The effect of differential root and shoot temperature on the nitrate reductase activity,assayed in vivo and in vitro in roots ofHordeum vulgare (barley)

There was a large increase in nitrate reductase activity (NAR) assayed both in vivo and in vitro in roots of barley plants (cv. Midas_ grown with roots at 10°C and shoots at 20°C, compared with whole plants grown at 20°C. There were diurnal fluctuations in NRA in roots from both treatments, but they were much greater in roots grown at 20°C, where NRA fell to a very low value in the dark period. The diurnal fluctuations in the malate content of the roots were also related to the root growth temperature. Plants with roots grown at the lower temperature had a higher malate content, especially in the dark period where it was 20 times greater than in plants with roots at 20°C. At all times there was a three-fold increase in soluble carbohydrate in cooled roots and diurnal fluctuations were much less pronounced than those of malate. Growth at low temperatures increased the total flux of amino N into the xylem sap and increased the proportion of reduced N in the total N flux. At certain times of day both 10°C- and 20°C-grown roots responded to exogeneous malate by increasing the flux of amino acid into the xylem sap, although this effect was always more pronounced in 20°C-grown roots.     

Acclimation of potassium influx in rye (Secale cereale) to low root temperatures

The influx of K⁺(⁸⁶Rb⁺) into intact roots of rye (Secale cereale L. cv. Rheidal) exposed to a differential temperature (DT) between the root (8° C) and shoot (20° C) is initially reduced compared with warm-grown (WG) controls with both shoot and root maintained at 20° C. Over a period of 3 d, however, K⁺-influx rates into DT plants are restored to levels similar to or greater than those of the WG controls, the absolute rates of K⁺ influx being strongly dependent upon the shoot/root ratio. Acclimation in DT plants results in a reduction of K⁺ influx into the apical (0–2 cm) region of the seminal root which is associated with a compensatory increase in K⁺ influx into the more mature, basal regions of the root. Values of V _max and apparent K _m for K⁺ influx into DT plants were similar to those for WG plants at assay temperatures of 8° C and 20° C except for an increase in the apparent K _m at 8° C. The influx of K⁺ from solutions containing 0.6 mol·m^-3 K⁺ into both WG and DT plants was found to be linearly related to assay temperature over the range 2–27° C, and the temperature sensitivity of K⁺ influx to be dependent upon shoot/root ratio. At high shoot/root ratios, the ratio of K⁺ influx at 20° C:K⁺ influx at 8° C for WG plants approached a minimum value of 1.9 whereas that for DT plants approached unity indicating that K⁺ influx into DT plants has a large temperature-insensitive component. Additionally, when plants were grown in solutions of low potassium concentration, K⁺ influx into DT plants was consistently greater than that into WG plants, in spite of having a greater root potassium concentration ([K⁺]int). This result indicates some change in the regulation of K⁺ influx by [K⁺]int in plants exposed to low root temperatures. We suggest that K⁺ influx into rye seedlings exposed to low root temperatures is regulated by the increased demand placed on the root system by a proportionally larger shoot and that the acclimation of K⁺ influx to low temperatures may be the result of an increased hydraulic conductivity of the root system.Abbreviations DT differential temperature pretreatment - [K⁺]int root potassium concentration - [K⁺]ext potassium concentration of nutrient medium - WG warm-grown pretreatment     

Phospholipid composition and fatty acid desaturation in the roots of rye during acclimatization of low temperature

When the roots of rye plants grown at 20°C were cooled to 8°C the concentration of phospholipid in them more than doubled over a 7 d period in comparison with that in roots remaining at 20°C. The relative abundance of lecithin (PC) declined while that of phosphatidyl ethanolamine (PE) increased; this change was completed after 2 d cooling. Labelling with ³²P suggested that turnover of phospholipids may be inhibited by low temperature. Acyl lipids contained an increased proportion of linolenic acid (18:3) and reduced proportion of linoleic acid (18:2) when roots were cooled at 8°C for 7 d. The ratio of these acids is a relatively more sensitive indicator of desaturation than is the double bond index. Cooling brought about no change in the abundance of the principal saturated acid, palmitic (16:0). In the first 3 days of cooling PC and PE desaturated markedly while there was no change in galactosyl and neutral lipids. Desaturation did not appear to be greatly sensitive to the concentration of dissolved O₂ and was only partly inhibited in 8°C solutions where the oxygen concentration was lowered to 0.5–2.0%. Positional analysis of acyl chains in PC and PE showed that more than 90% of all 16:0 is associated with position I while 65% of the 18:2+18:3 is associated with position II. When roots are cooled the abundance of 18:3 increases in both chains but the relative distribution of saturated and unsaturated fatty acids remains constant in positions I and II. At both 20°C and 8°C there is a high probability that a saturated chain in position I will be paired with the polyunsaturated one in position II.Abbreviations PC Lecithin - PE phosphatidyl ethanolamine - TLC thin layerchromatography - BHT butylatedhydroxytoluene     

Evaluation of the Relative Ureide Content of Xylem Sap as an Indicator of N(2) Fixation in Soybeans: GREENHOUSE STUDIES

The use of the relative ureide content of xylem sap [(ureide-N/total N) × 100] as an indicator of N₂ fixation in soybeans (Merr.) was examined under greenhouse conditions. Acetylene treatments to inhibit N₂ fixation were imposed upon the root systems of plants totally dependent upon N₂ fixation as their source of N and of plants dependent upon both N₂ fixation and uptake of exogenous nitrate. Significant decreases in the total N concentration of xylem sap from plants of the former type were observed, but no significant decrease was observed in the total N concentration of sap from the latter type of plants. In both types of plants, acetylene treatment caused significant decreases in the relative ureide content of xylem sap. The results provided further support for a link between the presence of ureides in the xylem and the occurrence of N₂ fixation in soybeans. The relative ureide content of xylem sap from plants totally dependent upon N₂ fixation was shown to be insensitive to changes in the exudation rate and total N concentration of xylem sap brought about by diurnal changes in environmental factors. There was little evidence of soybean cultivars or nodulating strains affecting the relative ureide content of xylem sap. `Ransom' soybeans nodulated with Rhizobium japonicum strain USDA 110 were grown under conditions to obtain plants exhibiting a wide range of dependency upon N₂ fixation. The relative ureide content of xylem sap was shown to indicate reliably the N₂ fixation of these plants during vegetative growth using a ¹⁵N method to measure N₂ fixation activity. The use of the relative ureide content of xylem sap for quantification of N₂ fixation in soybeans should be evaluated further.     

Chemical forms of aluminum in xylem sap of tea plants (Camellia sinensis L.)

To identify the chemical forms of aluminum (Al) transported from roots to shoots of tea plants (C. sinensis L.), ²⁷Al-nuclear magnetic resonance and ¹⁹F NMR spectroscopy were used to analyze xylem sap.The concentration of Al in collected xylem sap was 0.29 mM, twice as high as that of F. Catechins were not detected in xylem sap. The concentration of malic acid in xylem sap was higher than that of citric acid, whereas the concentration of oxalic acid was negligible.There were two signals in the ²⁷Al NMR spectra of xylem sap, a larger signal at 11 ppm and a smaller one at −1.5 ppm. The former signal was consistent with the peak for an Al-citrate model solution, suggesting that an Al-citrate complex was present in xylem sap. Although the latter signal at −1.5 ppm was thought to indicate the presence of an Al-F complex (at 1.7 ppm) in xylem sap, there was only one signal at −122 ppm in the ¹⁹F NMR spectrum of xylem sap, indicating that the main F complex in xylem sap was F⁻.These results indicate that Al might be translocated as a complex with citrate, while Al-malate, Al-oxalate and Al-F complexes are not major Al complexes in xylem sap of tea plants.     

Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench)

 The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv. Jianxi) that accumulates Al in its leaves. The Al concentration in the xylem sap was 15-fold higher in the plants exposed to AlCl₃ than in those exposed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AlCl₃ solution and 10-fold higher after a 2-h exposure. The Al concentration in the xylem sap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that Al uptake by the root is a passive process, and La³⁺ competes for the binding sites for Al³⁺ on the plasma membrane. The form of Al in the xylem sap was identified by ²⁷Al-nuclear magnetic resonance analysis. The chemical shift of ²⁷Al in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in the xylem sap was citric acid, indicating that Al was translocated in the form of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate occurs before Al is released to xylem. Received: 10 December 1999 / Accepted: 3 February 2000     

Effects of Growth and Assay Temperatures on Unidirectional K+ Fluxes in Roots of Rye (Secale cereale)

The effects of growth and assay temperature on unidirectionalK⁺ fluxes in excised roots of rye (Secale cereale cv. Rheidol)were studied using ⁸⁶Rb⁺ as a tracer. Both K⁺ influx to thevacuole, estimated as K⁺ uptake between 3 and 12 h after transferof unlabelled roots to radioactive solution, and movement ofK⁺ to the xylem were determined directly. Other fluxes weredetermined on excised roots of plants, which had been labelledwith ⁸⁶Rb⁺ since germination, by conventional triple exponentialefflux analysis. When assayed at 20°C, roots of plants previously grown at20°C(WG roots) had lower rates of net K⁺ uptake than rootsof low temperature-acclimated plants, grown with a temperaturediferential between roots (87°C) and shoots (20°C) eithersince germination (DG roots) or for 3 d prior to experiments(DT roots). This resulted from a greater unidirectional K⁺ effluxacross the plasma membrane and a reduced K⁺ flux to the xylemin WG roots, compared to DG or DT roots, rather than a decreasein unidirectional K⁺ influx or a decrease in the net K⁺ fluxto the vacuole. Indeed, although WG roots had lower rates ofK⁺ influx and K⁺ efflux across the tonoplast at 20°C thanDG or DT roots, roots of plants from all growth temperaturetreatments showed an equivalent net K⁺ flux to the vacuole. Although all unidirectional K⁺ fluxes in roots from plants grownunder all temperature regimes were reduced by lowering the temperatureof the root, these fluxes were differentially affected in rootsof plants from contrasting growth temperature treatments. Rapidcooling to 8°C of WG roots resulted in a lower rate of K+influx and a transient increase in K⁺ efflux across both theplasma membrane and tonoplast, compared to DG and DT roots.Furthermore, since the K⁺ flux to the xylem was lower in WGroots, the net K⁺ uptake at 8°C into WG roots was considerablyreduced compared to DG and DT roots. These results suggest thatlow temperature-acclimation of K⁺ fluxes in rye roots may involvea reduction in the temperature sensitivity of K⁺ influx anda curtailment of K⁺ efflux across both the plasma membrane andtonoplast at low temperatures. Key words: K⁺influx, K⁺ efflux, low temperature, potassium, rye (Secale cereale cv. Rheidol)     

Der Einfluss verschiedener Salze und verschiedener Inhibitoren auf den Wurzeldruck von Zea mays

The effect of different salts and inhibitors on the root pressure of Zea mays.— The influence of various salt solutions and inhibitors on the exudation rate has been investigated with young excised primary roots of Zea mays. The following results were obtained. — The effect of chlorides on the exudation rate was higher than the effect of sulphates K⁺ and Na⁺ effected higher flux rates than Ca²⁺ and Mg²⁺ The highest exudation rate was obtained with KCl. — In comparison to an isotonic Lutrol-solution (a liquid condensed polyethylenoxid) a 0.5525 molar KCl-solution, applicated on the root stump, increased the exudation rate considerably. — Metabolic inhibitors and anaerobic conditions decreased the exudation rate. — Experiments, concerning the influence of metabolic inhibitors on the exudation and on the Rb-uptake showed a highly significant positive correlation (r =+0.72***) between the exudation rates and the Rb-concentrations in the exudates. The Rb-accumulation in the root tissue was not correlated to the exudation rate. — The experimental data agree with the concept of a transversal water transport in the root tissue, effected by osmotic forces. The root pressure is based on the osmotic gradient between the xylem sap and the outer solution. This gradient is built up by the metabolic secretion of ions into the xylem sap. It is supposed that the transversal water transport in the roots mainly goes through the free space of the cortex.     

Temperature-dependent water and ion transport properties of barley and sorghum roots : I. Relationship to leaf growth

Root temperature strongly affects shoot growth, possibly via “nonhydraulic messengers” from root to shoot. In short-term studies with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) seedlings, the optimum root temperatures for leaf expansion were 25° and 35°C, respectively. Hydraulic conductance (L_p) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 25°C, remaining high with further warming. In sorghum, the L_p of intact plants and of detached roots peaked at 35°C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in L_p. Extreme temperatures greatly decreased ion flux, particularly K⁺ and NO₃⁻, to the xylem of detached roots of both species. Removing external K⁺ did not alter short-term K⁺ flux to the xylem in sorghum but strongly inhibited flux at high temperature in barley, indicating differences in the sites of temperature effects. Leaf growth responses to root temperature, although apparently “uncoupled” from water transport properties, were correlated with ion fluxes. Studies of putative root messengers must take into account the possible role of ions.     

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.     

Visualisation of xylem sap flow direction in isolated fine lateral roots and estimation of the xylem sap osmotic potential

Xylem sap outflow from fine lateral roots (FLRs) isolated from hydroponically grown young maize (Zea mays L.) plants was visualized by local brightening of test solutions contrasted with purified Indian ink particles. Flow into the vessels was indicated by the adsorption of Evans Blue in their walls. The fraction of the FLRs able to exude xylem sap in a mineral medium with 30 mM mannitol decreased with increasing incubation time. This change was strongly retarded, when the FLRs were incubated in a medium containing glucose instead of mannitol. There was a broad range of variation of the osmotic potential of the test solutions (Ψ_so), wherein the fraction of the FLRs showing an initially reversed flow of the xylem sap varied between zero and unity. A median (M) of the osmotic potential of the xylem sap in FLRs (Ψ_sx) was estimated. It represents the value of Ψ_so that was lower than Ψ_sx in half of the roots of a sample before their transfer to the test solutions (Ψ_sxo). M was dependent on the osmotic potential of the medium used for growth or pre-incubation of the FLRs. Its value was not dependent on the molecular size of the osmolytes used to adjust Ψ_so, including dextran 8, which is excluded from cell walls. In all of the studied plants, M was lower than the osmotic potential of the xylem sap collected from the root before isolation of the FLRs. To explain this finding it is assumed that FLRs with Ψ_sxo > M had a higher hydraulic conductivity and a larger volume contributed to the exuded sap than those with Ψ_sx < M.     

Effect of nitrate on water transfer across roots of nitrogen pre-starved maize seedlings

The addition of 10 mM KNO₃ to the solution bathing the roots of young nitrogen-starved seedlings of Zea mays L. enhanced root water transfer within 15 h, compared with 10 mM KCl addition. The free exudation flux was 2.2–3.9 times higher in excised KNO₃-treated roots than in KCl-treated ones. Cryo-osmometry data for xylem sap suggested that, compared with chloride, nitrate treatment increased the steady solute flux into the xylem, but did not modify the osmotic concentration of sap. Root growth was not significantly modified by nitrate within 15 h. Root hydraulic conductances were measured by using either hydrostatic-pressure or osmotic-gradient methods. During hydrostatic experiments, the conductance (k_p), which is thought to refer mainly to the apoplasmic pathway, was 1.6 times larger in KNO₃-than in KCl-treated plants. From experiments in which polyethylene glycol (PEG) 8000 was used as external osmolyte, osmotic conductances (k_s) were found to be smaller by 5–20 times than k_p for the two kinds of plants. The KCl-treated roots were characterized by a low k_s which was the same for influx or efflux of water. By contrast, KNO₃-treated roots exhibited two distinct conductances k_s1 and k_s2, indicating that influx of water was easier than efflux when the water flow was driven by the osmotic pressure gradient. Infiltration of roots with KNO₃ solution supported the idea that nitrate might enhance the efficiency of the cell-to-cell pathway. The low k_s value of KCl-treated roots and the existence of two contrasting k_s values (k_s1 and k_s2) for KNO₃-treated roots are discussed in terms of reversible closing of water channels.     

Organic acids are not specifically involved in the nitrate-enhanced Zn hyperaccumulation mechanism in Noccaea caerulescens

Nitrogen form has been shown to affect Zn uptake, translocation and storage in the Zn-hyperaccumulating plant Noccaea caerulescens but the biochemical processes are not fully understood. Organic acids and amino acids have been implicated in Zn transport and storage. This study aimed to examine the effect of N form on concentrations of organic acids and amino acids and how these metabolites correlated with Zn hyperaccumulation. Plants were grown in nutrient solution with NO₃⁻, NH₄NO₃ or NH₄⁺, supplied with 50 or 300 μM Zn, and buffered at either pH 4.5 or 6.5. The metabolomic profile was determined by gas chromatography mass spectroscopy. The concentration of Zn in shoots, xylem and roots was greatest for the NO₃⁻, pH 6.5 and 300 μM Zn treatments. For all N forms, the lower growth-medium pH raised xylem sap pH but had no influence on Zn concentration or exudation rate of the xylem sap. Nitrate enhanced organic acid production while NH₄⁺ increased amino acid production. Organic acids in the xylem were more responsive to changes in growth-medium pH than N form, and did not correlate with Zn concentration in shoots, roots or xylem. Serine might be directly involved in Zn hyperaccumulation. Phosphoric acid was associated with reduced Zn accumulation in the shoots. Malic acid was not detected in the shoots but responded to cation uptake more than to Zn specifically in the roots. Citric acid responded to cation uptake more than to Zn specifically in the shoots but did not correlate with Zn concentration in the roots or the xylem sap, or any other cations in the roots. In conclusion, organic acids in N. caerulescens are not specifically involved in Zn hyperaccumulation but are involved in regulating pH in the xylem and cation–anion balance in plants.     

Spontaneous Phloem bleeding from cryopunctured fruits of a ureide-producing legume

The vasculature of the dorsal suture of cowpea (Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N₂. Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K⁺ content and high ratio of Mg²⁺ to Ca²⁺. Fruit cryopuncture sap became labeled with ¹⁴C following feeding of [¹⁴C]urea to leaves or adjacent walls of the fruit, of ¹⁴CO₂ to the pod gas space, and of [¹⁴C] asparagine or [¹⁴C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of ¹⁴C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of ¹⁴C distribution in phloem sap suggested that [¹⁴C]allantoin was metabolized to a greater extent in its passage to the fruit than was [¹⁴C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO₃-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO₃-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of ¹⁵NO₃ to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant.     

Root-zone temperature and salinity: Interacting effects on tillering,growth and element concentration in barley

The effects of root-zone salinity (0, 30, and 60 mmol L^–1 of NaCl) and root-zone temperature (10, 15, 20, and 25°C) and their interactions on the number of tillers, total dry matter production, and the concentration of nutrients in the roots and tops of barley (Hordeum vulgare L.) were studied. Experiments were conducted in growth chambers (day/night photoperiod of 16/8 h and constant air temperature of 20°C) and under water-culture conditions. Salinity and root temperature affected all the parameters tested. Interactions between salinity and temperature were significant (p<0.05) for the number of tillers, growth of tops and roots, and the concentration of Na, K, P in the tops and the concentration of P in the roots. Maximum number of tillers and the highest dry matter were produced when the root temperature was at the intermediate levels of 15 to 20°C. Effect of salinity on most parameters tested strongly depended on the prevailing root temperature. For example, at root temperature of 10°C addition of 30 mmol L^–1 NaCl to the nutrient solution stimulated the growth of barley roots; at root temperature of 25°C, however, the same NaCl concentration inhibited the root growth. At 60 mmol L^–1, root and shoot growth were maximum when root temperature was kept at the intermediate level of 15°C; most inhibition of salinity occurred at both low (10°C) and high (25°C) root temperatures. As the root temperature was raised from 10 to 25°C, the concentration of Na generally decreased in the tops and increased in the roots. At a given Na concentration in the tops or in the roots, respective growth of tops or roots was much less inhibited if the roots were grown at 15–20°C. It is concluded that the tolerance of barley plant to NaCl salinity of the rooting media appears to be altered by the root temperature and is highest if the root temperature is kept at 15 to 20°C.     

Changes in the Composition of Xylem Sap during Development of the Spadix of Skunk Cabbage (Symplocarpus foetidus)

The spadix of skunk cabbage, Symplocarpus foetidus, is thermogenic and maintains an internal temperature of around 20 °C even when the ambient air temperature drops below freezing. This homeothermic heat production is observed only during the stigma stage, and thereafter ceases at the male stage when pollen is shed. To clarify the regulatory mechanism by which the stigma stage-specific heat production occurs in the spadix, sugars, organic acids, and amino acids in xylem sap were analyzed and compared with those of post-thermogenic plants. Interestingly, no significant difference was observed in the total volume of xylem sap per fresh weight of the spadix between thermogenic (31.2±24.7 μl h^?1g^?1) and post-thermogenic (50.5±30.4 μl h^?1g^?1) plants. However, concentrations of sugars (sucrose, glucose, and fructose), organic acids (malate and succinate), and amino acids (Asp, Asn, Glu, Gln, Gly, and Ala) in xylem sap decreased remarkably in post-thermogenic plants. Our results indicate that the composition of the xylem sap differs during the development of the spadix of S. foetidus.     

Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO(3)-grown) Cowpea [Vigna unguiculata (L.) Walp.

The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO₃ levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO₃ reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO₃ showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO₃. These data, and the absence in the plants of significant pools of stored NO₃, indicated that shoots were major organs of NO₃ assimilation, especially at levels of NO₃ (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO₃ plants and non-nodulated plants fed 10 or 20 millimolar NO₃, the levels of NO₃ which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO₃-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO₃-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO₃-fed plants. Theoretical considerations suggest that the elevated CO₂ output of nodulated roots was due not only to CO₂ loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO₃-fed plants. Data are compared with previously published information from other legumes.     

Transport and Partitioning of CO(2) Fixed by Root Nodules of Ureide and Amide Producing Legumes

Nodulated and denodulated roots of adzuki bean (Vigna angularis), soybean (Glycine max), and alfalfa (Medicago sativa) were exposed to ¹⁴CO₂ to investigate the contribution of nodule CO₂ fixation to assimilation and transport of fixed nitrogen. The distribution of radioactivity in xylem sap and partitioning of carbon fixed by nodules to the whole plant were measured. Radioactivity in the xylem sap of nodulated soybean and adzuki bean was located primarily (70 to 87%) in the acid fraction while the basic (amino acid) fraction contained 10 to 22%. In contrast, radioactivity in the xylem sap of nodulated alfalfa was primarily in amino acids with about 20% in organic acids. Total ureide concentration was 8.1, 4.7, and 0.0 micromoles per milliliter xylem sap for soybean, adzuki bean, and alfalfa, respectively. While the major nitrogen transport products in soybeans and adzuki beans are ureides, this class of metabolites contained less than 20% of the total radioactivity. When nodules of plants were removed, radioactivity in xylem sap decreased by 90% or more. Pulse-chase experiments indicated that CO₂ fixed by nodules was rapidly transported to shoots and incorporated into acid stable constituents. The data are consistent with a role for nodule CO₂ fixation providing carbon for the assimilation and transport of fixed nitrogen in amide-based legumes. In contrast, CO₂ fixation by nodules of ureide transporting legumes appears to contribute little to assimilation and transport of fixed nitrogen.     

Effect of Defoliation or Excision of Growing Axillary Shoots on the Composition of Labeled Products of Photosynthesis in the Leaves and Xylem Sap of Kidney Bean

The content of ¹⁴C in the products of photosynthesis of the source leaf and xylem sap was investigated in kidney bean (Phaseolus vulgaris L.) plants during the stage of mass tillering. ¹⁴C partition was measured a day after two-minute photoassimilation of ¹⁴CO₂ by an individual mature leaf located in the middle part of the shoot. The source-sink relations were disturbed by the excision of all mature leaves (except the source leaf) or all growing axillary shoots. The leaves or growing axillary shoots were excised 15 min after leaf feeding with ¹⁴C₂. A day later, in plants with excised growing axillary shoots, the content of ¹⁴C in the source leaf was by 18% higher and in those with removed leaves by 15% lower than in control plants. The next day after the excision of growing axillary shoots, radioactivity of the xylem sap increased; after defoliation, both the volume of the xylem sap and its specific radioactivity decreased. In the xylem sap of defoliated plants, the proportion of ¹⁴C in malate decreased more than six times, whereas the proportion of ¹⁴C in amino acids somewhat increased (1.5 times). In two days, the volume of the xylem sap exuded by treated plants became the same as in control plants and its radioactivity decreased almost by an order of magnitude but essentially did not differ in the both types of treatment. It is concluded that the processes occurring in the roots are governed by photosynthesis but its regulatory effect is limited by a photoperiod and largely depends on changes in the ratio between biosynthesis of amino acids in the roots and leaves.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 518–521.Original Russian Text Copyright © 2005 by Chikov, Bakirova, Batasheva, Sergeeva.     

The Effects of Temperature on the Exudation Process of Excised Primary Roots of Zea mays

The fluid exudation rates and the ionic compositions of theexudates of excised maize roots have been determined in bathingmedia of 0.1 mM, 1.0 mM, 10.0 mM, and 50.0 mM KCl, each containing0.1 mM CaCl₂, at temperature intervals between 10 °C and30 °C.Analysis of these data in terms of an osmotic modelfor excised root exudation shows that the observed temperaturevariation in fluid exudation rate is accounted for by the observedtemperature variation in the osmotic driving force, the saltconcentration difference from xylem fluid to bathing medium.Temperature variation in the osmotic permeability of the rootand of the non-osmotic water flow are not significantly differentfrom zero.