Involvement of cellulose synthesis in actions of gibberellin and kinetin on cell expansion. Gibberellin-coumarin and kinetin-coumarin interactions on stem elongation

In azuki bean (Azukia angularis = Vignia angularis) epicotylsections, 5 ? 10^–4 M coumarin inhibited the incorporationof radioactivity from [U^–14C]glucose into the cellulosefraction by 35% in the absence of indole-3-acetic acid (IAA)and by 40% in the presence of 1 ? 10^–4 M IAA. There wasno inhibitory effect on the incorporation of radioactivity intothe other fractions. Coumarin at 5 ? 10^–4 M reversed thepromoting effect of 1 ? 10^–5 M gibberellin A₃ (GA) andthe inhibitory effect of 1 ? 10^–5 M kinetin on IAA-inducedelongation of sections with no significant effects on IAA-inducedelongation. Neither GA nor kinetin had any appreciable effectson cellulose synthesis. No inhibition of cellulose syntheiswas observed with 1 ? 10^–3 M colchichine, which has beenreported to have effects similar to those of coumarin on GA-or kinetin-affected stem elongation. Coumarin at 5 ? 10^–4M was ineffectual in breaking up wall microtubules, while adisrupting effect on wall microtubules was clearly demonstratedwith 3 ? 10^–4M colchicine. From these results, the possible involvement of cellulose synthesisin cell expansion controlled by GA or kinetin was suggested. (Received August 3, 1973; )     

Rate of Uptake of Potassium by Three Crop Species in Relation to Growth

Barley, ryegrass, and fodder radish were grown in flowing nutrientsolutions at four potassium concentrations, [K_e⁺], from 0.05to 4 mg I^–1. During the first 2 weeks after germinationthe response to [K_e⁺] (fodder radish > barley > ryegrass)depended on the potential relative growth rate, the ratio ofroot surface area to plant weight, and on the K⁺ flux into theroots. Subsequently, there was no effect of [K_e⁺] on growthrate within the range tested. The K⁺ flux decreased from 4–23? 10^–12 mol cm^–2 s^–1 in the first 2 weeksafter germination, when it was concentration-dependent, to 2–5? 10^–12 mol cm^–2 s^–1 after 4–5 weeks,when it became independent of [K_e⁺] down to 0.05 mg 1^–1.The results explain the importance of high [K_e⁺] and rapid rootgrowth during the first 2 weeks after seed germination.     

Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings VI. Promotive and inhibitory effects of kinetin

The interaction of kinetin with IAA and GA₃ on the elongationof hypocotyl sections of Cucumis sativus L. cv. National Picklingwas studied. Kinetin in the concentration range of 10^–7M to 10^–4 M markedly inhibited IAA-induced elongation,while in a lower range from 10^–10 M to 10^–8 M, itsynergistically enhanced IAA-induced elongation. Kinetin alonein this range had no effect. A 5-to 15-min pulse treatment seemsenough to induce the maximum effect for both inhibition andpromotion. Since the magnitude of the maximum inhibition dependedon the concentration and not on the duration of treatment, thereaction in the cell caused by kinetin seemed to be completedwithin a short period. Washing of the sections with distilledwater after kinetin treatment (30 min) did not significantlyeliminate the kinetin effect. This probably indicates that thebinding of kinetin molecules to a supposed acceptor is not reversible.Interaction of kinetin with GA₃ in their pretreatment effectson IAA-induced elongation shows that in the inhibitory concentrationrange, the kinetin effect was partly overcome by GA₃, and thatin the promotive range, the magnitude of the enhancement wasdetermined by kinetin regardless of the presence of GA₃. Theeffect of kinetin seems to dominate over that of GA₃ indicatingthat the modes of their pretreatment effects differ from oneanother. (Received June 24, 1977; )     

The Conductance of the Plasmalemma for CO2

Permeability coefficients (P_S values) for CO₂ of the plasmamembrane (PM) of the unicellular green algae Eremosphaera viridis,Dunaliella parva, and Dunaliella acidophila, and of mesophyllprotoplasts isolated from Valerianella locusta were determinedfrom ¹⁴CO₂ uptake experiments using the rapid separation ofcells by the silicone oil layer centrifugation technique. Theexperimental P_S values were compared with calculated numbersobtained by interpolation of Collander plots, which are basedon lipid solubility and molecular size, for D. parva cells,mesophyll protoplasts isolated from Spinacia oleracea, mesophyllcells and guard cells of Valerianella, and guard cell protoplastsisolated from Vicia faba. The conductivity of algal plasma membranes for CO₂ varies between0.1 and 9 ? 10^–6 m s^–1, whereas for the plasmalemmaof cells and protoplasts isolated from leaves of higher plantsvalues between 0.3 and 11 ? 10^–6 m s^–1 were measured.By assuming that these measurements are representative for plantsand algae in general, it is concluded that the CO₂ conductivityof algal PM is of the same order of magnitude as that of thehigher plant cell PM. P_s values of plasma membranes for CO₂are lower than those for SO₂, but are in the same order of magnitudeas those measured for H₂O. On the basis of these results itis concluded that theoretical values of about 3000 ? 10^–6m s^–1 believed to be representative for higher plant cells(Nobel, 1983) and which are frequently used for computer-basedmodels of photosynthesis, lack experimental confirmation andrepresent considerable overestimations. However, with severalsystems, including higher plant cells, the conductance of thePM for CO₂ was significantly higher in light than in darkness.This suggests that in light, additional mechanisms for CO₂ uptakesuch as facilitated diffusion or active uptake may operate inparallel with diffusional uptake. Key words: Conductivity, CO₂, permeability coefficient, photosynthesis, plasmalemma     

EFFECTS OF VARIOUS FACTORS ON THE INCREASE OF {alpha}-AMYLASE ACTIVITY IN RICE ENDOSPERM INDUCED BY GIBBERELLIN A3

The increase of -amylase activity in embryoless rice endosperminduced by the addition of gibberellin A₃ was examined undervarious conditions with an aim to establish a bioassay methodfor gibberellins. Sterilized embryoless rice endosperms were incubated in a testtube containing 0.2–1.0 ml of test solution for 4 daysat 30. -Amylase activity in the endosperm was determined bymeasuring digestion of added starch. The increase of amylaseactivity during the incubation was not affected by the additionof various vitamins, amino acids, organic acids, protease, sucrose,indoleacetic acid or kinetin. Helminthosporol, helminthosporicacid and sclerin (1–10 µg/ml) had weak promotingeffects. Under appropriate conditions, 10^–5 µg/mlof gibberellin A₃ could be detected. In double logarithmic plot,the increase in the enzyme activity was proportional to thegibberellin A₃ concentration in the range from 10^–5 to10^–2 µg/ml. (Received April 13, 1966; )     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. cv. Bien venu in Flowing Solution Culture: II. UPTAKE FROM SOLUTIONS CONTAINING NH4NO3

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. CV. Bien venu in flowing solution culture.II. Uptake from solutions containing NH₄NO₃.—J. exp. Bot.38: 53–66 The effects of root temperature on uptake and assimilation ofNH₄⁺ and NO₃^– by oilseed rape (Brassica napus L. CV. Bienvenu) were examined. Plants were grown for 49 d in flowing nutrientsolution at pH 6?0 with root temperature decrementally reducedfrom 20?C to 5?C; and then exposed to different root temperatures(3, 5, 7, 9, 11, 13, 17 or 25?C) held constant for 14 d. Theair temperature was 20/15?C day/night and nitrogen was suppliedautomatically to maintain 10 mmol m^–3 NH₄NO₃ in solution.Total uptake of nitrogen over 14 d increased threefold between3–13?C but was constant above 13?C. Net uptake of NH₄⁺exceeded that of NO₃^– at all temperatures except 17?C,and represented 47–65% of the total uptake of nitrogen.Unit absorption rates of NH₄⁺ and of 1?5–2?7 for NO₃^–suggested that NO₃^– absorption was more sensitive thanNH₄⁺ absorption to temperature. Rates of absorption were relativelystable at 3?C and 5?C compared with those at 17?C and 25?C whichincreased sharply after 10 d. Tissue concentration of N in theshoot, expressed on a fresh weight basis, was independent ofroot temperature throughout, but doubled between 3–25?Cwhen expressed on a dry weight basis. The apparent proportionof net uptake of NO₃^– that was assimilated was inverselyrelated to root temperature. The results are used to examinethe relation between unit absorption rate adn shoot:root ratioin the context of short and long term responses to change ofroot temperature Key words: Brassica napus, oilseed rape, root temperature, nitrogen uptake     

The Uptake of Gaseous Ammonia by the Leaves of Italian Ryegrass

Lockyer, D. R. and Whitehead, D. C. 1986. The uptake of gaseousammonia by the leaves of Italian ryegrass.—J. exp. Bot.37: 919–927. Plants of Italian ryegrass (Lolium multiflorum Lam.) grown insoil with two rates of added ¹⁵N-labelled nitrate were exposed,in chambers, for 40 d to NH₃ in the air at concentrations of16, 118 and 520 µg m^–3. At the highest concentrationof NH₃, this source provided 47?3% of the total nitrogen inplants grown with the lower rate of nitrate addition (100mgN kg^–1 dry soil) and 35?2% with the higher rate (200mgN kg^–1 dry soil) At the intermediate concentration ofNH₃, the contributions to total plant N were 19?6% and 10?8%,respectively, at low and high nitrate while, at the lowest concentrationof NH₃, they were 5?1% and 32%. Most of the N derived from theNH₃ remained in the leaves, but some was transported to theroots. The amount of N derived from the NH³ that was presentin the leaves was not reduced by washing the leaves in waterat pH 5?0 before harvesting, indicating that the N was assimilatedby the plant and not adsorbed superficially. Rates of uptakeof NH₃ per unit leaf area ranged from 1?7 µg dm^–2h^–1 at a concentration of 16 µg m^–3 to 29?0µg dm^–2 h^–1 at a concentration of 520 µgm^–3 and with the lower rate of nitrate addition. Increasingthe supply of nitrate to the roots slightly reduced the rateof uptake of NH₃ per unit leaf area. Uptake of N from the higherrate of nitrate was reduced at the highest concentration ofNH₃ in the air. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.     

Root Hydraulic Conductivity of Two Cactus Species in Relation to Root Age, Temperature, and Soil Water Status

The effects of root age, temperature, and soil water statuson root hydraulic conductivity (L_P) were investigated for twocactus species, Ferocactus acanthodes and Opuntia ficus-indica.The volumetric flux density of water was measured for excisedroot segments, either using negative hydrostatic pressures appliedto the proximal end or using reverse flow of water from theroot to the soil. For both species, L_P at 20 ?C increased withroot age, average values reaching a maximum of 3.9 ? 10^–7m s^–1 MPa^–1 for F. acanthodes and 5.2 ? 10^–7m s^–1 MPa^–1 for O.ficus-indica at 11 to 17 weeksof age; L_P subsequently declined with increasing root age forboth species. L_P was maximal at a temperature of about 10 ?Cfor the youngest roots (1–3 weeks), this optimum shiftingto 40 ?C for 8-week-old roots of both species. For older roots(up to 1.5-years-old), L_P increased with temperature from 0?C to 50 ?C, with a Q₁₀ of 1.3 between 20 ?C and 30 ?C. At asoil water potential (_soil) of –0.016 MPa, root L_P wasindependent of the direction of water flow for both species.Depending on root age, L_P declined 45- to 500-fold for F. acanthodesand 90- to 800-fold for O.ficus-indica as _soil was reduced from–0.016 to –1.06 MPa, consistent with a rectifier-likebehaviour with respect to water movement between soil and roots.Incorporation of such responses into water uptake models shouldlead to a better understanding of root function. Key words: Ferocactus acanthodes, Opuntia ficus-indica, water potential, tension, reverse flow     

Effect of Temperature and External pH on the Cytoplasmic pH of Chara corallina

Cytoplasmic pH (pH_c) in Chara corallina was measured (from [¹⁴C]stribution)as a function of external pH (pH₀)and temperature. With pH₀near 7, pH_c at 25?C is 7.80; pH_cincreases by 0.005 pH units?C^–1 temperature decrease, i.e. pH_c at 5 ?C is 7.90. WithpH? near 5.5, the increase in pH_c with decreasing temperatureis 0.015 units ?C^–1 between 25 and 15?C, but 0.005 units?C^–1 between 15 and 5?C. This implies a more precise regulationof pH_c with variations in pH_o at 5 or 15 ?C compared with 25?C. The observed dp H_c/dT is generally smaller than the –0.017units ?C^–1 needed to maintain a constant H⁺/OH^–1,or a constant fractional ionization of histidine in protein,with variation in temperature. It is closer to that needed tomaintain the fractional ionization of phosphorylated compoundsor of CO₂–HCO₃^– The value of dpH_c/dT has importantimplications for several regulatory aspects of cell metabolism.These include (all as a function of temperature) the rates ofenzyme reactions, the _H+ at the plasmalemma(and hence the energy available for cotransport processes),and the mechanism for pH_c regulation by the control of bidirectionalH⁺ fluxes at the plasmalemma.     

Some Properties of Ribulose Bisphosphate Carboxylase Extracted from Tomato Leaves

Ribulose bisphosphate carboxylase (EC 4.1.1.39 [EC] ) activity wasvery low in tomato leaf extracts unless prepared in the presenceof Mg²⁺, and polyclar AT. With young leaves, but not with fully-expanded leaves, the RuBP carboxylase activityextracted was increased by prolonged illumination of the leaves(2 h). The main effect of the light treatment was to increasethe specific activity of the enzyme but there was also a smallincrease in RuBP carboxylase protein. Tomato leaf RuBP carboxylasein extracts had specific activities in the range 0.2–0–6µmol CO₂ min^–1 mg^–-1 total protein extracted,or 0.5–1.2 µmol CO₂ min^–1 mg^–1 RuBPcarboxylase, and an apparent K_m (CO₂) at 20 ?C of 9.3 ? 1.2µM (using a of 6.407). Key words: Tomato leaf, RuBP carboxylase, Properties     

Modification of the CO2 Responses of Maize Stomata by Abscisic Acid and by Naturally-Occurring and Synthetic Cytokinins

Fragments of maize leaves were incubated at controlled temperatureand irradiance either on distilled water or on one of threeconcentrations of cytokinin (10^–1, 10^–2 and 10^–3mol m^–3). The effects of zeatin or kinetin on stomatalaperture were determined by stripping abaxial epidermis fromthe fragments after incubation and immediately measuring stomatalapertures under the microscope. At each cytokinin concentrationleaf pieces were incubated at 5 or 350 µmol mol^–1CO₂ with or without ABA (10^–1 mol m^–3). At 5.0 µmolmol^–1 CO₂ increasing the concentrations of zeatin hada negligible effect upon stomatal aperture. When air containing350 umol mol^–1 CO₂ was bubbled through the incubationsolutions, apertures of stomata incubated on water were morethan halved. Increasing cytokinin concentrations reduced theeffect of CO₂ on stomata and incubation on 10^–1 mol m^–3zeatin completely removed any CO₂ response. The addition ofABA restored the effect of CO₂, even at the highest cytokininconcentration. Key words: Maize, CO₂ response, ABA, Cytokinins     

Phytotoxicity of Phthalate Plasticisers: 2. SITE AND MODE OF ACTION

The effects of phthalate esters on chlorophyll a₂ fluorescencein radish plants (Raphanus sativus L. cv. Cherry Belle) wereexamined Fluorescence yield was increased in those plants exposedto an aerial concentration of 120 ng dm^–3 dibutyl phthaiatc(DBP) at a rate of 3.0 dm³ min^–1 for 13 d. Comparisonof fluorescence enhancement ratios and F_red/F_ox, suggests thatDBP inhibits photosynthesis in radish plants at a site afterQ_A. Both DBP and diisobutyl phthalate (DIBP) strongly inhibiteduncoupled (PS2+PS1) electron transport rates in thylakoids isolatedfrom spinach. At a chlorophyll concentration of 10 µgcm^–3 the concentrations of DBP and DIBP exhibiting 50%inhibition were 44 mmol m^–3 and 42 mmol m^–3 respectively.Basal electron transport rates were also inhibited, with 87mmol m^–3 of DBP or DIBP producing 50% inhibition. Measurementof photosystcm 1 activity suggested that the main site of actionof these phthalates was localized at a site near the reducingside of photosystem 2. Key words: Phthalate, plasticiser, chlorophyll, fluorescence, photosynthesis, inhibition     

EFFECTS OF CHLORAMPHENICOL AND KINETIN ON UPTAKE AND INCORPORATION OF AMINO ACIDS BY TOBACCO LEAF DISKS

The effects of chloramphenicol and kinetin on uptake and incorporationof ³⁵S-methionine and some ₁₄C-amino acids have been investigatedin leaf-disks of Nicotiana rustica in light and dark. Chloramphenicolin a concentration of 1 mg per ml inhibits the uptake of aminoacids from 30 to 60 per cent compared with the water control.The incorporation of amino acids into bulk protein is stronglyinhibited in light (40 to 70 per cent), but only to a smalldegree in dark (10 to 20 per cent), as revealed also by ¹⁴CO₂-photosynthesisof the disks and following treatment with chloramphenicol indark. The stimulating effect of kinetin on uptake and incorporationof amino acids is dependent upon its concentration (10^–5to 10^–6 M ; but 10^–4 M solution inhibits stronglyboth uptake and incorporation). The stimulation seems to influencemore incorporation than uptake processes. Possible interactionsof chloramphenicol and kinetin in the protein metabolism oftobacco leaves have been discussed. (Received April 27, 1964; )     

Intracellular pH Regulation in the Giant-Celled Marine Alga Chaetomorpha darwinii

The vacuolar pH (pH_v) and the cytoplasmic pH (pH_c) of the marinegiant-celled green alga Chaetomorpha darwinii were measuredby pH microelectrode techniques on extracted vacuolar sap, andby the [^I4C]DMO distribution method respectively. Equilibrationof DMO occurred with a half-time of about 2 h, with an apparentP_DMO of 3.6 x 10^–5 cm s^–1, but the vacuolar concentrationof free, undissociated DMO was always less than the externalconcentration. The explanation offered for freshwater giant-celledalgae of net DMO^– leakage across the plasmalemma cannotapply to Chaetomorpha darwinii, since electrically-driven DMO^–exit from the cytoplasm should be similar across the plasmalemmaand the tonoplast in these cells with large, vacuole-positivepotential differences across the tonoplast. pHc was accordinglycomputed assuming either tonoplast or plasmalemma equilibrationof DMO, with correction for DMO metabolism within the cell.pH_c was 8.0–8.3 in the light in artificial seawater (pH_oabout 8.0), was some 0.5 units lower in the dark, and was slightlylower with an external pH of 7. Vacuolar pH was 6.5–6.9,without consistent effects of illumination or of external pHof 7 rather than 8. While µ_H+ at the tonoplast was similarto that in giant-celled freshwater algae (although with a greatercontribution from relative to pH), µ_H+ at the plasmalemmawas less than 8 kJ mol^–1, i.e. less than one-third ofthe value in freshwater green algae. µ_Na+ was some 13kJ mol^–1 at the plasmalemma. The possibility that theprimary active transport process at the plasmalemma of Chaetomorphadarwinii (and certain other marine algae) is Na⁺ efflux ratherthan H⁺ efflux is discussed.     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Acclimation of NO-3 fluxes to low root temperature by Brassica napus in relation to NO-3 supply

Acclimation of NO^–₃ transport fluxes (influx, efflux)in roots of oilseed rape (Brassica napus L. cv. Bien venu) andtheir sensitivity to growth at low root temperature was studiedin relation to external NO^–₃ supply, defined by constantconcentrations ranging from sub- to supra-optimal with respectto plant growth rate. Plants were grown from seed in flowingnutrient solutions containing 250 mmol m^–3 NO^–₃at 17°C for 20d, and solution temperature in half the cultureunits was then lowered decrementally over 3 d to 7°C. Threedays later plants were supplied with NO^–₃ at 1, 10, 100or 1000 mmol m^–3 maintained for 18 d. Dry matter productionwas decreased more by low root zone temperature than low [NO^–₃]_e. Root specific growth rates were inversely related to [NO^–₃]_eand shoot:root ratios increased with time at [NO^–₃]_e between10–1000 mmol m^–3. Net uptake of NO^–₃ at 17°Cwas twice that at 7°C, and at both temperatures it doubledwith increasing [NO^–₃]_e between 1–10 mmol m^–3with further small increases at higher [NO^–₃]_e. Mean unitabsorption rates of NO^–₃ between 0–6 d and 6–14d were linearly related (r² of 0.79–0.99) to log₁₀[NO].Steady-state Q₁₀ (7–17°C) for uptake between 0–6d were 0.91, 1.62, 1.27, and 1.10, respectively, at [NO^–₃]_eof 1, 10, 100, and 1000 mmol m^–3, compared with correspondingvalues of 0.98, 1.38, 1.68, and 1.89 between 6–14 d. Thedata indicated that net uptake rates at 7 and 17°C divergedover time at high [NO^–₃]_e. Short-term uptake rates from1 mol m^–3 NO^–₃ measured at 17°C were higherin plants grown with roots at 7°C than at 17°C; for7°C plants there was a strong inverse linear relationship(r²=0.94) between uptake rate and treatment log₁₀ [NO^–₃]_ewhilst rates in 17°C plants were independent of prior [NO^–₃]_e. Rates of NO^–₃ influx and efflux under different steady-stateconditions of NO^–₃ supply and root temperature were calculatedfrom dilution of ¹⁵N added to culture solutions. Efflux wassubstantial relative to net uptake in all treatments, and wasinversely related to [NO^–₃]_e at 17°C but not at 7°C.Ratios of influx: efflux ranged from 1.6–2.9 at 17°Cand 1.3–1.8 at 7°C, indicating the proportionatelygreater impact of efflux at low root temperature. Ratios ofefflux: net uptake were 0.53–1.56 at 17°C and 1.21–3.58at 7°C. The apparent sensitivities of influx and effluxto steady-state root temperature varied with [NO^–₃]_e.Both fluxes were higher at 17°C than 7°C in the presenceof 100–1000 mmol m^–3 NO^–₃ but the trend wasreversed at 1–10 mmol m^–3 NO. Concentrations oftotal N measured in xylem exudate were at least 2-fold higherat 7°C compared with 17°C, attributable mainly to higherconcentrations of NO^–₃ glutamine and proline. The resultsare discussed in terms of acclimatory and other responses shownby the NO^–₃ transport system under conditions of limitingNO^–₃ supply and low root temperature. Key words: Brassica napus, nitrate supply, efflux, influx, root temperature, xylem exudate     

Some Experimental and Theoretical Observations Concerning Mass Flow in the Vascular Bundles of Heracleum

The theory and practice of applying the thermodynamics of irreversibleprocesses to mass-flow theories is presented. Onsager coefficientswere measured on cut and uncut phloem and cut xylem strandsof Heracleum muntegazzimum. In 0.3 N sucrose + 1 mN KC1 theyare as follows. In phloem, L_EE = 5 ? 10–⁴ mho cm^–1,L_pE = 9 ? 10^–6 cm³ s^–1 cm^–2 volt^–1 cm,and L_PP = 0.16 cm³ s^–1 cm^–2 (J cm^–3)^–1cm. In uncut phloem strands L_EE is about 1 ? 10^–3 mhocm^–1. In xylem in 2 x 10^–3 N KCI, L_pp = 50 to 225,L_PE = 2 ? 10^–4, and L^EE = 4 ? 10^–3. The measurementsare tentative since the blockage of the sieve plates is an interferingfactor, but if they are valid they lead to the conclusion thatneither a pressure-flow nor an electro-kinetic mechanism envisaginga ‘long distance’ current pathway can be the majormotive ‘force’ for transport in mature phloem. Measurementsof biopotentials along conducting but laterally detached phloembundles of Heracleum suggest, nevertheless, that there may bea small electro-osmotic component of at least 0.1 mV cm^–1endogenous in the phloem.     

The Effect of Selected Growth-Promoting and Growth-Inhibiting Substances on the Extension of Segments of Etiolated Tomato Seedling Hypocotyls

The effects of a number of growth-promoting and growth-inhibitingsubstances, including two fungal toxins, were studied on theextension of segments of etiolated tomato seedling hypocotyls.The bioassay was sensitive to small quantities of NaF, coumarinand ₂, ₄-DNP and inhibition was observed at all concentrations.₂, ₄-DNP or Iodoacetate stimulated growth at concentrationsbetween 1? 10^–4 and 5 ? 10^–6M. or 1 ? 10^–6and 1 ? 10^–7M. respectively. Inhibitor experiments inbuffered nutrient solution were approximately 10 per cent. moresensitive than those in deionized water. By means of paper partition chromatography small quantitiesof two fungal toxins, fusaric and alternaric acid were chromatographedand bioassayed. The effect of fussric acid (5, n-butyI picolinicacid) on hypocoty1 growth was detected at concentrations aslow as 1 ? 10^–5M. Experiments with recongnized growth-promoting substances showedthat Kinetin inhibited growth at concentrations up to 1 ?10^–8M.in both light and dark. IAA inhibited growth up to 1 ? 10^–6M.At 1 ? 10^–7 and 1 ? 10^–8 only small increases occurredwith IAA and the effect of light was negligible. Gibberellicacid (GA₂)stimulated growth at concentrations from 10^–3to 10^–7M. and significant increases up to 17 per cent.were recorded in the light. Since the light induced inhibitionwas only partly restored, the existence of some other naturallight sensitive growth substance is suggested. The value ofthe bioassay as a method for estimating natural growth-inhibitingand growth-promoting substances is discussed.     

Effect of Temperature on Nitrogenase Activity in White Clover

Single, clonal plants of white clover were grown without inorganicnitrogen in four contrasting day/night temperature regimes,with a 12 h photoperiod, in controlled environments. Root andnodule respiration and acetylene reduction activity were measuredin a flow-through system during both day and night for plantsacclimated to day/night regimes of 23/18, 15/10 and 10/5 ?C.Similar measurements were made on plants acclimated to 20/15?C and stepwise at temperatures from 4 to 33 ?C. Peak rate of ethylene production, nitrogenase-linked respirationand basal root + nodule respiration increased approximatelylinearly from 5 to 23 ?C both in temperature-acclimated plantsand in plants exposed to varying measurement temperatures. Themeasured attributes did not vary significantly between day andnight. Temperatures above 23–25 ?C did not further enhancethe rate of ethylene production, which remained essentiallythe same up to the maximum measured temperature of 33 ?C. The measurements of nitrogenase-linked respiration between 5and 23 ?C, during both day and night, demonstrated a constant‘energetic cost’ of acetylene reduction of 2.9 µmolCO₂ µmol C₂H₄^–1,. Over the same temperature range,the approximate activation energy of acetylene reduction was60 kJ mol^–1. The integrated day plus night nitrogenase-linkedrespiration accounted for 13.4–16% of the plant‘snet shoot photosynthesis in a single diurnal period: there wasno significant effect of temperature between 5 and 23 ?C. Key words: Trifolium repens, white clover, temperature, N₂ fixation, respiration