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PubMed1968

Diurnal pattern of water potential in woody plants.

The dynamic relationship between the rates of water loss and uptake controls plant water status. Marked diurnal variations in water potential of both leaves and fruit occurred in all plants studied. Variations in water status during the day were most clearly related to changes in evaporative demand of the air and were different for the east and west sides of a tree. At night, the plant water potential reflected the soil moisture status.Changes in the water potential of pear fruit were correlated with changes in fruit diameter. Since water loss from fruit occurred mostly through the pedicel into the xylem of the tree, the fruit could be used as a crude gauge of xylem water potential, which also showed dramatic changes during the day.

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PubMed1968

Relationship of boron to gibberellic Acid-induced proliferation in debudded tobacco plants.

Stem applications of gibberellic acid (GA) to debudded tobacco plants (Nicotiona tabacum L., var. One Sucker) produce stem swellings that involve intense proliferation of, primarily, xylem tissue. Withholding boron from GA-treated debudded plants greatly reduces the GA-induced proliferation. This response offers a system for directly demonstrating the effect of boron on xylem formation and lignification that is unconfounded by the effect of boron on elongation growth.

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PubMed1971

Water potential gradient in a tall sequoiadendron.

With an elevator installed in a 90-meter tall Sequoiadendron to collect the samples, xylem pressure potential measurements were made approximately every 15 meters along 60 meters of the tree's height. The measured gradient was about -0.8 bar per 10 meters of height, i.e., less than the hydrostatic gradient. Correction of the xylem pressure potential data by calibration against a thermocouple psychrometer confirmed this result. Similar gradients are described in the literature in tall conifers at times of low transpiration, although a different sampling technique was used. If the data in the present study and those supporting it are typical, they imply a re-evaluation of either the use of the pressure chamber to estimate water potential or the present theories describing water transport in tall trees.

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PubMed1973

Some factors regulating auxin translocation in intact bean seedlings.

Several factors which influence the translocation patterns of stem-injected indoleacetic acid, 2,4-dichlorophenoxyacetic acid, and 2,4,5-trichlorophenoxyacetic acid in bean seedlings (Phaseolus vulgaris L. cv. Stringless Greenpod) were characterized. The acropetal translocation of auxin from the site of injection is markedly sensitive to concentration in the range of 1.0 to 5.0 micrograms per plant. The antiauxin p-chlorophenoxyisobutyric acid enhanced translocation of 2,4,5-trichlorophenoxyacetic acid to the growing shoots and primary leaves. Translocation to the roots was unaffected by p-chlorophenoxyisobutyric acid while leaching of 2,4,5-trichlorophenoxyacetic acid into the nutrient solution was enhanced slightly. Steam girdling experiments revealed that translocation to the primary leaves was in the xylem. The protein synthesis inhibitor, cycloheximide, inhibited accumulation of 2,4,5-trichlorophenoxyacetic acid in young shoots, epicotyls, and roots and enhanced accumulation in the primary leaves. The relative exchangeability of auxin between xylem and phloem is discussed in terms of regulation of auxin movements in intact bean seedlings.

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PubMed1973

Organic acids and iron translocation in maize genotypes.

Translocation of Fe was studied in WF9 (Fe-efficient) and ys(1)/ys(1) (Fe-inefficient) maize (Zea mays L.) genotypes. Iron-deficient WF9 translocated more Fe to the tops than Fe-deficient ys(1)/ys(1). Malate and citrate contents of root saps increased nearly 2-fold and aconitate increased over 4-fold in both genotypes as Fe of nutrient solutions increased from 0.1 to 3 milligrams per liter. Relative acid contents in root saps were as follows: malate > aconitate > citrate. Citric acid concentrations in stem exudates were nearly the same as in root sap. Malic acid concentrations were considerably lower in exudates than in root saps, and only a trace of aconitic acid was detected in the exudates. The concentration of Fe was 7-fold higher in exudate of WF9 than in exudate of ys(1)/ys(1) and the concentration of exudate P was about the same for both genotypes.Electropherograms of WF9 stem exudates showed that (59)Fe moved toward the anode as (59)Fe-citrate. Exudates of ys(1)/ys(1) contained insufficient (59)Fe to produce radiographs. When (59)Fe was added in vitro to ys(1)/ys(1) stem exudate, the (59)Fe moved as (59)Fe-citrate, indicating that sufficient citric acid was present in the exudate to chelate the Fe. Effectiveness of citric, isocitric, trans-aconitic, and malic acids in moving (59)Fe electrophoretically in acetate, citrate, isocitrate, trans-aconitate, and malate buffers was studied. Malic, acetic, and trans-aconitic acids were ineffective in moving Fe from the origin. Citric acid moved Fe anodically whenever present on the electropherogram and successfully competed with the other acids for Fe.Results with ys(1)/ys(1) roots indicate an absence of an efficient mechanism for transporting Fe from cortical cells to the xylem. If Fe can reach the xylem stream, the ys(1)/ys(1) genotype should be as efficient as WF9 in moving Fe to the leaves.

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PubMed1974

Effect of low temperature on amino Acid metabolism in wintering poplar: arginine-glutamine relationships.

Analyses of free amino acids in poplar (Populus gelrica) were carried out throughout a year to see the effect of low temperature on a system regulating amino acid metabolism in the tree. The results indicated that during the wintering phase arginine was the major amino acid both in bark and xylem, particularly in xylem, and that at the time of budding and growing glutamine and glutamate became dominant. Changes in the relative levels of glutamine (plus glutamate) and arginine to the total amino acids of the alpha-ketoglutarate family indicated the presence of a regulatory system annually controlling the synthesis between glutamine (plus glutamate) and arginine. The system appeared to be governed and sensitized by low temperatures. Neither a transition of the synthesis from arginine to glutamine (plus glutamate) nor budding occurred in the poplars which spent the winter months in a greenhouse.

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PubMed1976

Electrochemical gradients and k and cl fluxes in excised corn roots.

The compartmental analysis method was used to estimate the K(+) and Cl(-) fluxes for cells of excised roots of Zea mays L. cv. Golden Bantam. When the measured fluxes are compared to those calculated with the Ussing-Teorell flux-ratio equation, an active inward transport of Cl(-) across the plasmalemma is indicated; the plasmalemma K(+) fluxes are not far different from those predicted for passive diffusion, although an active inward transport cannot be precluded. Whether fluxes across the tonoplast are active or passive depends upon the vacuolar potential which is unknown. Assuming no electropotential gradient, the tracer flux ratios are fairly close to those predicted for passive movement. However, if the vacuole is positive by about 10 millivolts relative to the cytoplasm, the data suggest active inward transport for K(+) and outward transport for Cl(-).Fluxes to the xylem exudate were found to be more accurately estimated from the specific radioactivity of the cytoplasm (symplasm) than from the external solution specific radioactivity. The electrochemical gradients for K(+) and Cl(-) between the xylem vessels and the surrounding stelar parenchyma indicate active K(+) and passive Cl(-) movement into the vessels. The data are interpreted as being in accord with radial transport through the symplast into living vessels.

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PubMed1976

Interaction between Osmotic- and Pressure-induced Water Flow in Plant Roots.

When the pressure gradient across a root alters, there is often an apparent change in the permeability of the root to water. Fiscus (Plant Physiol. 1975. 55: 917-922) has suggested that this can be explained by a simple two-compartment model which takes into account rates of solute uptake into the xylem. A method of testing actual data against the Fiscus model is proposed; this shows that in some cases the apparent changes in permeability cannot be explained by the model. The model is not adequate to predict the amounts of solute reaching the xylem by passive drag: a three-compartment model would be more realistic.

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PubMed1976

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: I. Regulation by Nitrate Flux.

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) were investigated in 8- to 14-day old maize (Zea mays L.) plants containing high nitrate levels while other environmental and endogenous factors were constant. The nitrate flux of intact plants was measured from the product of the transpiration rate and the concentration of nitrate in the xylem. NRA decreased when the seedlings were deprived of nitrate. The nitrate flux and the leaf nitrate content also decreased. When nitrate was resupplied to the roots, all three parameters increased.Attempts to alter the nitrate flux by varying transpiration rates were unsuccessful due to a relatively constant rate of delivery of nitrate to the xylem as transpiration rates fell. However, cooling the roots resulted in a decrease in the nitrate flux. Plants with a lower nitrate flux rapidly lost NRA, although the leaf nitrate content was initially unaffected. If the roots remained cool for a long enough time, the leaf nitrate content eventually decreased. Rewarming the roots increased the nitrate flux, leaf nitrate content, and NRA to control levels. When the nitrate flux in excised shoots was varied in three separate ways, decreasing the nitrate flux to the leaves resulted in a rapid decrease in NRA, although leaf nitrate contents were unchanged.These experiments show that the nitrate flux to the leaves from the roots plays a much larger regulatory role than the leaf nitrate content in controlling the level of NRA in intact plants.

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PubMed1978

In Situ Estimates of Variable Plant Resistance to Water Flux in Ilex opaca Ait.

Xylem pressure potentials and stomatal diffusion resistances were measured in the field in Ilex opaca Ait. during days which differed in temperature and vapor pressure deficit. Water flux into leaves was calculated by combining the field data with laboratory determinations of the relation between tissue water deficit and water potential. Estimates of apparent plant resistance were then calculated from fluxes and differences between soil water potential and xylem tension. The resistance depended strongly on water flux, dropping by a factor of over 7 from low to high water flow rates. This extends the generality of variable plant resistances measured in controlled environment studies to I. opaca as it occurs naturally in the field. The relation of apparent plant resistance to water flux as estimated in this study can be useful in simulation models which calculate water uptake to leaves as a flux driven by a difference in soil and leaf water potentials across a resistance between the bulk soil and the leaf.

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PubMed1978

Pattern of cell division and wound vessel member differentiation in coleus pith explants.

When pith parenchyma explants are taken from Coleus blumei plants and cultured on an agar medium containing sucrose and indoleacetic acid wound vessel members differentiate in 10 days. The time course of wound xylem appearance and an auxin requirement suggest that this uncomplicated system is responding in a manner comparable to wounded Coleus plants and cultured stem segments.Histological examination and cell size comparisons confirm that parenchyma cells divide before differentiating. When colchicine is used to prevent mitosis no tracheary elements differentiate. Following the time course of this cytodifferentiation histologically shows that xylem differentiates from cells that are the products of several cell divisions.

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PubMed1978

Application of the chemiosmotic hypothesis to ion transport across the root.

The evidence on how ions accumulated in the root symplasm are released to the xylem vessels is examined. It is suggested that Mitchell's chemiosmotic hypothesis as applied to ion transport might account for the process. A model based on this hypothesis shows the symplasm as an osmotic unit connecting two isolated solutions, but with no significant difference in proton motive force across the unit. If it is assumed that the resistance to transport by plasmalemma uniports and antiports differs in the cortical and stelar ends of the symplasm, the model will provide for an influx of ions to the xylem. The reported properties of isolated steles suggests that the porters (carriers) do have properties in accord with the model.

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PubMed1979

Susceptibility of plants to vascular disruption by macromolecules.

The xylem of alfalfa (Medicago sativa L.) was found to be susceptible to vascular obstruction by picomole quantities of dextrans. Not all parts of the xylem were equally susceptible to this plugging. The quantity of dextran of 2 x 10(6) molecular weight required to stop vascular flow was 8 picomoles in petiole junctions and 0.4 picomole in leaflet veins. Vascular flow through stems was greatly reduced but not stopped, even by over 150 picomoles of the dextran. The ability of dextrans to interfere with vascular conductance was directly correlated with their molecular weight. Dextrans of molecular weight less than 250,000 had little ability to stop vascular flow.

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PubMed1979

Economy of Carbon and Nitrogen in a Nodulated and Nonnodulated (NO(3)-grown) Legume.

Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO(3)-fed and nodulated, N(2)-fed plants of white lupin (Lupinus albus L.). The NO(3) regime used (5 millimolar NO(3)) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO(3)-fed plants was judged to be reduced in roots. Empirically based models of C and N flow demonstrated that patterns of incorporation of C and N into dry matter and exchange of C and N among plant parts were essentially similar in the two forms of nutrition. NO(3)-fed and N(2)-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO(3) supply from NO(3)-fed plants led to substantial changes in assimilate partitioning, particularly in increased translocation of N from shoot to root. Nodulated plants showed a lower (57%) conversion of C or net photosynthate to dry matter than did NO(3)-fed plants (69%), and their stems were only half as effective as those of NO(3)-fed plants in xylem to phloem transfer of N supplied from the root. Below-ground parts of symbiotic plants consumed a larger share (58%) of the plants' net photosynthate than did NO(3)-fed roots (50%), thus reflecting a higher CO(2) loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO(3)-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO(2) output of the nodulated root involved a slightly greater expenditure for N(2) than for NO(3) assimilation, a small extra cost due to growth and maintenance of nodule tissue, and a considerably greater nonassimilatory component of respiration in root tissue of the symbiotic plant than in the root of the NO(3)-fed plant.

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PubMed1980

Nitrate uptake by roots as regulated by nitrate assimilation in the shoot of castor oil plants.

Ricinus communis was used to test the Ben Zioni-Dijkshoorn hypothesis that NO(3) uptake by roots can be regulated by NO(3) assimilation in the shoot. The rate of the anion charge from assimilated NO(3) (-) (and SO(4) (2-)) was followed in its distribution between organic acid anion accumulation and HCO(3) (-) efflux into the nutrient solution. In plants adequately supplied with NO(3) (-), HCO(3) (-) efflux accounted for between 56 and 63% of the anion charge. When the plants were subjected to a low NO(3) regime HCO(3) (-) excretion accounted for only 23% of the charge. A comparison of mature plants growing for a 10-day period at the two levels of NO(3) nutrition revealed that the uptake of NO(3) (-) at the higher level was increased 3-fold, whereas K uptake was unaltered. To trace ion movement within the plant, the ionic constituents of xylem and phloem sap were determined. In xylem sap these constituents were found to be predominantly K(+), Ca(2+), and NO(3) (-), whereas in the phloem sap they were mainly K(+) and organic acid anions. Results have been obtained which may be interpreted as providing direct evidence of NO(3) uptake by roots regulated by NO(3) reduction in the tops, the process being facilitated by the recirculation of K(+) in the plant.

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PubMed1981

4-methyleneglutamine in peanut plants: dynamics of formation, levels, and turnover in relation to other free amino acids.

Neither 4-methyleneglutamine nor 4-methyleneglutamic acid were found in free or bound form in ungerminated peanut seeds (Arachis hypogaea L.). Both, however, were formed soon after germination; whereas, 4-methyleneglutamic acid appeared slightly before 4-methyleneglutamine, the former remained at a low concentration while the level of 4-methyleneglutamine rose rapidly between 2 and 10 days of germination and declined slowly thereafter. Free proline and glutamine followed a pattern similar to 4-methyleneglutamine; on the other hand, asparagine increased for at least 20 days but other free amino acids remained at relatively low, constant levels. In mature peanut plants, 4-methyleneglutamine occurred in all parts except developing pods, was virtually the only free amino acid in xylem sap, and constituted about 70% of the total soluble nitrogen of sap. In contrast, 4-methyleneglutamic acid was found only in leaves and stems in highly variable amounts.Levels of 4-methyleneglutamine were largely independent of the processes of nodulation and nitrogen fixation. This amide was degraded much more slowly in nitrogen-deficient plants than were any of the other free amino acids and did not become labeled when a photosynthesizing plant was exposed to (14)CO(2) for 24 hours. The results indicate that 4-methyleneglutamine is the principle carrier of nitrogen in the xylem of peanut plants but its carbon skeleton appears to turn over very slowly.

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PubMed1981

Cadmium distribution and chemical fate in soybean plants.

The distribution and chemical behavior of Cd(2+) in tissues and its chemical form in xylem water of soybean plants (cv. Williams) were investigated. Following root absorption, Cd is strongly retained by roots, with only 2% of the accumulated Cd being transported to leaves; as much as 8% was transported to seeds during seed filling. In vivo xylem exudates contained two anionic Cd complexes in addition to inorganic forms of Cd. Once accumulated in root and leaf tissues, Cd rapidly equilibrated between the insoluble, soluble, and organelle fractions. Of the solubles, which contain 50% of the Cd, >50% was associated with components of >10,000 molecular weight, and <8% was associated with <500 molecular weight components. Cadmium accumulated in soybean seeds was primarily associated with cotyledons. Fractionation of seeds showed the soy proteinate and soy whey to contain 32 and 50% of the accumulated Cd, respectively.

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PubMed1982

Nitrate Reduction by Roots of Soybean (Glycine max [L.] Merr.) Seedlings.

Studies were conducted with 9 to 12 day-old soybean (Glycine max [L.] Merr. cv. Williams) seedlings to determine the contribution of roots to whole plant NO(3) (-) reduction. Using an in vivo -NO(3) (-) nitrate reductase (NR) assay (no exogenous NO(3) (-) added to incubation medium) developed for roots, the roots accounted for approximately 30% of whole plant nitrate reductase activity (NRA) of plants grown on 15 mm NO(3) (-).Nitrogen analyses of xylem exudate showed that 53 to 66% of the total-N was as reduced-N, depending on the time of day of exudate collection. These observations supported enzyme data that suggested roots were contributing significantly to whole plant NO(3) (-) reduction. In short-term feeding studies using (15)N-NO(3) (-) significant and increasing atom percent (15)N excess was found in the reduced-N fraction of xylem exudate at 1.5 and 3 hours after feeding, respectively, which verified that roots were capable of reducing NO(3) (-).Estimated reduced-N accumulation by plants based on in vivo -NO(3) (-) NR assays of all plant parts substantially over-estimated actual reduced-N accumulation by the plants. Thus, the in vivo NR assay cannot be used to accurately estimate reduced-N accumulation but still serves as a useful assay for relative differences in treatment conditions.

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