The Effects of Aluminium on the Growth of White Clover dependent upon Fixation of Atmospheric Nitrogen

Jarvis, S. C. and Hatch, D. J. 1985. The effects of aluminiumon the growth of white clover dependent upon fixation of atmosphericnitrogen.–J. exp. Bot. 36: 1075–1086. The effects of aluminium (Al^{3 +}) on the growth of white cloverdependent upon symbiotic fixation of atmospheric nitrogen wereexamined at concentrations that may be encountered in solutionsfrom soils of low pH. Well established plants were grown inflowing solution culture with carefully maintained concentrationsof Al and of P and with pH held constant at a value (4.5) atwhich insoluble precipitates are unlikely to form. After 3 weeksof treatment, there were major visual differences between treatmentsin both shoots and roots. Although added Al at 25, 50 and 100mmol m^–3 reduced dry weight, the differences between theplants were not significant. There were, however, some considerabledifferences in P and Ca contents between the treatments. Muchof the Al held by the roots was displaced when plants were transferredto solutions containing either scandium or gallium. As wellas the effects on the plant, Al had a considerable influenceon the fixation of atmospheric nitrogen. At concentrations of50 and 100 mmol m^–3 Al³⁺ nodule initiation was reducedand there was a much reduced nodule nitrogenase activity perunit of root. Even at 25 mmol m^–3 Al³⁺, when nodule numberswere not reduced, nitrogenase activity was adversely affected. Key words: —Aluminium, Trifolium repens, nitrogen fixation     

Effectiveness of Lotus Root Nodules: II. RELATIONSHIP BETWEEN ROOT NODULE EFFECTIVENESS AND 'IN VITRO' SENSITIVITY OF FAST-GROWING LOTUS RHIZOBIA TO FLAVOLANS

An extract from the roots of Lotus pedunculatus plants was foundto contain a compound toxic towards fast-growing Lotus rhizobia.This compound was identified as a flavolan, which has a prodeiphinidin:procyanidin ratio of 75:25. A fast-growing strain of Rhizobium(NZP2213) which forms ineffective root nodules on L. pedunculatuswas four times more sensitive to this flavolan (ED₅₀ = 25 ?gml^–1) than another strain (NZP2037, ED₅₀ = 100 ?g ml^–1)which forms effective root nodules on this species. The rootsof another Lotus species, L. tenuis, on which both strains ofRhizobium form effective root nodules, also contained a flavolan( 95% procyanidin) but both strains were relatively insensitiveto this flavolan (EDED₅₀ = 350 to 500 ?g ml^–1) L. pedunculatusplants bearing ineffective root nodules contained two to threetimes more flavolan in their roots (5–7 mg g^–1 fr.wt.)than uninoculated control plants. Experiments with seven otherLotus species and with hybrid plants developed between L. pedunculatusand L. tenuis showed a relationship between the prodeiphinidin:procyanidin ratio of the flavolan in their roots and the effectivenessof root nodules formed on these plants by NZP2213. Quantitativebinding studies of the flavolan from L. pedunculatus to NZP2037and NZP2213 indicated that, while the affinity constants forbinding were similar for both strains, the surface of strainNZP2037 contained four times more binding sites than NZP2213,possibly correlating with this strain's ability to toleratehigher concentrations of this flavolan. It is suggested thatthe differential sensitivity of these two strains of Rhizobiumto flavolans is related to their ability to form effective rootnodules on Lotus species.     

Ammonia emission from young barley plants: influence of N source, light/dark cycles and inhibition of glutamine synthetase

Barley (Hordeum vulgare L. cv. Golf) plants were grown at twodifferent relative addition rates; 0.1 and 0.2 d^–1 ofnitrate. Three to five days before measurements started theplants were transferred to a nutrient solution with 2 mM nitrateor ammonium. The ammonium-grown plants showed increased ammoniumlevels in both shoots and roots and also increased ammoniumconcentrations in xylem sap. Ammonia emission measured in cuvettes connected to an automaticNH₃ monitor was close to zero for nitrate-grown plants but increasedto 0.59 and 0.88 nmol NH₃ m^–2 S^–1 for plants transferredto ammonium after growing at RA=0.2 and 0.1 d^–1, respectively.In darkness, NH₃ emission decreased together with photosynthesisand transpiration, but increased rapidly when the light wasturned on again. Addition of 0.5 mM methionine sulphoximine (MSO) to the plantscaused an almost complete inhibition of both root and shootglutamine synthetase (GS) activity after 24 h. Ammonia emissionincreased dramatically and photosynthesis and transpirationdecreased in both nitrate- and ammonium-grown plants as a resultof the GS inhibition. At the same time plant tissue and xylemsap ammonium concentrations increased, indicating the importanceof GS in controlling plant ammonium levels and thereby NH₃ emissionfrom the leaves. Key words: Hordeum vulgare, ammonia emission, ammonium, glutamine synthetase, nitrogen nutrition, photosynthesis, transpiration     

Root to Shoot Communication in Maize Plants of the Effects of Soil Drying

Seedlings of Zea mays L. (John Innes hybrid) were grown withroots divided between two containers such that part of the rootsystem could reduce the water potential of the soil in its immediatevicinity while the rest of the root system was well suppliedwith water. When compared to plants rooted in two pots of moistsoil, drying of part of the root system resulted in partialclosure of stomata, even though leaf water potential, turgorand abscisic acid (ABA) content remained unaffected. When leafpieces were removed from the two groups of plants and incubatedunder conditions favourable for stomatal opening, stomata ofthe ‘half-watered’ plants still showed restrictedapertures. Incubation in kinetin (10 mmol m^–3) or zeatin(100 mmol m^–3) reversed the closure of stomata stimulatedby soil drying. These results suggest that a continuous supplyof cytokinin from roots may be necessary to sustain maximalstomatal opening and an interruption of this supply due to soildrying may act as an indicator of inhibited root activity, resultingin restricted stomatal opening and thereby restricted wateruse. Key words: Zea mays L., Soil drying, Stomata, Roots     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

Modification of the root plasma membrane lipid composition of cadmium-treated Pisum sativum

The effects of in vivo Cd treatments on pea root plasma membrane(PM) lipid composition were studied. In the long-term experiment,plants were supplied with Cd: moderate stress (10 µM)or strong stress (50 µM) for 10 d. Growth of root andshoot was severely affected in 50 µM Cd-treated plants,as evidenced by the approximately 7-fold reduction in theirRelative Growth Increment (RGI). Treatment with Cd (10 µM)resulted in changes to the lipid composition of the pea rootPM, including increases in the degree of unsaturation of phospholipid-associatedfatty acids and in the relative amount of stigmasterol (30–42%).This change was accompanied by a reduction in sitosterol content(26.8 to 17.4 µg mg^–1 protein). However, the sterolcomposition was not altered in plants treated with 50 µMCd for 10 d. The content of phosphatidylethanolamine and phosphatidylcholine(major phospholipids present in pea root PM) decreased as Cdlevel increased, but the ratio between them remained unaffected.In the short-term experiment, plants exposed to Cd (50 µM)accumulated less sitosterol (from 27.7 to 14.0 µg g mg^–1protein) over 72 h, but no significant effect on other measuredlipids was observed. The physiological repercussions of changesin plasma membrane lipid composition, as a result of Cd exposureare discussed. Key words: Cadmium, lipids, pea, Pisum sativum, plasma membranes     

Agrobacterium rhizogenes Mediated Transformation of the Forage Legumes Medicago sativa and Onobrychis viciifolia

Three cultivars of M. sativa and one cultivar of O. viciifoliawere evaluated for their response to inoculation with A. rhizogenesstrain A4T (containing pRiA4b). A cultivar-dependent responsewas observed in M. sativa with 94%, 25%, and 4% of infectedstem explants producing transformed roots in the cultivars Vertus,Regen-S, and Rangelander, respectively. In O. viciifolia cv.Hampshire Giant, an explant-dependent response was observedwith 78% and 50% of seedling cotyledon and hypocotyl explantsresponding, respectively. Leaf explants failed to produce transformedroots. Transformed roots showed plagiotropic and negativelygeotropic growth on hormone-free agar MS medium. Productionof transgenic shoots from O. viciifolia root cultures occurredspontaneously. Recovery of transgenic plants from M. salivacv. Rangelander was achieved by transfer of callus (inducedon UM medium containing 2·0mg dm^–3 2,4-D and 0·25mg dm^–3 kinetin) to MS medium containing 0·5 ingdm^–3 BAP and 0·05 mg dm^–3 NAA. Cultured rootsof both species synthesized opines (agropine and mannopine).Extensive morphological variation was observed in plants ofM. sativa (clone Al) and O. viciifolia (clone A4Tl) establishedin the glasshouse. DNA sequences homologous to TL-DNA and TR-DNAwere present in root clones and regenerated plants. Key words: Agrobacterium rhizogenes, Medicago sativa, Onobrychis viciifolia, transformed roots, transgenic plants     

Partition of 14C Assimilate between Organs and Fractions of Contrasting Varieties of Carrot during Initiation of the Storage Root

This work examines the differences in partition and activityof ¹⁴C in two varieties of carrot (Daucus carota L.) contrastingin shoot to storage root ratio at maturity. Plants were grownin a controlled environment of 20 ?C and 500 µmol m^–2s^–1. During initiation of the storage root (10–25d from sowing) plants were exposed to ¹⁴CO₂ for 1 h and theradioactivity in ethanol-soluble and -insoluble fractions ofshoots, storage and fibrous roots estimated at various timesup to 48 h after exposure. Between 35% and 40% of radioactivityinitially present in the plants was respired during the first24 h and 25–35% of that remaining after 24 h was foundin the roots, depending on age. The proportion found in thestorage region remained fairly constant between 15 and 25 dand was smaller than at 10 d. In the variety with a larger proportionof storage root at maturity (cv. Super Sprite), there was agreater proportion of label in both ethanol-soluble and -insolublefractions of the storage region soon after storage root initiationhad begun than in the variety with a smaller proportion of storageroot at maturity (cv. Kingston). There was no varietal differencein specific activities of the storage roots, but fibrous rootsof cv. Super Sprite showed a greater specific activity thanin cv. Kingston. Differences in shoot to storage root ratiomay thus be associated with characteristics of the fibrous roots.Partition and specific activities are discussed in relationto the initiation and development of the storage organ. Key words: Daucus carota, carrot, assimilate, partition, ¹⁴C, storage root     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: IV. TRANSLOCATION AND REMOBILIZATION OF NITROGEN

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grownunder nitrogen limitation in solution culture until near maturity.Three different nitrogen addition regimes were used: in the‘HN’ culture, the relative rate of nitrate-N additionwas 0·08 d^–1 until day 48 and then stepwise decreasedto, finally, 0·005 d^–1 during late grain-filling;the ‘LN’ culture received 45% of the nitrogen addedin HN; the ‘CN’ culture was maintained at RA 0·0375d^–1 throughout growth. At four different growth stages(vegetative,anthesis, and twice during grain-filling), ¹⁵N-nitrate was fedto the plants. In some cases (‘split root cultures’),label was fed only to one-half of the root system. These wereharvested directly after labelling, whereas ‘standardcultured’ plants were harvested at termination of theexperiment (day 148). Absorption of added nitrate was nearlycomplete in the HN and LN cultures, and translocation of nitrogenwithin the plants could thus be studied independently of differencesin nitrate absorption. Cycling of nitrogen absorbed by vegetativeplants accounted for up to 50% of the nitrogen recovered inthe roots. The sink strength of the roots for cycling nitrogen,however, declined during post-anthesis growth, and net lossof nitrogen from both roots and vegetative shoot tissue occurredconcomitantly with incorporation of labelled ¹⁵N-nitrogen. Thenitrogen of the vegetative shoot tissue was substantially lesslabelled than the nitrogen entering the ears, indicating thattranslocation of recently absorbed nitrogen to ears occurs withminor prior exchange with the bulk nitrogen of shoots. In caseswhere the sink strength of the ears was weak, as in LN-culturedLaevigatum (due to high frequency of sterile flowers) and inCN-cultured Golf, nitrogen translocated from roots appearedto be incorporated into the vegetative shoot tissue. There werealso indications that a fraction of the remobilized nitrogenwas actually lost from the plants in these cases. It is concludedthat the root remains efficient in translocation of nitrogento the aerial parts throughout ontogeny and that nitrogen takenup during grain–filling is preferentially directly translocatedto the developing grains. The further translocation of nitrogenreceived by vegetative shoot parts to ears appears mainly relatedto the potential of the ear to accumulate nitrogen. Nitrogenabsorbed/remobilized in excess of the sink strength of the earsis either invested in continued shoot growth, or is irreversiblylost from the plants. Key words: Barley, ¹⁵N-labelling, post-anthesis, remobilization, translocation     

Reciprocal Ammonium Transport Into and Out of Plant Roots: Modifications by Plant Nitrogen Status and Elevated Root Ammonium Concentration

Wheat and oat were grown for 20 d on a nitrate-containing solution(nitrogen-replete plants) or for the last 6 d of this periodon a nitrate-free solution (nitrogen-depleted plants). Exposureof the nitrogen-depleted plants on day 20 to nitrate-free solutionscontaining 500 mmol m^–3 ammonium (96 A% ¹⁵N) resultedin a cumulative net influx of ¹⁵N-ammonium over an 8 h periodthat was appreciably greater than that of the nitrogen-repleteplants. Both the initial rate and the more restricted rate afterthe first hour were enhanced by nitrogen deprivation. In thenitrogen-replete plants, cumulative net efflux of endogenous¹⁴N-ammonium was approximately equivalent to net ammonium uptakeduring the first hour, and was essentially complete after 1–2h. Pretreating nitrogen-depleted plants for 5 h in 500 mmolm^{–3 15}N-ammonium (99 A% ¹⁵N) resulted in root ammoniumconcentrations of 12.7?1.1 and 16.0?0.4 µmol for wheat and oat, respectively. Subsequent net efflux of ¹⁵N-ammoniumto 500 mmol m^–3 exogenous ¹⁴N-ammonium exceeded theseinitial amounts within 2 h. Increasing ambient ¹⁴N-ammoniumto 5000 mmol m^–3 increased net ¹⁵N-ammonium efflux suchthat net loss of the maximal original amount in the root tissuewas exceeded within 0.75 h. The data for both species indicatesubstantial reciprocal transfers of ammonium into and out ofroots of ammonium-treated plants and a significant degradationof recently synthesized products of ammonium assimilation concurrentwith ammonium assimilation. Key words: Accumulation, ammonium, efflux, oat, root, uptake, wheat     

Growth and swainsonine production of Swainsona galegifolia (Andr.) R. Br. untransformed and transformed root cultures

Untransformed and transformed root cultures of Swainsona galegifollawere established for swainsonine production. Transformed rootsgrew faster and produced higher swainsonine levels (62.3 µgg^–1 DW) than untransformed roots (23.6 ,µg g^–1DW) or roots of intact plants (8.7 µg g^–1 DW). Transformationof a number of plant genotypes using A. rhizogenes strain LBA9402 showed that plant genotype Influences swainsonine levelin transformed roots but that a wide range of swainsonine levelscan be induced by separate transformation events in the samegenotype. Enhancement of swainsonine production was attemptedby treatment with sugars and induction of polyploid roots. Key words: Agrobacterium rhizogenes, root cultures, Swainsona galegifolia, swainsonine     

Relationship Between the Development and Growth of Rye (Secale cereale L.) and the Potassium Concentration in Solution

The development and growth of rye (Secale cereale L. cv. Rheidol)was studied in seedlings grown hydroponically in complete nutrientsolutions containing between 10 and 600 µM K⁺. The phyllochron(defined as the interval between the appearance of successiveleaves) was used as a developmental timescale to compare plants.The pattern of both shoot and root development was strictlyordered on a phyllochron basis and was unaffected by solutionK⁺ concentration, with the exception that tillers in plantsgrown at the lowest K⁺ concentrations were occasionally eithernot initiated or aborted at an early stage of development. However,both the rate of leaf appearance on the main stem and successivetillers and the rate of tiller appearance were slower in plantsgrown at lower K⁺ concentrations. The rate of leaf appearanceon the main stem was reduced to below 90% of its maximal valueat solution concentrations below about 50 µM K⁺. Plantrelative growth rate (RGR) was also reduced by lowering theK⁺ concentration of the nutrient solution and fell to below90% of its maximal value at solution concentrations below about200 µM K⁺. There was a complex relationship between tissueK⁺ concentration and the K⁺ concentration of the nutrient solution,which differed between leaves and root. Leaf K⁺ concentrationincreased steadily from about 50 µmol g^-1 f. wt to about200 µmol g^-1 f. wt as solution K⁺ concentration was increasedfrom 10 to 400 µM. In contrast, root K⁺ concentrationexhibited a sigmoidal dependence on solution K⁺ concentration,maintaining a minimal value of approximately 20 µmol g^-1f. wt at concentration below 100 µM K⁺, then increasingprogressively to about 120 µmol g^-1 f. wt at a solutionconcentration of 600 µM K⁺. The 'critical' leaf K⁺ concentration,i.e. the concentration at which either plant RGR or plant developmentwas reduced 90% of its maximal value, was 86 µmol g^-1f. wt for plant RGR and 150 µmol g^-1 f. wt for plant development.The 'critical' root K⁺ concentration was 24 µmol g^-1 f.wt K⁺ for both RGR and development. A decline in tissue K⁺ concentrationbelow these thresholds reduced plant growth considerably. RootK⁺ concentration was a sensitive indicator of the K⁺ statusof the plant with respect to potential growth since plant growthdeclined abruptly as root K⁺ concentration approached its 'critical'value, whereas plant growth showed a less defined relationshipwith shoot K⁺ concentration.Copyright 1993, 1999 Academic Press Critical K⁺ concentration, development, potassium, relative growth rate (RGR), rye, Secale cereale L. cv. Rheidol     

The Occurrence of 1,3-{beta}-Glucanase in Healthy and Verticillium-infected, Resistant and Susceptible Tomato Plants

Leaf, stem, and root extracts of near-isogenic tomato plantscv. Craigella, resistant and susceptible to Verticillium albo-atrum,showed constitutive 1,3-ß-glucanase activity whichincreased following inoculation with the pathogen. Partiallypurified enzyme extracts were obtained by dialysing a 30–80%ammonium sulphate fraction of the tissue brei. The enzyme hadpH and temperature optima of 5?5 and 44 ?C respectively, withhigh activity between 50 and 60 ?C. The response to laminarinconcentration was linear between 1?2 and 7?5 mg ml^–1.Root inoculation of susceptible plants with 10⁶ propagules ml^–1V. albo-atrum led to a umform 300 per cent increase in all steminternodes except the terminal one, which was 500 per cent ofthe controls. No spatial relationship of enzyme activity tothe localization of fungus within the stem was apparent. Petioles,leaves, and roots of susceptible infected plants similarly showedan increase in activity but less than that in stems. Changedlevels of stern enzyme activity at different times after inoculationwere associated with reductions in the number of vessels containinghyphae. Extracts of plants of the resistant isoline showed increasedglucanase activity over controls, but this was substantiallylower than that in susceptible plants and was associated withthe greatly reduced mycelial colonization in resistant plants. It is concluded that single gene resistance in tomato to Verticilliumis not associated with innately higher levels of 1,3-ß-glucanasein healthy plants. The increased activity in infected plantsis proportional to the overall quantity of pathogen in the plantor of pathogenic metabolites.     

The Effect of Triacontanol on Peroxidase, IAA, and Plant Growth in Pisum sativum var.'Alaska' and 'Little Marvel'

The present study investigates the effects of triacontanol (CH₃(CH₂)₂₈CH₂OH),on plant growth (root and stem), peroxidase activity (apicalmeristem tissue), and auxin destruction (apical meristem tissue)in ‘Little Marvel’ dwarf (LM) and ‘Alaska’peas (AP). Triacontanol inhibited root growth in LM comparedto untreated controls. However, root growth in AP tissue wasenhanced by 1.0 mg I^–1 triacontanol and inhibited by allother treatments, in comparison to untreated controls. Wateruptake in triacontanol-treated AP plants was greater than inuntreated controls, with the converse being the case for LM.Triacontanol treatment caused an increase in peroxidase activityin both LM and AP plants compared to untreated controls. Interms of (1–¹⁴C)IAA destruction, GA₃ + 0.01 mg 1^–1triacontanol caused appreciable auxin breakdown (40%) in LMtissue, with GA₃ + 0.1 mg 1^–1 triacontanol giving a 43%decrease compared to untreated controls. In AP tissue, 10 µMGA3 increased auxin destruction by 188% whereas 0.1 mg I^–1triacontanol caused a 20% decrease compared to untreated controls.The effects of triacontanol on root and stem growth, peroxidaseactivity, and auxin destruction appear to be cultivar-specific,with respect to LM and AP varieties of peas.     

Hormonal responses to partial drying of the root system of Helianthus annuus

Plants of Helianthus annuus were pot-grown in soil, with approximately30% of the root system protruding through the base. After 7d, the upper part of the root system of half of the plants wasexposed to drought (internal roots) while the lower part waskept in aerated nutrient solution (protruding root). The treatmentrapidly reduced the internal roots' water content from 26.1to 21.9 g g^–1 dry weight (DW), while in protruding rootsof stressed plants it slowly and continuously decreased from31.9 to 25.2 g g^–1 DW. Leaf water content rapidly decreasedin treated plants from 7.4 to 6.4 g g^–1 DW in the first2d and then reached a plateau. In stressed plants leaf stomatalresistance was significantly higher in the first 3 d while leafwater potential was lower only on the last day. Abscisic acid (ABA) concentration in treated plants increasedsignificantly compared to the controls. In treated internalroots, ABA rose from the first day, reaching a maximum of 1.48±0.49nmol g^–1 DW after 3 d. In treated protruding roots a maximumof 0.99±0.09 nmol g^–1 DW was reached after 1 d.ABA concentration in the xylem sap increased 2 d and 3 d afterthe start of soil drying, with a maximum of 113±12nmoll^–1 during the third day. The ABA rise in the leaves oftreated plants was less significant. Indol-3yl-acetic acid (IAA) concentration in internal rootsof treated plants reached a maximum of 22.54±3.34 nmolg^–1 DW on the third day, then decreased dramatically.The protruding root system of control plants showed a maximumvalue of 16.05±1.77 nmol g^–1 DW on the sixth day. Little difference in cytokinin content of xylem sap was notedbetween control and treated plants. Hormonal variations in different parts of the plant are discussedin relation to drought stress. Key words: Soil drying, roots, ABA, IAA, cytokinins     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

The influence of salinity on the utilization of root anaplerotic carbon and nitrogen metabolism in tomato seedlings

In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO₃^– reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO₃^– from the root solution was strongly inhibitedby salinization. Consequently, NO₃^– concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO₃^–, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO₃^– reduction in salinized plants. Lowerstem K⁺ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K⁺–shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH¹⁴CO₃ followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H¹⁴CO₃^– incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H¹⁴CO₃^– incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more ¹⁴C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO₃^– into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity     

Partial characterization of farnesyl and geranylgeranyl diphosphatases induced in rice seedlings by UV-C irradiation

This study reports the identification and characterization ofallyl diphosphatases (EC 3.1.7.1) in plants by using rice seedlings.Two distinct Mg²⁺-independent allyl diphosphatases, which convertfarnesyl diphosphate (FDP) and geranylgeranyl diphosphate (GGDP)into farnesol and geranylgeraniol, respectively, were inducedin rice seedlings irradiated with UV-C. Farnesyl diphosphatase(FDPase) and geranylgeranyl diphosphatase (GGDPase) are locatedin the microsomal fraction. The relative specific activity ofFDPase was much higher than the specific activity of GGDPase.FDPase activity was inhibited by GGDP (50% inhibition at 5 µM)and geranyl diphosphate (50% inhibition at 100 µM).In contrast, GGDPase activity was inhibited 50% by 50 µMisopentenyl diphosphate or 100 µM FDP. The optimalpH for FDPase was 6.3 and for GGDPase was 7.9. ¹ Corresponding author: E-mail, kback@chonnam.chonnam.ac.kr;Fax, +82-62-530-2169.     

Growth of Zea mays L. plants with their seminal roots only. Effects on plant development, xylem transport, mineral nutrition and the flow and distribution of abscisic acid (ABA) as a possible shoot to root signal

Maize (Zea mays L.) was grown in quartz sand culture eitherwith a normal root system (controls) or with seminal roots only(‘single-rooted’). Development of adventitious rootswas prevented by using plants with an etiolated mesocotyl andthe stem base was positioned 5–8 cm above the sand. Eventhough the roots of the single-rooted plants were sufficientlysupplied with water and nutrients, the leaves experienced waterdeficits and showed decreased transpiration as trans plrationalwater flow was restricted by the constant number of xylem vesselspresent in the mesocotyl. As a consequence of this restriction,transpirational water flow velocities in the metaxylem vesselsreached mean values of 270 m h^–1 and phloem transportvelocities of 5.2 m h^–1. Despite limited xylem transportmineral nutrient concentrations in leaf tissues were not decreasedin single-rooted plants, but shoot and particularly stem developmentwas somewhat inhibited. Due to the lack of adventitious rootsthe shoot:root ratio was strongly increased in the single-rootedplants, but the seminal roots showed compensatory growth comparedto those in control plants. Consistent with decreased leaf conductance,ABA concentrations in leaves of single-rooted plants were elevatedup to 10-fold, but xylem sap ABA concentrations in these plantswere lower than in controls, in good agreement with the well-wateredconditions experienced by the seminal roots. Surprisingly, however,ABA concentrations in tissues of the seminal roots of the single-rooted plants were clearly increased compared to the controls,presumably due to increased ABA import via phloem from the water-stressedleaves. The results are discussed in relation to the role ofABA as a shoot to root signal. Key words: Zea mays, seminal roots, plant development, xylem transport, mineral nutrition, ABA, shoot-to-root signal     

Effects of Low Oxygen on Photosynthesis, Translocation and Growth in Green Pepper (Capsicum annuum)

Green pepper (Capsicum annuum cv. Bell Boy) plants were exposedin chambers to low (2%) oxygen and controlled carbon dioxideconcentrations. Vegetative and fruiting plants showed short-termincreases in net photosynthesis in low oxygen or elevated carbondioxide (up to 900 µl CO₂ l^–1). Photosynthesis ofyoung vegetative plants increased in low oxygen in the short-termbut there was no long-term benefit. Low oxygen enhancement ofphotosynthesis declined with time and after 10 d, leaf areaand root dry weight were less than in plants grown in normalair. Labelled assimilates were translocated from leaves to otherregions at similar rates in low oxygen and normal air. Low oxygenreduced respiratory losses from leaves and reduced the proportionof soluble carbohydrate converted to polysaccharide in all plantparts. Thus, low-oxygen environments decrease the utilisationof assimilates which then may lead to inhibition of photosynthesis. Capsicum annuum, photosynthesis, photorespiration, translocation, utilization of assimilates