Concentrations In Phloem Research Articles

Inhibition of maize root growth by high nitrate supply is correlated with reduced IAA levels in roots

The plant root system is highly sensitive to nutrient availability and distribution in the soil. For instance, root elongation is inhibited when grown in high nitrate concentrations. To decipher the mechanism underlying the nitrate-induced inhibition of root elongation, the involvement of the plant hormone auxin in nitrate-dependent root elongation of maize was investigated. Root growth, nitrogen and nitrate concentrations, and indole-3-acetic acid (IAA) concentrations in roots and in phloem exudates of maize grown under varying nitrate concentrations were analyzed. Total N and nitrate concentrations in shoots and roots increased and elongation of primary, seminal and crown roots were inhibited with increasing external nitrate from 0.05 to 5mM. High nitrate-inhibited root growth resulted primarily from the reduced cell elongation and not from changes in meristem length. IAA concentrations in phloem exudates reduced with higher nitrate supply. Inhibition of root growth by high nitrate was closely related to the reduction of IAA levels in roots, especially in the sections close to root tips. Exogenous NAA and IAA restored primary root growth in high nitrate concentrations. It is concluded that the inhibitory effect of high nitrate concentrations on root growth may be partly attributed to the decrease in auxin concentrations of roots.

Phenolic predictors for Norway spruce resistance to the bark beetleIps typographus(Coleoptera: Scolytidae) and an associated fungus,Ceratocystis polonica

Changes in phloem phenolic content of Norway spruce (Picea abies (L.) Karst.) clones were followed during the first 12 days of the reaction induced by phloem artificial inoculation with Ceratocystis polonica Siem., a bark beetle (Ips typographus L.) associated fungus. The aim was to confirm our previous results concerning the mechanisms of this reaction and the possible predictors of Norway spruce resistance to bark beetles and their associated fungi. The induced reaction was characterized by a slight decrease of tanning ability and an increase of (+)-catechin concentration, which confirmed our previous observations. The relative resistance of the clones was first predicted using the predictors previously proposed. In addition, the first axis of the principal component analysis describing the phenolic content of all clones was used as a synthetic predictor (resistance axis). Related variables were also tested as predictors. Actual resistance of each clone was then measured, using mass inoculations of C. polonica, and was compared with the predictions. Four predictors were so validated: the resistance axis, tanning ability and isorhapontin concentration in uninoculated phloem, and (+)-catechin concentration in the phloem 6 days after its inoculation. Phloem phenolic composition could thus be used to predict Norway spruce resistance to bark beetles and their associated fungi.

Mineral nutrition and transport in xylem and phloem ofBanksia prionotes(Proteaceae), a tree with dimorphic root morphology

Xylem sap of proteoid roots, lateral roots, sinker root and age classes of trunk segments, and phloem sap of mid-trunks were collected from trees of Banksia prionotes (Proteaceae) in native habitat on highly impoverished sands in south-western Australia. Proteoid roots were major exporters of phosphate, K + and amino acids during the wet winter season and showed in vitro nitrate reductase activity during periods of soil nitrification. Other parts of the root served as general sources of Na + , Cl - , Mg 2+ , Ca 2+ , and SO 4 2- . Lateral root xylem sap was more concentrated in virtually all solutes than that of sinker roots, even during the dry summer following senescence of proteoid roots. Gradients in xylem sap concentration up the main trunks suggested lateral abstraction and storage of incoming phosphate in basal stem parts during winter and a subsequent release to the xylem in summer. Phloem sap was many times more concentrated in nutrient ions than xylem sap, and, like xylem sap, showed unusually low K + and H 2 PO 4 - relative to Na + , Cl - and SO 4 2- , suggesting a sparing role of the latter three ions in meeting ionic requirements of transport. Amino acid analyses showed higher overall concentrations in phloem than xylem sap, but much lower proportions of total amino N as glutamine and asparagine in the former sap. This suggested utilization of xylem-derived amide N by leaves for growth and synthesis of phloem-mobile amino acids.

The Regulation of Gelation of Phloem Exudate from Cucurbita Fruit by Dilution, Glutathione, and Glutathione Reductase

The average glutathione equivalent concentration in phloem exudate collected from squash fruit (Cucurbita moschata [Duchesne] Poir. var Butternut) and pumpkin fruit (Cucurbita pepo [L.] var Jack-o-lattern) was 1.02 and 0.60 millimolar, respectively. Glutathione reductase (EC 1.6.4.2) activity in phloem exudate from squash and pumpkin fruit averaged 0.48 and 1.74 micromole NADPH oxidized per minute per milliliter, respectively. Protein concentrations in fruit phloem exudates averaged 67 milligrams per milliliter for squash and 57 milligrams per milliliter for pumpkin. The phloem-specific P-proteins account for most of the protein content of exudate. Pure exudate from fruit does not gel for hours or days, but when diluted with neutral or alkaline aqueous solutions, exudate gels rapidly. Exudate solutions undergo biphasic pH changes with dilution. We suggest that P-protein undergoes conformational change upon dilution, exposing titratable groups and sulfhydryl residues. Oxidation of the latter forms the intermolecular disulfide bridges of the gel. The gelation of diluted exudate is regulated by factors (oxygen, pH, glutathione, NADPH) which affect the maintenance of reduced sulfhydryl residues and the activity of glutathione reductase. While these factors may also act in vivo to regulate redox conditions in phloem, their relationship to hypothetical sol/gel transitions or motile and nonmotile phases in the transport conduit is unknown.

Effect of NaCI Salinity on Growth, Development, Ion Distribution, and Ion Translocation in Castor Bean (Ricinus communis L.)

Summary Castor bean ( Ricinus communis L.) was grown over a wide range of external salinities (up to 160 mol m -3 NaCl) and effects on growth, ion distribution, and composition of xylem and phloem sap were studied. NaCl above 40 mol m -3 caused severe changes in plant development with reduction in growth, suppressed branching, and reduced size and epinasty of leaves, but even at 160 mol m -3 NaCl Ricinus survived and produced viable seeds. In presence of external salt internal concentrations of Na + and Cl - increased, but Na + concentration in stem and petiole tissues were substantially higher than in leaf blades. Conversely, K + concentrations in petioles or stems decreased while those of leaf blades remained high. Na + concentrations in xylem and phloem saps increased with external NaCl salinity but very slightly so, while K + concentrations in both long distance transport fluids were maintained at high levels. Na + concentrations in phloem sap collected from young petioles were 2-5 times lower than in those obtained from petioles of older leaves or from the stem base. Apparently Na + was removed from xylem and phloem sap by stem and petiolar tissues. The resulting non-uniform distribution of K + and Na + within the shoot as a whole and the leaf was maintained over extended exposures to high external salinity. The importance of these processes for salinity tolerance of Ricinus are discussed.

Boron Mobility and Nutrition in Broccoli (Brassica oleracea var. italica

Broccoli (Brassica oleracea var. italica cv Premium Crop) plants were germinated in soil, transferred to vermiculite three weeks later and grown in the glasshouse, then either supplied continuously with boron levels ranging from 0.0 (deficient) to 12.5 (toxic) mg l−1 of nutrient solution or transferred from 2.5 to 0.0 mg B l−1 at the initiation of inflorescence development. At commercial maturity the concentrations of various inorganic and organic solutes in phloem exudates and xylem saps, as well as plant characteristics and elemental composition of the various plant parts, were determined. Under deficient B levels leaf midrib and stem corkiness were evident, together with signs of stem pith breakdown, symptoms which resemble the initiation of the hollow stem disorder. The xylem sap B concentration declined by about 50 % when B was not supplied or was removed after a period of adequate supply; the phloem concentration was unaffected. Also, the decreasing B concentration gradients from mature transpiring tissues to young developing sinks disappeared. Therefore, it is concluded that when B is deficient, it is retranslocated from source leaves in the phloem stream supplying the developing leaves and inflorescence. The data also suggested that at toxic levels B undergoes extensive lateral transfer, probably from xylem to xylem, thereby enhancing the B concentration of developing sinks. The B regime influenced dry-matter partitioning, retranslocation of some elements, and the synthesis and distribution of amino acids and sugars, reflecting the general nature of B involvement in plant processes.

Phloem translocation of weak acids in ricinus communis

AbstractUptake of weak acids by roots of Ricinus communis and their translocation to shoots increased as the pH of the nutrient solution decreased, the behaviour being closely similar to that observed for the same compounds in barley. Phenoxyacetic acids (p Ka ˜3) with log Kow, for the undissociated acid of between 1 and 3 were readily taken up and translocated in phloem to the apical leaf and the upper stem of R. communis following injection into the petioles of mature leaves; strong retention of these chemicals in the stem rapidly depleted their concentration in phloem down the stem so that very little was found in roots. A more polar and a more lipophilic phenoxyacetic acid were poorly translocated from the petioles, and flamprop (pKa 3.7, log Kow, 2.9) was predominantly xylem‐mobile. Maleic hydrazide (pKa 5.65, log Kow, −0.6) was taken up from the petioles rather slowly but was readily phloem‐translocated from the mature leaves, and little was retained by the stem, permitting appreciable amounts to reach the roots and nutrient solution. Concentrations of these weak acids in leaf phloem were up to ten times higher than in the adjacent leaf cells, and could be predicted reasonably well assuming that accumulation in phloem was purely a physico‐chemical process occurring due to the ion‐trap effect, taking values obtained from barley roots for the ratio of the permeabilities of membranes to the undissociated and dissociated forms of the acids.

Trace Elements in Tree Rings: Evidence of Recent and Historical Air Pollution

Annual growth rings from short-leaf pine trees in the Great Smoky Mountains National Park show suppressed growth and increased iron content between 1863 and 1912, a period of smelting activity and large sulfur dioxide releases at Copperhill, Tennessee, 88 kilometers upwind. Similar growth suppression and increases of iron and other metals were found in rings formed in the past 20 to 25 years, a period when regional fossil fuel combustion emissions increased about 200 percent. Metals concentrations in phloem and cambium are high, but whether they exceed toxic thresholds for these tissues is not known.

The Kinetics of Phloem Loading of Valine in the Shoot of a Nodulated Legume (Lupinus albusL. cv. Ultra)

Phloem loading of several amino acids (dand L-Val, Arg, Asn, Asp, Leu) was studied in shoots of L. albus using a phloem bleeding technique on both intact plants and detached shoots fed via the transpiration stream. Val was singled out for intensive study due to the minimal amount of metabolism it underwent in the shoot. For the amino acids studied, the relationship between xylem, phloem, and leaflet concentrations was determined by the interaction of rates of xylem supply, metabolism, and export. At elevated xylem fluid concentrations, low rates of loading of D-Val into the phloem and little metabolism in the tissues resulted in high levels in the leaflets. For other amino acids (Arg, Asp, Leu) rapid metabolism in the leaflets prevented a build-up in concentration in either phloem or leaflets. Asn was rapidly transferred to the phloem, thus high levels in the xylem lead to high concentrations in the phloem without greatly affecting leaflet concentrations. L-Val responded in a manner intermediate between Asn and D-Val. A detailed study of L-Val showed it to be loaded into the phloem against a concentration gradient in both stem and leaflets. Some of this Val originated from the transpiration stream at both locations but in the leaflets as much as 64% of the Val originated from other sources, e.g. recent photosynthesis. L-Val supplied to the phloem in the stem was derived from a large storage pool and did not come directly from the xylem fluid. As a consequence the rate of stem loading was independent of short-term fluctuations in the xylem fluid Val concentration. L-Val entering the leaf lets in the xylem initially bypassed the large storage pool and was loaded directly into the phloem. However, after 350 min the pools had reached an equilibrium and rate of phloem loading was dependent on total leaflet concentration.