Evaluation of the 15N Natural Abundance Method and Xylem Sap Analysis for Assessing N2 Fixation of Understorey Legumes in Jarrah (Eucalyptus marginata Donn ex Sm.) Forest in S.W. Australia

Hansen, A. P. and Pate, J. S. 1987. Evaluation of the ¹⁵N naturalabundance method and xylem sap analysis for assessing N₂ fixationof understorey legumes in jarrah (Eucalyptus marginata Donnex Sm.) forest in S.W. Australia.—J. exp. Bot 38: 1446–1458. Nodulated seedlings of Acacia pulchella, A. alata and A. extensawere grown in glasshouse sand culture under a range of levels(0–16 mol m³) of nitrate, supplied as ¹⁵NO₃, or as unenrichedlaboratory grade nitrate (¹⁵N value 5·5%_o). Nitrate at8·0 mol m ³ or above was highly inhibitory to growthof all species. Using ¹⁵N dilution analysis of the ¹⁵N enrichedcultures to measure symbiotic dependency, it was shown that¹⁵N values of the parallel unenriched cultures increased innear linear fashion from close to zero in fully symbiotic plantsto values close to that of the supplied NO₃ in plants experiencingnitrate levels (4·0 mol m³ or above) inhibiting N₂ fixationby over 90%. Xylem sap analyses (0·4 mol m³ NO₃ treatments)showed asparagine as the major nitrogenous solute, relativelylittle spill-over of free nitrate, and no evidence of majorshifts in balance of amino compounds with increasing dependenceon nitrate. This essentially invalidated use of the techniqueas a field assay for N₂ fixation by the species. ¹⁵N values for total N of soil sampled at 64 widely distributedsites in jarrah forest ranged from – 2·15 to +5·4(mean +2·1). Comparable values for soil mineral N (NH⁺₄and NO₃) were +0·3 to + 14·2 (mean +5·1).¹⁵N values of the total plant N of the legumes and of non-N₂-fixingreference species were also highly variable between sites, withlittle evidence of reference plant N accurately reflecting the¹⁵N abundance of soil nitrogen, or of visibly well nodulatedlegume components showing consistently lower ¹⁵N values thantheir companion reference plants. At one site it was possibleto compare ¹⁵N values of first season seedling legumes withpreviously published estimates of their progressive N₂ fixationusing C₂H₂ reduction assays. It was concluded that heterogeneity in ¹⁵N discrimination ofsoil within the ecosystem precluded effective use of the ¹⁵Nnatural abundance technique for assessing legume N₂ fixation. Key words: Acacia spp., ¹⁵N natural abundance,, xylem sap analysis,, nitrogen fixation.     

Effects and Interactions of Ozone and the Anti-ozonant EDU at Different Stages of Radish {Raphanus sativus L.) Development

The use of the anti-ozonant ethylenediurea (EDU) to assess croplosses due to atmospheric ozone (O₃), is controversial becauseof incomplete knowledge on the optimal dosage, the extent ofprotection provided and potential effects on plant performanceof EDU per se. In a greenhouse experiment, radish (Raphanussativus L.) cv. ‘Cherry Belle’ plants were treatedwith EDU as a soil drench (150 mg1¹, 60 ml plant¹) at threestages of development: early, maximum and late hypocotyl thickening.Two days later the plants were exposed to O₃, mimicking an ambientpattern (70 nl 1¹ O₃, 7 h d¹, 5 d, and up to 120 nl 1¹ O₃ on2 d), or remained in filtered air. Assessment of leaf injury showed complete protection from ozone-inducedleaf damage at all stages of growth due to EDU. EDU did causeslight leaf margin necrosis and hyponastic leaf deformationirrespective of the atmospheric treatment, but biomass and relativegrowth rate (RGR) of the shoot and leaf number duration werenot altered and leaf area duration was increased when comparedto control plants. EDU did not affect dry weight accumulationnor RGR of the hypocotyl. Ozone-induced reductions in growthof below-ground organs (hypocotyl and root), which were mostprominent in the intermediate and early stage of growth, werelargely suppressed by the EDU-treatment. Decreases of hypocotylbiomass due to O₃ were transient and significant increases inRGR of the hypocotyl during the post-exposure period indicateda substantial capability of compensation during the period ofrecovery. Key words: Radish (Raphanus sativus), ozone, anti-ozonant, ethylenediurea (EDU), development, phytotoxicity     

The Composition of Phloem Exudate and Xylem Sap from Broccoli (Brassica oleracea var. italica) Supplied with NH4+, NO""3 or NH4NO3

Shelp, B. J. 1987. The composition of phloem exudate and xylemsap from broccoli (Brassica oleracea var. italica) suppliedwith NH⁺₄, NO₃ or NH₄NO₃.—J. exp. Bot. 38: 1619–1636. The detailed composition of xylem sap and exudate from stemincisions of attached inflorescences of broccoli (Brassica oleraceavar. italica) was compared in plants supplied with NH⁺₄, NO₃or NH₄NO₃. A phloem origin for the exudate was suggested fromthe high levels of sugars (71–133 mg cm^-3), amino acids(8·1-26·7 mg cm³) and K. (2·3–3·8mg cm³), the low levels of NO₃ and Ca, the high C: N (w/w) ratios(8·3–33), and the alkaline pH (7·2–7·3).In contrast, the xylem sap was mildly acidic (pH 5·6–6·0),and possessed lower levels of all organic and inorganic solutesbut NO₃ and Ca, and lower ratios of K: Ca, Mg: Ca and C: N (0·6–4·4). Glutamine was the predominant o-phthalaldehyde-reactive aminocompound in both transport fluids with the next most abundantamino acids dependent on sap type and N-form. Together witharginine, -aminobutyric acid, which was found only in the xylemstream, was enhanced by NH⁺₄compared to NO₃ -nutrition suggestingthat glutamate metabolism was stimulated in the roots. Underlimiting N the amino acid concentrations in the transport fluidswere greater with NH⁺₄ than with NO₃. NO₃ reduction occurredin both the root and shoot with the latter site predominatingover the entire N range (0-300 mol m³). Even though the compositionof nitrogenous solutes in the xylem was dependent on cultivarand N source, the composition of the phloem streams supplyingthe developing inflorescence was relatively unaffected. The data on the element composition of organs and phloem sapare interpreted to suggest that, in spite of the restrictedmobility of some elements such as B and Mn, a significant proportionof their total supply to developing sinks is carried in thephloem stream. Key words: Transport fluid composition, plant nutrition, phloem mobility.     

Wheat Response to CO2 Enrichment: Growth and CO2 Exchanges at Two Plant Densities

Du Cloux, H. C, André, M., Daguenet, A. and Massinuno,J. 1987. Wheat response to CO₂ enrichment: Growth and CO₂ exchangesat two plant densities.—J. exp. Bot. 38: 1421–1431. The vegetative growth of wheat (Triticum aestivum L., var. Capitole)was followed for almost 40 d after germination in controlledconditions. Four different treatments were carried out by combiningtwo air concentrations of CO₂, either normal (330 mm³ dm ³)or doubled (660 mm³ dm ³) with two plant densities, either 200plants m ² or 40 plants m ². Throughout the experiment the CO₂gas exchanges of each canopy were measured 24 h d¹. These provideda continuous growth curve for each treatment, which were comparedwith dry weights. After a small stimulation at the start (first13 d), no further effect of CO₂ enrichment was observed on relativegrowth rate (RGR). However, RGR was stimulated throughout theexperiment when plotted as a function of biomass. The finalstimulation ol dry weight at 660 mm³ dm ³ CO₂ was a factor of1·45 at high density and 1·50 at low density,contrary to other studies, no diminution of this CO₂ effecton dry weight was observed over time. Nevertheless, at low density,a transient additional enhancement of biomass (up to 1·70)was obtained at a leaf area index (LAI) below 1. This effectwas attributed to a different build up of the gain of carbonin the case of an isolated plant or a closed canopy. In theformer, the stimulation of leaf area and the net assimilationrate are both involved; in the latter the enhancement becomesindependent of the effect on leaf area because the canopy photosynthesisper unit ground area as a function of LAI reaches a plateau. Key words: Triticum aestuum, L. var. Capitole, Vegetative growth, Canopy     

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napusL.) in Flowing Solution Culture: II. CATION-ANION BALANCE

Macduff, J. H., Hopper, M. J., Wild, A. and Trim, F. E. 1987.Comparison of the effects of root temperature on nitrate andammonium nutrition of oilseed rape (Brassica napusL.) in flowingsolution culture. II. Cation-anion balance.—J. exp. Bot.38: 1589-1602. The effects of root temperature and form of N nutrition (NH4or NOJ) on the mineral composition of the plant, the balanceof inorganic cation-anion uptake and on the apparent net effluxof H ⁺/OH^–ions from the roots were studied with 49-d-oldoilseed rape (Brassica napusL. cv. Bien venu) in flowing solutionculture. Plants were pre-treated for 14 d at a root temperatureof 5 °C prior to constant root temperatures of 3, 7, 11or 17°C for 14 d, with a common shoot temperature of 20/15°Cday/night. Nitrogen was supplied as NH⁺₄4 or NO^–₃ at 10mmol m³. Values of Q₁₀ (7-17°C) for mean unit absorptionrates of all the major nutrient ions (K⁺ , Mg⁺⁺ , NH⁺₄, SO₄,H₂PO₄, NO₃), except Ca⁺⁺, were > 2.0 over the first 5 d oftreatment but thereafter were < 1.5; the apparent effectof temperature on uptake rates diminished with time. Under NH⁺₄nutrition, inorganic cation uptake (Mg^{+ +} + K⁺+Ca^{+ +} +NH⁺₄)exceeded inorganic anion uptake (SO4 ^–₄+ H₂PO₄) over 14d at all temperatures, with the proportion of cation uptakeas NH4 remaining constant (0.67-0-68) irrespective of root temperature.The net efflux of H ⁺ from the roots approximately balancedNH⁺₄ uptake (1:1) over 14 d at each temperature and also balancedthe difference between the total uptake of inorganic cationsand inorganic anions. Under NO₃ nutrition, the sum of the netefflux of OH and the change in the carboxylate contents of plantsover 14 d approximately balanced the sum of NO₃ and SO ^–₄reduced in the plant. The majority of the negative charge associatedwith the reduction of NO₃ and SO ^–₄ was apparently effluxedas OH, but this fraction was lower at low root temperatures.The results are discussed in terms of mechanisms that have beenproposed to regulate the internal pH of plants. Key words: Brassica napus, oilseed rape, root temperature, cation-anion balance, H⁺/OH efflux.     

Stomatal Control of Photosynthesis of Eucalyptus globulus Labill. Trees under Field Conditions in Portugal

Pereira, J. S., Tenhunen, J. D. and Lange, O. L. 1987. Stomatalcontrol of photosynthesis of Eucalyptus globulus Labill. treesunder field conditions in Portugal.—J. exp. Bot. 38: 1678–1688. Stomatal behaviour of adult leaves of Eucalyptus globulus treeswas studied under field conditions in Portugal. In the absenceof severe plant water stress stomata were open when the summedtotal of photosynthetically active photon flux density incidenton both leaf surfaces was above 100 µmol m²s¹ and leafconductance to water vapour reached 245 mmol m ² s¹ on a total(both epidermes) leaf area basis. The stomata of both leaf epidermesresponded similarly to changes in solar radiation and waterstress. Water stress resulted in decreasing daily maxima inleaf conductance as predawn leaf water potential decreased.Maximal leaf conductance decreased to less than 50 mmol m ²s ¹ when predawn leaf water potential decreased below —1·0MPa. At similar values of predawn leaf water potential stomatawere more closed as the leaf to air water vapour partial pressuredifference increased. The effect of increasing air dryness onstomata was greatest at high predawn leaf water potential. Dailymaxima in photosynthetic rates and in leaf conductance werelinearly related to one another in spring and summer. Both decreasedwith increase in leaf water stress. In autumn and winter, increasesin leaf conductance occurring under natural conditions duringthe course of the day were not necessarily accompanied by increasesin net photosynthesis. Stomata were more closed in the afternoonthan in the morning at the same rates of net photosynthesis,temperature or leaf to air water vapour partial pressure difference. Key words: Eucalyptus globulus,, photosynthesis, stomata, water stress.     

Plant Characteristics and Nutrient Composition and Mobility of Broccoli (Brassica oleracea var. italica) Supplied with NH4+, NO3"" or NH4NO3

Shelp, B. J. 1987. Plant characteristics and nutrient compositionand mobility of broccoli (Brassica oleracea var. italica) suppliedwith NH⁺₄, NO₃ or NH₄NO₃.—J. exp. Bot. 38: 1603–1618. The effects of varying NH⁺₄, NO₃ or NH₄NO₃ concentration onthe final plant characteristics, element composition, and accumulationof NO₃-N, NH⁺₄-N and organic-N were evaluated in broccoli (Brassicaoleracea var. italicacv. Futura and Premium Crop) plants culturedin vermiculite under greenhouse conditions supplemented withlight. NH⁺₄-grown plants were stunted and exhibited signs ofmarginal necrosis on the old leaves, accompanied by an accumulationof NH4. The tissue levels of N, P, Mn, Cu, Zn and B were generallyincreased by NH⁺₄ versus NO₃ nutrition whereas the reverse wastrue for Ca; Mg and K were only slightly affected, if at all.These results are attributed to: changes in element availabilityresulting from reduced rhizosphere pH due to NH⁺₄uptake ratherthan NO ₃uptake; competition of Ca uptake by NH⁺₄; and dilutionof N by increased vegetative growth with NO₃-nutrition. Theelement concentrations of N, P or K were similar in all tissueswhereas Ca, B and Mn were markedly less in the florets and youngleaves compared to mature leaves; this supports literature indicatingthat the former elements are phloem-mobile whereas the latterare not. Assuming that the nutrient supply for mature leavesis delivered principally via the xylem stream, the data suggestthat nutrients for developing leaves and florets are suppliedpredominantly in the phloem. If so, under our experimental conditions.Zn and Cu were also readily mobile in the phloem whereas Mgmovement was restricted. NH₄⁺ versus NO₄⁺ J nutrition alteredthe distribution of these elements. The two broccoli cultivarstested under the greenhouse environment differed in NH⁺₄ toleranceand in the distribution of K and Cu suggesting there was a geneticbasis for cultivar variation in mineral acquisition and redistribution. Key words: Plant nutrition, phloem mobility, elemental composition.     

Sugar Efflux from Attached Seed Coats of Glycine max (L.) Men.

Ellis, E. C. and Spanswick, R. M. 1987. Sugar efflux from attachedseed coats of Glycine max (L.) Merr.—J. exp. Bot. 38:1470—1483. Sugar efflux (sucrose + glucose) from attached seed coats ofGlycine max (L.) Merr. was measured at high sampling rates toimprove the kinetic characterization of seed coat exudation.This study confirms that sugar efflux in seed coats has at leasttwo components, and demonstrates that the concentration of mannitolosmoticum bathing the seed coat may influence one or both ofthese components. High leaf irradiance increased sugar effluxrelative to a low leaf irradiance at the same mannitol concentration.A high concentration of mannitol (500 mol m³) enhanced sugarefflux relative to a medium concentration (100 mol m³) underboth high and low leaf irradiance. A low mannitol concentration(10 mol m³) stimulated sugar efflux (relative to 100 mol m³)to a greater extent when leaf irradiance was high. Rapid changesin mannitol concentration produced immediate stimulations ofsugar efflux. Effects of osmoticum on sugar efflux are explainedby simultaneous turgor-mediated effects on import of sucroseby the phloem and retrieval of apoplastic sucrose, presumablyby seed coat parenchyma.     

Alternative Respiration and Heat Production in Ripening Banana Fruits (Musa paradisiaca van Mysore Kadali)

The involvement of alternative respiration in thermogenesisduring the ripening of banana {Musa paradisiaca var. MysoreKadali) fruits, attached to a bunch, has been examined. Thetemperature of the youngest (unripened) banana fruit increasedfrom 27·0 ± 0·2°C to 30·8±0·1°C and the total respiration (in nmo1 oxygen min¹ g¹ drywt.) increased from 1·39·6 ± 5·5to 167·3 ± 7·0 at the fully ripened stage.Although the capacity for alternative respiration showed littlechange, the actual operation of this pathway increased from38 to 73% (p= 0·38 to 0·73) during ripening. Similarresults were obtained in fruits along the central axis at differentstages of ripening. It is suggested that alternative respirationmay contribute to the temperature rise observed in ripeningbanana fruit. Key words: Alternative respiration, tehrmogenesis, fruit ripening     

Comparative Ecophysiology and Anatomy of Terrestrial and Epiphytic Anthurium bredemeyeri Schott in a Tropical Andean Cloud Forest

Water relations and anatomy of a casual epiphyte were studiedat La Carbonera, a tropical cloud forest. Anthurium bredemeyerigrowing as an epiphyte and in its terrestrial form were studiedto find differences due to their different habits. Both formsmaintained relatively high leaf conductances (0·12 to0·15 mol m² s¹) when leaf ater potential was relativelyhigh (above – 0·5 MPa). A lowering of the leafwater potential (below – 0·5 MPa) during the dryseason, significantly affected leaf conductances in both terrestrialand epiphytic forms, the latter one to a greater degree. Interms of anatomy, a reduction in stomatal density was observedin the epiphyte, although no other differences were observed.The results show how the epiphyte was affected to a greaterdegree by a decrease in water availability during the dry seasoncompared to the terrestrial form. Key words: Anthurium bredemeyeri, epiphyte, water relations, anatomy     

A comparison of the impacts of various nitrogen sources on acid-base balance in C3 Triticum aestivum L. and C4 Zea mays L. plants

This work aimed to study the impacts of acquisition and assimilationof various nitrogen sources, i.e. NO₃^–, NH₄⁺ or NH₄NO₃,in combination with gaseous NH₃ on plant growth and acid-basebalance in higher plants. Plants of C₃ Triticum aestivum L.and C₄ Zea mays L. grown with shoots in ambient air in hydroponicculture solutions with 2 mol m^–3 of nitrogen source asNO₃^–, NH₄⁺ or NH₄NO₃ for 21 d and 18 d, respectively,had their shoots exposed either to 320 µg m^–3 NH₃or to ambient air for 7 d. Variations in plant growth (leaves,stubble and roots), and OH^– and H⁺ extrusions as wellas the relative increases in nitrogen, carbon and carboxylatewere determined. These data were computed as H⁺/N, H⁺/C, (C-A)/N,and (C-A)/C to analyse influences of different nitrogen sourceson acid-base balance in C₃ Triticum aestivum and C₄ Zea maysplants. Root growth in dry weight gain was significantly reduced bytreatment with 320 µg m^–3 NH₃ in Triticum aestivumand Zea mays growing with different N-forms, whereas leaf growthwas not significantly affected by NH₃. In comparison with C₃Triticum aestivum, non-fumigated C₄ Zea mays had low ratiosof OH^–/N in NO₃^–3-grown plants and of H⁺/N in NH₄⁺- and NH ₄NO₃-grown plants. Utilization of NH₃ from the atmospherereduced both the OH^–N ratios in NO₃^– -grown plantsand the H⁺/N ratio in NH₄⁺ - and NH₄NO₃ -grown plants of bothspecies. Furthermore, Zea mays had higher ratios of (C-A)/Nin NH₄⁺ - and NH₄NO₃-grown plants than Triticum aestivum. Thismeans that C₄ Zea mays had synthesized more organic anion perunit increase in organic N than C₃ Triticum aestivum plants.Within both species, different nitrogen sources altered theratios of (C-A)/N in the order: NH₄NO₃>NH₄⁺>NO₃^–.Fumigation with NH₃ increased organic acid synthesis in NO₃^–- and NH₄⁺ - grown plants of Triticum aestivum, whereas it decreasedorganic acid synthesis in Zea mays plants under the same conditions.Furthermore, these differences in acid-base regulation betweenC₃ Triticum aestivum and C₄ Zea mays plants growing with differentnitrogen sources are discussed. Key words: Acid-base balance, ammonia, ammonium, nitrate, ammonium nitrate, C₃ Triticum aestivum L., C₄ Zea mays L.     

Development of Oilseed Rape Buds, Flowers, and Pods In Vitro

A technique for growing buds, flowers, and pods of oilseed rape(Brassica napus L. cv. Haplona) on stem explants in vitro hasbeen developed. Open flowers and young pods underwent normaldevelopment on a basal medium of minerals, vitamins, and sucrosebut the development of buds was less successful. Young buds(3 mm long) did not develop and only limited development ofthe older buds (5 mm long) took place. Some 3 mm-long buds wereinduced to develop to open flowers by adding naphthyl aceticacid or gibberellic acid. Pod and seed set in open flowers werenot affected by adding plant growth substances to the medium,but pod elongation and pod dry weight were promoted by gibberellicacid, 105 M, and benzyl amino purine, 10⁷ M, respectively. Reducingthe supply of sucrose or minerals to open flowers reduced seedset, pod elongation and pod weight but did not affect pod set.The physiological significance of the results is discussed. Key words: In vitro cultures, oilseed rape, pod development, flower development     

Nitrate Nutrition of Grasses from Steady-State Supplies in Flowing Solution Culture following Nitrate Deprivation and/or Defoliation: II. ASSIMILATION OF AND SHORT-TERM EFFECTS ON UPTAKE

Plants of two genotypes of Lolium perenne L. cv. S23 and a L.perenne ? L. multiflorum Lam. hybrid cv. Augusta were grownin flowing solution culture. N was suppled in one treatmentat 10 mmol m^–3 NO^–3 throughout (HN), and in another(LN) the N supply was terminated after 10 d for 11 d. When was re-supplied both LN and HN plants were leftentire or defoliated. The two genotypes showed similar responsesto all treatments. The concentration of N in shoot dry matterdeclined from 4.4% to 2.0% and in the root from 2.8% to 1.0%over the 11 d of N deprivation, with 95% of the initially present being assimilated during this period. LN plantsassimilated 10% more of their total uptake than did HN plants. The in vitro nitrate reductase activity(NRA) was 10- to 50-fold higher in the youngest fully-expandedleaves than in roots and declined in the leaves during N deprivation.Between 2–6 d after defoliation, there was a large increasein NRA in leaves of HN (but not LN) plants. After defoliationof HN plants, net uptake from 10 mmol m^–3 declined to negligible levels within 15 h, but in defoliatedLN plants it increased to levels similar to those of entireHN plants (15–20 µmol h^–1 g^–1 fr. wt.root) within 8 h. When was re-supplied to entire LN plants, uptake of increased to levels similar to those of entire HN plants within 2.3 h, butdid not markedly exceed that of HN plants for at least 10 h.Net uptake of by LN plants during depletion of stirred static nutrient solutions containing 1.0 mol m^–3 lagged behind that by HN plants by 70–100 min, but the maximum unit absorption rate was similar for LNand HN plants (5–7 µmol h^–1 g^–1 fr.wt. root). The nature of the short-term demand for uptake following recovery from the stresses of defoliation andN starvation is discussed. Key words: Lolium perenne, Lolium multiflorum, N-deficiency, defoliation, nitrate uptake, nitrate reductase, N-assimilation     

Cellular Cobalt Fluxes in Roots and Transport to the Shoots of Wheat Seedlings

Macklon, A. E. S. and Sim, A. 1987. Cellular cobalt fluxes inroots and transport to the shoots of wheat seedlings.—J.exp. Bot. 38: 1663–1677. From compartmental analysis of radioisotope elution measurements,concentrations and fluxes of cobalt were estimated for corticalcells of wheat (Triticum aestivum L. cv. Fenman) seedling roots,relative to a complete nutrient solution containing 20 mmolm³ Co^{2 +} . The water free space, Donnan free space and cytoplasmcontained little Co relative to the high levels accumulatedin the vacuole, resulting in time courses of Co uptake whichwere essentially linear in continuous light. In light/dark cyclesthere was almost complete inhibition of uptake during the darkperiods. Comparison of estimated concentrations of Co in cytoplasmand vacuole with values to be expected, if passive diffusionalone was operative, indicated that active transport was occurringin an outward direction at the plasmalemma and into the vacuoleat the tonoplast. Transport to the shoot, which was unaffectedby transpirational water flux through the plant, reached about10% of root absorbed Co, most of which was stored in the corticalcell vacuoles and removed from the transport pathway. Between0·02 and 20 mmol m ³ Co in external solution, uptakewas linearly related to concentration on a log/log plot, andthe proportion of transported Co was constant. The implicationsof these findings are discussed. Key words: Cobalt, transmembrane fluxes, transport.     

Short-term Effects of Ferulic Acid on Ion Uptake and Water Relations in Cucumber Seedlings

Ferulic acid (FA) is commonly found in soils and is consideredan allelochemical. Studies have suggested that FA and otherphenolic acids decrease plant growth in part by decreasing theabsorption of mineral nutrients and water. However, no studieshave examined these parameters in a single experimental systemto investigate how FA affected both ion uptake and plant-waterrelations in whole plants. Using intact cucumber (Cucumis sativusL. cv. Early Green Cluster) seedlings, we examined short-termeffects of FA on ion uptake kinetics, transport promoters andinhibitors, and water relations as indicated by a pressure-volumeanalysis. We found that after 3 h of treatment, 200 µMFA inhibited net ion uptake, particularly NO₃, and promotednet K⁺ efflux from seedling roots. The addition of fusicoccin,a K⁺ transport promoter, counteracted the inhibitory effectof FA on net K⁺ uptake. Concurrent treatment of seedlings withFA and tetraethylammonium, a channel-blocking salt, reducedaverage K⁺ efflux by 66%. Treatment of seedlings with FA alsodecreased leaf water potential (₁ and turgor pressure (P_T).However, decreased ₁ and P_T were not caused by changes in theosmotic properties of the symplast or stomatal conductance.A decrease in water absorption is a likely explanation for theloss of P_T observed. The results of our experiments indicatethat both ion uptake and plant-water relations can be adverselyaffected by FA. Key words: Cucumis sativus, ferulic acid, allelopathy, ion uptake, water relations     

Ion-Exchange Properties of Isolated Cell Walls of Brown Algae: The Interstitial Solution

Kloareg, B., Demarty, M. and Mabeau, S. 1987. Ion-exchange propertiesof isolated cell walls of brown algae: the interstitial solution.—J.exp. Bot. 38: 1652–1662. Isolated cell walls of Pelvetia canaliculata (Dene) et Thur.,Laminaria digitata (L.) and other intertidal brown algae wereequilibrated in seawater and various mixtures of sodium andcalcium chlorides. After elution with distilled water, the interstitialsolution was assayed for ionic composition. With respect tothe external solution, it was less concentrated and containedlower proportions of divalent cations. Such modifications wererelated to the divalent ionic fraction and the total anion concentrationof the external solution. No significant differences were foundamong the brown algae investigated in spite of the differingcomposition of their walls. Results were analysed accordingto the polyelectrolyte condensation model. When the concentrationof the external solution was high (1000 meq dm³), experimentaldata were qualitatively in good agreement with the predictionsof the model. Key words: Cell walls, ion-exchange, Phaeophyta.     

The Uptake of Nitrate by Lolium perenne from Flowing Nutrient Solution: I. EFFECT OF CONCENTRATION

Experiments with simulated swards of perennial ryegrass (Loliumperenne L.) show the relationship between concentration in flowing nutrient solution, nitrate uptake,plant growth, and the chemical composition of roots and shoots.Rates of uptake exceeding 1 g N m^–2 d^–1 were maintainedat concentrations in solution down to 0•02 mg N l^–1. Short-term studies confirmed that at such lowconcentrations the plants were able to maintain rates of uptakeof about 85% of maximum. Between 0•2 and 200 mg N l^–1the concentration of in solution had little effect on rate of uptake or plant growth. With at 1000 and 2000 mg N l^–1 there was a marked reductionin weight of the shoots and, more particularly, in the lengthand tensile strength of the roots. There were several significanttrends in mineral composition of the plants (notably in S, Ca,Mg) which were apparently correlated with increasing concentrationof in solution.     

Relative Uptake Rates of Inorganic Nutrients by NO3- and NH4+ Grown Ricinus communis and by two Plantago Species

Allen, S., Thomas, G. E. and Raven, J. A. 1986. Relative uptakerates of inorganic nutrients by and grown Ricinus communis and by two Plantago species.—J. exp. Bot. 37: 419–428. The relative rates of uptake and assimilation of C, N, P, S,Cl^–, K⁺ , Na⁺ Ca²⁺ and Mg²⁺ by and grown Ricinus conimunisand by NH₄NO₃- grown Plantago lanceolata and P. major were calculatedfrom data presented elsewhere. Results showed that for grown Ricinus the short term relativeuptake rates, for each nutrient X did not change significantly over the steady-state periodof exponential growth. The average gave , the mean relative uptake rate during exponential growth, for each nutrient. The amountof each nutrient taken up from a nutrient solution over a periodof time could, therefore, be calculated. For and -grown R. communis,the relative uptake rate of each nutrient was a constant fractionof the relative rate of carbon assimilation. It is suggestedthat this is typical of plants of cauline habit. For both Plantago spp., the relative rates of nitrogen uptakeand assimilation fell significantly during the exponential growthphase It is suggested that this could be a characteristic ofthe growth habit of the rosette plant. Key words: Relative uptake rates, Ricinus, Plantago, ammonium, nitrate, cauline, rosette     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.