron limitation. It is concluded that, in this plant species, mannitol concentrations in the leaf phloem may be involved in the promotion of BR under inadequate external supply of boron.

Evidence of phloem boron transport in response to interrupted boron supply in white lupin (Lupinus albusL. cv. Kiev Mutant) at the reproductive stage

The present study investigates whether previously acquired boron (B) in mature leaves in white lupin can be retranslocated into the rapidly growing young reproductive organs, in response to short-term (3 d) interrupted B supply. In a preliminary experiment with white lupin in soil culture, B concentrations in phloem exudates remained at 300-500 microM, which were substantially higher than those in the xylem sap (10-30 microM). The high ratios of B concentrations in phloem exudates to those in the xylem sap were close to values published for potassium in lupin plants. To differentiate 'old' B in the shoot from 'new' B in the root, an experiment was carried out in which the plants were first supplied with 20 microM (11)B (99.34% by weight) in nutrient solution for 48 d after germination (DAG) until early flowering and then transferred into either 0.2 microM or 20 microM (10)B (99.47% by weight) for 3 d. Regardless of the (10)B treatments, significant levels of (11)B were found in the phloem exudates (200-300 microM in 20 microM (10)B and 430 microM in 0.2 microM (10)B treatment) and xylem sap over the three days even without (11)B supply to the root. In response to the 0.2 microM (10)B treatment, the translocation of previously acquired (11)B in the young (the uppermost three leaves), matured, and old leaves was enhanced, coinciding with the rise of (11)B in the xylem sap (to >15 microM) and phloem exudates (430 microM). The evidence supports the hypothesis that previously acquired B in the shoot was recirculated to the root via the phloem, transferred into the xylem in the root, and transported in the xylem to the shoot. In addition, some previously acquired (11)B in the leaves may have been translocated into the rapidly growing inflorescence. Phloem B transport resulted in the continued net increment of (11)B in the flowers over 3 d without (11)B supply. However, it is still uncertain whether the amount of B available for recirculation is adequate to support reproductive growth until seed maturation.