Virus particles in guttate and xylem of infected cucumber (Cucumis sativus L.)

Particles of plant viruses representing ten genera were visualised by electron microscopy in samples of guttate from systemically infected cucumber (Cucumis sativus L.) plants. For the majority of viruses tested, guttation samples tested positively by ELISA and infectivity was also demonstrated. Virus particles of zucchini yellow mosaic potyvirus were also observed in xylem in sections of infected root tissue. Since guttate originates from xylem exudate, this study demonstrates the potential for xylem tissue to act as a conduit for a diverse group of plant viruses in planta     

Biochemical Changes in Root Exudate and Xylem Sap of Tomato Plants Infected with Meloidogyne incognita

Under two monoxenic culture techniques of growing plants (filter paper and silica sand cultures), sugar in root exudate from Meloidogyne incognita-infected tomato increased 133 to 836% over controls. In contrast, amino acids were moderately reduced 52 to 56%. Chromatographic analysis showed that galled root exudate contained three sugars, twelve amino acids, and three organic acids, whereas healthy root exudate contained four sugars, fifteen amino acids, and four organic acids. Polysaccharide was responsible for the large increase of sugars in galled root exudates. The concn and the absolute amount of total sugars in the infected plant xylem sap were greater than in healthy plant xylem sap up to 6 wk after inoculation, whereas amino acids were moderately lower than in controls throughout the test period. Chromatographic analysis showed that xylem sap from both healthy and infected plants at 4 wk after inoculation contained four sugars and five organic acids. We identified 18 and 17 amino acids in the healthy and infected plant xylem sap, respectively. The concn of sugar increased as the nematode inoculum increased at 2, 4 and 6 wk after inoculation. The amino acids in all samples from the infected plant moderately decreased with an increase of nematode inoculum. We suggest that changes in total sugars and amino acids, of infected plant xylem sap and root exudate are a probable mechanism by which tomato plants are predisposed to Fusarium wilt.     

Low-temperature pretreatment of the root system of Brassica rapa L. plants: effects on the xylem sap exudation and on the nitrate absorption rate

Brassica rapa plants were exposed for a 52 h period (as pretreatment) to a differential temperature (DT) between roots (5°C) and shoots (20°C), while control plants were maintained with both shoot and roots at 20°C (warm grown = WG). Measured at 20°C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration flows were similar in pretreated and control plants. Both transpiration and exudation flows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate flow from the pretreated roots was associated with a two-fold increase in nitrate uptake rate. The relationship of the cold-induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.     

Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K⁺ concentrations and K⁺ transport to the shoot was studied in both nutrient-solution and field-grown tomato plants ( Lycopersicon esculentum ).
K⁺ concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient-solution plants, the most rapid decline occurred during the vegetative growth phase, while in field-grown plants, the xylem K⁺ concentrations remained high during an-thesis and then subsequently declined. The rapid decline in nutrient-solution plants might be related to a decrease in the absorptive efficiency of the root system. In field-grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K⁺ concentrations. The volume (ml h⁻¹ plant⁻¹) and the net rates of K⁺ exudation (mmol h⁻¹ plant⁻¹) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K⁺ concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K⁺ contribute to a reduction in leaf free space K⁺ concentration which would explain the decline in tomato leaf K⁺ concentrations.     

The concentration of xylem sap constituents in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.)

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.     

Factors which affect the amount of inorganic phosphate, phosphorylcholine, and phosphorylethanolamine in xylem exudate of tomato plants

Phosphate in the xylem exudate of tomato (Lycopersicon esculentum) plants was 70 to 98% inorganic phosphate (Pi), 2 to 30% P-choline, and less than 1% P-ethanolamine. Upon adding ³²Pi to the nutrient, Pi in xylem exudate had the same specific activity within 4 hours. P-choline and P-ethanolamine reached the same specific activity only after 96 hours. The amount of Pi in xylem exudate was dependent on Pi concentration in the nutrient and decreased from 1700 to 170 micromolar when Pi in the nutrient decreased from 50 to 2 micromolar. The flux of 0.4 nmoles organic phosphate per minute per gram fresh weight root into the xylem exudate was not affected by the Pi concentration in the nutrient solution unless it was below 1 micromolar. During 7 days of Pi starvation, Pi in the xylem exudate decreased from 1400 to 130 micromolar while concentrations of the two phosphate esters remained unchanged.

The concentration of phosphate esters in the xylem exudate was increased by addition of choline or ethanolamine to the nutrient solution, but Pi remained unchanged. Upon adding [¹⁴C]choline to the nutrient, 10 times more [¹⁴C]P-choline than [¹⁴C]choline was in the xylem exudate and 85 to 90% of the ester phosphate was P-choline. When [¹⁴C]ethanolamine was added, [¹⁴C]P-ethanolamine and [¹⁴C]ethanolamine in the xylem sap were equal in amount. P-choline and P-ethanolamine accumulated in leaves of whole plants at the same time and the same proportion as observed for their flux into the xylem exudate. No relationship between the transport of P-choline and Pi in the xylem was established. Rather, the amount of choline in xylem exudate and its incorporation into phosphatidylcholine in the leaf suggest that the root is a site of synthesis of P-choline and P-ethanolamine for phospholipid synthesis in tomato leaves.

     

青枯病抗性不同的番茄品种根际拮抗菌拮抗能力差异研究

以植物基因型影响根际微生物的多样性为依据,通过平板拮抗试验,从番茄的感病品种和抗病品种、感病品种的严重发病植株和轻微发病植株根际筛选到36株抬抗菌,不同来源的抬抗菌群落具有不同的特征。严重发病的感病品种植株根际的拮抗菌群落多样性低,仅筛选到1个拮抗菌株,而轻微发病的感病品种和未发病的抗病品种植株根际筛选到的拮抗菌株数较多;抗病品种根际桔抗菌表现出强拮抗能力,而感病品种根际拮抗菌的拮抗能力较弱。测定植株根面和根颈处导管内青枯菌的种群数量、3株拮抗菌的防病效果,发现导管内青枯茵数量与防病效果基本一致,探讨了导管内青枯菌数量作为拮抗菌防病效果指标的可能性。     

Xylem exudate yield from detopped maize plants as an estimator of root pruning

Current methods of studying roots are either labour intensive or require expensive equipment. In 1986 and 1987 root pruning treatments were given to maize plants grown indoors. In both years the amount of xylem exudate collected from plant stumps shortly after root pruning increased with the dry weight of roots attached to the plant. The measurement of xylem exudation is presented as an alternative for conventional laborious root-study methods.     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Effect of calcium on transport of cations to the xylem exudate of tobacco

Summary Roots of detopped tobacco plants (Nicotiana tabacum var. Virginia Gold) were exposed to Na, K, and Ca salts or to water, and cation transfer to xylem vessels was measured. In some cases plants had been exposed to Na in addition to regular nutrient solutions before detopping. Calcium in the external medium greatly depressed the transport of Na from the external medium to the xylem vessels and it often stimulated the transfer of K from the external medium to the xylem vessels. The K/Na ratio in the exudate thus was dependent upon the Ca content of the external medium under these conditions. In contrast, externally applied Ca or Ca deficiency had very little effect on the transfer of preaccumulated K and Na from compartments within roots to the xylem vessels. The K/Na ratio in the exudate under these conditions was not related to Ca levels nor to mild Ca deficiency. The ratios decreased with time after detopping regardless of Ca level. Intact plants accumulated more Na than did root systems of detopped plants in a 6-day period.Riverside University of CaliforniaSoil Science and Agricultural Engineering     

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.     

The effect of local application of fertilizer on the content of cytokinins in the xylem sap of maize

Cytokinin content in xylem sap was higher in plants grown under local supply of fertilizers as compared to those grown under homogenous distribution of nutrients in soil. The separate assay of cytokinins in xylem exudate from split root system showed that roots, which were in contact with fertilizer mainly contributed to cytokinins transported from roots to shoots. This revised version was published online in July 2006 with corrections to the Cover Date.     

Xylem sap proteins

Xylem sap from apple (Malus domestica Borkh), peach (Prunus persica Batsch), and pear (Pyrus communis L.) twigs was collected by means of pressure extrusion. This sap contained a number of acidic peroxidases and other proteins. Two other sources of xylem sap used in this study were stem exudates and guttation fluid. Similar peroxidases were also found in stem exudates and guttation fluids of strawberry (Fragaria x ananassa Duch.), tomato (Lycopersicum esculentum L.), and cucumber (Cucumis sativus L.). Isoelectric focusing activity gels showed that two peroxidases (isoelectric point [pl] 9 and pl 4.6) were present in initial stem exudates collected in the first 30 minutes after excision. Subsequent samples of stem exudate collected contained only the pl 4.6 isozyme. The pl 4.6 peroxidase isozyme was also found in root tissue and guttation fluid. These observations suggest that roots produce and secrete the pl 4.6 peroxidase into xylem sap. Cucumber seedlings were treated with 100 microliters per liter ethylene for 16 hours and the exudate from decapitated hypocotyl stumps was collected over a 3 hour period. Ethylene increased the peroxidase activity of stem exudates and inhibited the amount of exudate released. These observations suggest that xylem sap peroxidase may play a role in plugging damaged vascular tissue.     

Transfer of amino acids and nitrate from the roots into the xylem of Ricinus communis seedlings

The loading of amino acids and nitrate into the xylem was investigated by collection and analysis of root-pressure exudate from the cut hypocotyl stumps of seedlings of Ricinus communis L. Glutamine was found to be the dominant amino acid in the exudate and also to be the amino acid which is transferred to the xylem most rapidly and accumulated to the greatest extent. The comparison between uptake and xylem loading showed significant differences in specificity between these two transport reactions, indicating a different set of transport systems. Nitrate is transferred to the xylem at a higher relative rate than any amino acid despite the great nitrate-storage capacity of the root system. Thus the supply of nitrate to Ricinus plants leads to enhanced nitrogen allocation to the shoots.     

Adaptation of potassium translocation into the shoot of maize (Zea mays) to shoot demand: Evidence for xylem loading as a regulating step

In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl₂ solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented.
From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.     

Iron-dependent changes of heavy metals,nicotianamine, and citrate in different plant organs and in the xylem exudate of two tomato genotypes. Nicotianamine as possible copper translocator

The influence of Fe nutrition on the distribution of the heavy metals Fe, Mn, Zn, and Cu and of the heavy metal chelators nicotianamine (NA) and citrate in 6 different shoot and 3 different root parts and in xylem exudate of a NA-containing tomato wild type and its NA-less mutant was investigated. Under the same Fe supply the mutant showed higher Fe, Mn, and Zn concentrations in all organs investigated, with exception of the shoot apex. The Cu concentration in the mutant was only in root parts higher than in the wild type but much lower in leaves. Analyses of xylem exudate showed that Fe, Mn, and Zn were readily translocated by both genotypes from the roots to the shoot at all levels of Fe supply, whereas in the absence of NA, Cu was only poorly transported. Citrate as main Fe chelator in the xylem was present in high concentrations in xylem exudate of the wild type under low Fe supply but in the mutant also at 10 M FeEDTA. NA occurred in xylem exudate of the wild type in concentrations high enough to chelate heavy metal ions.Generally, high Fe supply induced a decrease of Mn, Cu, and Zn concentrations in all organs of the wild type whereas high concentrations were observed in most cases under Fe deficiency. A positive correlation between Fe supply and NA concentration existed only in the shoot apex and in the xylem exudate of wild type plants. From the correlation between Cu and NA translocation and from the high stability constant of the NA-Cu-complex (log K=18.6) it is concluded that NA is a chelator for Cu in the xylem, whereas the translocation of Fe, Mn, and Zn is independent of NA.     

Gibberellin-like substances in peach tissue: Separation by thin-layer electrophoresis and chromatography

Summary Thin-layer electrophoresis in conjunction with thin-layer chromatography proved an excellent method for separating gibberellin-like compounds extracted from small quantities of peach tissue. Eleven compounds active in the barley-endosperm gibberellin bioassay were separated from peach shoot tips. Two of these were possibly identical with known gibberellins but the remainder were distinctly different. Of the latter, five were acidic, one was neutral, and three were basic. The most active compound in shoot tips may have been GA₁ or GA₃ but in seeds the most active compound resembled no known gibberellin.     

Stress induction of abscisic acid in maize roots

Moderate water stresses in the range 0 to −0.6 MPa applied with PEG 6000 to excised roots of Zea mays L. var. LG 11 induced increases of up to four-fold in the amount of abscisic acid (ABA) determined in the tissue after a 12 h period of xylem exudation. The ABA concentration of xylem exudate collected after a 2 h water stress also increased by up to four-fold. Salt stresses, induced with NaCl solutions, resulted in similar increases in the ABA concentrations. ABA concentrations in both root tissue and xylem exudate were highest 4 h after removal of the stress and then declined over a subsequent 8 h period. These results are interpreted in support of the concept that root-produced ABA may have a role in the fine control of the plant's water balance.     

Root exudates from lentil (Lens culinaris medic) seedlings in relation to wilt disease

Summary Root exudates from healthy and diseased lentil plants (Lens culinaris) have been investigated in relation to the wilt disease caused byFusarium oxysporum f.lentis. In all ten amino acids and five sugars have been detected. The spore germination of the pathogen in root exudates indicated that 21-days root exudate was inhibitory. Glycine and phenylalanine were detected in 21-days exudate and were found to have an inhibitory effect upon the germination of the spores of the pathogen which may partly explain the lesser disease incidence when plants of more than 3 weeks are inoculated with the pathogen.     

Nitrogen status of cotton subjected to two short term periods of waterlogging of varying severity using a sloping plot water-table facility

Summary Cotton is reported to be susceptible to waterlogging, and there is evidence that some of the symptoms shown by waterlogged plants are due to impaired uptake of nitrogen. To investigate this for cotton, the nitrogen nutrition of a field-grown crop was monitored when the plants were subjected to two short term periods of waterlogging of varying severity using a sloping plot water-table facility. Growth of severely waterlogged cotton decreased after 4 days in the first and second floodings, and these plants were wilted by the end of the first flooding but not the second. Waterlogging resulted in decreased concentrations of total-N and especially NO ₃ ^– –N in the petiole and lamina of the youngest fully-expanded leaf. Uptake of N by waterlogged plants occurred, but was not as great as for well-aerated plants. The nitrate reductase activity of leaves was much lower in waterlogged plants. Stumps of detopped waterlogged plants did not exude sylem sap at the end of the first flooding, suggesting impaired solute uptake due to damaged roots. However, xylem exudate was obtained from stumps of waterlogged plants at the end of the second flooding, indicating adaptive changes to the root systems of these plants. Although cotton is reported to reduce little NO ₃ ^– –N in its roots, analysis of xylem exudate showed that about half of the N exported by roots was as amino compounds. The concentration of amino compounds in xylem exudate from severely waterlogged plants was higher than in well-aerated plants. It was concluded that the growth reduction in waterlogged cotton was due partly to induced N-deficiency.