Influence of chromium(lll) on root-associated Fe(lll) reductase in Plantago lanceolata L.

The application of low chromium concentrations (5 M CrCI₃)decreased chlorophyll content of iron-deficient plants, butdid not cause visual toxicity symptoms in iron-sufficient plants.No significant plant growth differentials were obtained in responseto the various treatments. Chromium application to iron-deficientPlantago lanceolata L. roots increased the activity of root-associatedFe(lll) reductase. This effect was evident only with acceptorsof the turbo reductase and was not observed in iron-sufficientplants. Chromium accumulated in the roots, but was poorly transportedto the leaves. In split-root experiments, which allowed onlya part of the root system to receive chromium, while the otherportion was grown in iron-free medium, roots subjected to eithertreatment showed an intermediate FeEDTA reductase activity withrespect to non-split control plants. It is concluded that amobile factor from the leaves modulates the degree of expressionof the iron stress response. Possible mechanisms by which chromiumaffects Fe metabolism are discussed. Key words: Plantago lanceolata, ferric iron reduction, chromium, iron nutrition, shoot-to-root communication     

Comparison of the Respiratory Metabolism of Plantago lanceolata L. and Plantago major L.

Bryce, J. H. and ap Rees, T. 1985. Comparison of the respiratorymetabolism of Plantago lanceolata L. and Plantago major L.—J.exp. Bot. 36 1559–1565. The aim of this work was to discover if the respiratory metabolismof the roots of Plantago lanceolata L. differed from that ofthe roots of Plantago major L. Measurements of oxygen uptakeand dry weight of excised root systems during growth of seedlingsprovided evidence that the two species differed in the amountof respiration needed to support a given increase in dry weight.Excised root systems were given a 6-h pulse in [U-¹⁴C]sucrosefollowed by a 16.5-h chase in sucrose. The detailed distributionof ¹⁴C amongst the major components of the roots at the endof the pulse and the chase revealed no significant differencebetween the two species. Patterns of ¹⁴CO₂ production from [1-¹⁴C],[2-¹⁴C], [3,4-¹⁴C], and [6-¹⁴C]glucose of excised root systemsfrom plants of three ages were similar for the two species.It is suggested that there is no conclusive evidence for anysignificant inherent difference in the respiratory metabolismof the roots of the two species. Key words: ¹⁴C sugar metabolism, respiration, roots, Plantago     

Effect of Root Zone Temperature and Shoot Demand on Uptake and Xylem Transport of Macronutrients in Maize (Zea mays L.)

The effect of shoot demand for nutrients on nutrient uptakeand translocation in the xylem exudate was studied in maizegrowing in nutrient solution at uniform shoot zone (24/2C,day/night), but different root zone temperatures (RZT: 12C,18C, 24C). The shoot base (apical shoot meristem and zoneof leaf extension) was either kept within or lifted above thecooling zone. In plants with their shoot base above the coolingzone (RZT: 12C and 18C) shoot growth was significantly increasedbut not root growth. Therefore, at suboptimal RZT shoot freshweight increment d^–1 g^–1 root fresh weight, whichwas taken as a parameter for the shoot demand for nutrientsper unit root, varied strongly depending on the temperatureof the shoot base (shoot base temperature, SBT). In short-term studies (2 h and 1 d after onset of temperaturetreatment) rates of nutrient (nitrogen, N; potassium, K; phosphorus,P; calcium, Ca) uptake or translocation in the xylem exudatewere markedly decreased at suboptimal RZT (12C, 18C), irrespectiveof the SBT. In long-term studies (3, 5, and 10 d after onsetof temperature treatment) uptake and translocation of K, N,and Ca, but not P, increased in plants at suboptimal RZT whenthe shoot demand was high (shoot base above the cooling zone)but decreased when the shoot demand was low (shoot base withinthe cooling zone). These results suggest, that the increase of translocation ratesof N, K, and Ca after long-term exposure to suboptimal RZT wasa consequence of a higher shoot demand per unit root fresh weightand not due to a direct temperature effect on the nutrient uptakesystem. Key words: Xylem exudate, nutrient translocation, root zone temperature, shoot demand, nutrient circulation     

Root-mediated ferric reduction-responses to iron deficiency, exogenously induced changes in hormonal balance and inhibition of protein synthesis

The effects of the auxin analogue 2,4-dichloro-phenoxyaceticacid (2,4-D), the translation inhibitor cycloheximide (CHM)and iron deficiency on electron transport activities in rootshave been investigated. Deprivation of an external Fe supplycaused an increase in capacity to reduce both FeEDTA and theartificial electron acceptor ferricyanide. With respect to theiron stress-induced redox activity, bean (Phaseolus vulgarisL.) roots reduced both substrates at equal rates, while rootsof Plantago lanceolata L. exhibited clearly higher rates withFeEDTA. The iron stress induced FeEDTA and ferricyanide reductionactivities were unequally affected by CHM in both species, reflectingthe differences in substrate specificity. Application of 2,4-D via the nutrient solution induced an increasein ferricyanide reduction rates in iron-sufficient roots ofP. lanceolata; the reduction of FeEDTA was only slightly affected.The presence of 2,4-D caused no further enhancement in reductionactivity of plants grown without Fe. However, iron-deficientplants exhibited a distinct developmental pattern of ferricreduction activities when treated with 2,4-D, resembling thedevelopment of 2,4-D-induced ferricyanide reduction by iron-sufficientplants. This information is taken as support that the reduction of ferricyanideand FeEDTA can in part be separately regulated. The data alsoseem to indicate that the amount of Fe²⁺ is the main cause forinduction or repression of ferric chelate reduction by rootsof iron-efficient plants. Key words: Phaseolus vulgaris, Plantago, ferric iron reduction, cycloheximide, 2,4-dichlorophenoxyacetic acid     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

The Effect of Light Intensity and Relative Humidity on Growth Rate and Root Respiration of Plantago lanceolata and Zea mays

Plants of Plantago lanceolata L. and Zea mays L., cv. ‘Campo’were grown at two levels of light intensity. Especially in theroots, the rate of dry matter accumulation decreased at lowlight intensity. The carbohydrate content of both roots andshoots of P. lanceolata was not affected by light intensity.The relative contribution of SHAM¹-sensitive respiration, thealternative chain, to total root respiration of both P. lanceolataand Z. mays, was not affected by light intensity during thedaytime. The alternative pathway was somewhat decreased at theend of the dark period, but not in the root tips (0–5mm) where it still contributed 56% in respiration. It was, therefore,concluded that photosynthesis is not a major factor in regulationof root growth in the species investigated. To see whether the effect of light intensity on root growthrate was via transpiration, plants of Z. mays were grown atdifferent air humidities. Both high humidity and low light intensityaffected the root morphology in such a way that the distancebetween the apex and the first laterals on the primary rootaxis increased. It is suggested that this effect on root morphologyis due to transpiration and the subsequent removal of root-producedinhibitors of lateral root growth; although light intensityalso affected the rate of dry matter accumulation of roots andthe rate was not affected by the humidity of the air. It is,therefore, concluded that the effect of light intensity on therate of dry matter accumulation of roots of Z. mays is not viaan effect on transpiration.     

Fe-deficiency stress response in Fe-deficiency resistant and susceptible subterranean clover: importance of induced H+ release

Plants can exhibit Fe-deficiency stress response when they areexposed to Fe-deficiency conditions. The relative importanceof the individual Fe-deficiency stress-response reactions, forexample, increased release of H⁺ from roots, enhanced root plasmamembrane-bound Fe³⁺ -reductase activity, and release of reductant,in Fe-deficiency resistance is not understood. To address thisproblem, the Fe-deficiency stress response of two cultivarsof subterranean clover (subclover), Koala (Trifolium brachycalycinumKatzn. and Morley) (Fe-deficiency resistant) and Karridale (T.subterraneum L.) (Fe-deficiency susceptible), were evaluated.The plants were cultured hydroponically at 0 (–Fe) and30 (+Fe) µM Fe³⁺ EDTA conditions. After 6 d Fe treatment,the –Fe Koala and Karridale decreased the pH of the nutrientsolution by 1.83 and 0.79 units, respectively, while the +Feplants increased the pH of the nutrient solution. The H⁺ -releaserate of the –Fe Koala determined 7 d after Fe treatmentinitiation was more than three times higher than that of the–Fe Karridale. The –Fe plants had a significantlyenhanced Fe³⁺ -reduction rate compared with the +Fe plants foreach cultivar, but the resistant cultivar did not exhibit ahigher root Fe³⁺ -reduction rate than the susceptible cultivarat each Fe treatment. Reductant release from the roots of subcloverwas negligible. These results indicate that Fe-deficiency-inducedH⁺ release may be the predominant factor influencing Fe-deficiencyresistance in subclover. Key words: Fe-deficiency, Fe³⁺ reduction, H⁺ release, stress response, Trifolium     

Effects of low root zone temperatures on the early stages of symbiosis establishment between soybean [Glycine max (L.) Merr.] and Bradyrhizobium japonicum

It is known that low root zone temperatures (RZT) have moreeffect on infection and early nodule development than on nitrogenfixation by soybean [Glycine max (L.) Merr.]. However, therehave been no studies regarding how the low RZT inhibit the infectionstages of soybean. Two controlled environment experiments wereconducted to examine the effect of low RZT on bacterial attachmentto, and infection thread penetration of, soybean root hairs.The experimental designs were (1) plants maintained at 25, 17.5or 15C RZT, or transferred from 25 or 17.5 to 15C RZT at either0.5, 1, 2, or 7d after inoculation (DAI), (2) early symbioticestablishment between soybean and Bradyrhizobium japonicum wasexamined microscopically under three RZT (15, 17.5 and 25C).These results indicated that (1) keeping plants at 25C only0.5 DAI prior to transfer to a 15C RZT accelerates the onsetof N₂ fixation at 15C RZT by 6 d, (2) at RZT between 25 and17.5C the infection processes were progressively delayed astemperature declined, (3) RZT less than 17C strongly inhibitedinfection steps, such that when RZT dropped 8.5C from 25 to17.5C infection initiation was delayed 1 d, while when RZTdropped only 2.5C from 17.5 to 15C, infection initiation wasdelayed another 2 d. Key words: Bradyrhizobium japonicum, low temperature, nodulation, soybean     

Ethylene Production by Fe-deficient Roots and its Involvement in the Regulation of Fe-deficiency Stress Responses by Strategy I Plants

Species that showed marked morphological and physiological responsesby their roots to Fe-deficiency (Strategy I plants) were comparedwith others that do not exhibit these responses (Strategy IIplants). Roots from Fe-deficient cucumber (Cucumis sativusL.‘Ashley’), tomato (Lycopersicon esculentumMill.T3238FER) and pea (Pisum sativumL. ‘Sparkle’) plantsproduced more ethylene than those of Fe-sufficient plants. Thehigher production of ethylene in Fe-deficient cucumber and peaplants occurred before Fe-deficient plants showed chlorosissymptoms and was parallel to the occurrence of Fe-deficiencystress responses. The addition of either the ethylene precursorACC, 1-aminocyclopropane-1-carboxylic acid, or the ethylenereleasing substance, Ethephon, to several Fe-sufficient StrategyI plants [cucumber, tomato, pea, sugar beet (Beta vulgarisL.),Arabidopsis(Arabidopsis thaliana(L.) Heynh ‘Columbia’), plantago(Plantago lanceolataL.)] promoted some of their Fe-deficiencystress responses: enhanced root ferric-reducing capacity andswollen root tips. By contrast, Fe-deficient roots from severalStrategy II plants [maize (Zea maysL. ‘Funo’), wheat(Triticum aestivumL. ‘Yécora’), barley (HordeumvulgareL. ‘Barbarrosa’)] did not produce more ethylenethan the Fe-sufficient ones. Furthermore, ACC had no effecton the reducing capacity of these Strategy II plants and, exceptin barley, did not promote swelling of root tips. In conclusion,results suggest that ethylene is involved in the regulationof Fe-deficiency stress responses by Strategy I plants.Copyright1999 Annals of Botany Company. Arabidopsis (Arabidopsis thaliana(L.) Heynch), barley (Hordeum vulgareL.), cucumber (Cucumis sativusL.), ethylene, iron deficiency, maize (Zea maysL.), pea (Pisum sativumL.), plantago (Plantago lanceolataL.), ferric-reducing capacity, sugar beet (Beta vulgarisL.), tomato (Lycopersicon esculentumMill.), wheat (Triticum aestivumL.).     

The Function of Trace Metals in the Nitorgen Metabolism of Plants

In recent years trace metals have been found to be associatedwith flavo-proteins, a number of which are key enzymes in thenitrogen metabolism of plants. Thus Mo is a constituent of nitratereductase which mediates the reduction of nitrate to nitritein some bacteria, fungi, and higher plants. During the enzymicreduction of nitrate a one-electron change is involved, Mo⁶⁺Mo⁵⁺.Nitrite and hyponitrite reductases both require Fe and Cu andflavin for maximum activity and are inhibited by uncouplingreagents so that a phosphorylation takes place at these tworeductive steps. Hydroxylamine reductase, a flavoprotein thatis Mn-dependent, but, as with nitrate reductase, is unaffectedby uncoupling reagents. An aromatic route may also exist inplants for the reduction of organo-nitro compounds, but it ismore likely that the aliphatic route is the primary reductivepadiway. Mo, V, W, and Fe may all be involved in the process of N₂ fixationin free living micro-organisms and in the root nodule bacteria.The suggestion that the enzyme hydrogenase is coupled to N₂fixation is not borne out by the results of valency experimentswith Mo and V which show that an active hydrogenase does notnecessarily result in a vigorous fixation process. Although Mo has been shown to be a constituent of hydrogenasefrom Clostridium pasteurianum, experiments in this laboratoryshow that a deficiency of iron only reduced the enzyme in thisorganism and in Azotobacter. Both Mo and Fe, however, are essentialfor N₂ fixation. In some strains of Azotobacter, V or W canpartially replace Mo for N₂ fixation.     

Genistein accumulation in soybean (Glycine max [L.] Merr.) root systems under suboptimal root zone temperatures

Genistein, as a plant-to-bacteria signal, plays an importantrole in the establishment of the soybean (Glycine max [L.] Merr.)-Bradyrhizobiumjaponicum nitrogen-fixing symbiosis. It is essential to thedevelopment of effective root nodules and responsible for inducingthe nod genes of B. japonicum. Because sub-optimal root zonetemperature (RZT) delays infection and early nodule development,and decreases plant nodule number, and genistein addition overcomessome of this, it is reasonable to hypothesize that suboptimalRZT disrupts the inter-organismal signal exchange by inhibitinggenistein synthesis. Four experiments were conducted to testthese hypotheses. The results of these studies indicated that:(1) when soybean plants were germinated and maintained at RZTsranging from 13 to 17C, root genistein concentration and contentper plant were lower than those of plants with roots maintainedat RZTs above 17C; (2) when plants were germinated at an optimalRZT (25 C) then transferred to RZTs below 17C, and acclimatedfor a few days, root genistein concentration and content perplant were higher than those of plants with roots maintainedeither at optimal RZT, or transferred to RZT above 17 C, althoughby the end of the experiment, the genistein concentration ofroot systems at below 17C RZT appeared to be declining to valuesbelow those of plants with above 17 C RZT; (3) the root genisteinconcentration increased before the onset of nitrogen fixationand decreased thereafter; and (4) part of the effect of RZTson genistein content per plant root system was from reductionsin genistein concentration at lower RZT5, and part was due todecreased plant root growth. Key words: Genistein, Glycine max, suboptimal temperature     

Reproductive Biology of Plantago: Shift from Cross- to Self-pollination

The breeding systems of an obligate outbreeder (Plantago lanceolata)and an obligate inbreeder (P. patagonica) were compared. Outbreedingin the former and inbreeding in the latter species is achievedby self-incompatibility and preanthesis cleistogamy, respectively.The difference in breeding system is accompanied by a differencein the pollen output, pollen to ovule ratio, stigma and anthersize, and seed weight. P. ovata, a third species known onlyin cultivated form, is intermediate. This species has two typesof plants, some of which are protogynous and others where stigmareceptivity and anther dehiscence synchronize. Although thedifferences between the two intraspecific variants are not large,they nevertheless suggest that the species is in evolutionaryflux. Domestication is known to have modified the breeding systemin many crop plants and there is evidence that, in P. ovata,a start in this direction has already been made. Plantago lanceolata, P. ovata, P. patagonica, protogyny, anther, stigma, pollen to ovule ratio     

Response to Waterlogging and Differing Sensitivity to Divalent Iron and Manganese in Clones of Dactylis glomerata L. derived from Well-drained and Poorly-drained Soils

Clonally propagated plants of Dactylis glomerata derived froma well-drained, heavily grazed cliff habitat (clone L) and froman undergrazed poorly-drained soil (clone A) were tested forwaterlogging tolerance in soil-culture. Water-logging did notaffect the two clones differentially, a result, which contrastedstrongly with that of a previous experiment in which simulatedgrazing (clipping to 20 cm) unexpectedly caused clone A to beless tolerant of waterlogging than clone L. Maximum leaf andleaf sheath length was reduced more by water-logging in cloneL than in clone A (P < 0.05). In solution-culture when providedwith factorial combinations of 0.5, 5 and 50 mg dm^–2 ofFe²⁺ and Mn²⁺ the shoot dry weight yield of the dry-soil clonewas reduced more than that of the wet-soil clone by 50 mg Fedm^–3 irrespective of Mn²⁺ concentration (P < 0.01)but the reduction of growth was less at higher Mn²⁺ concentrations.Fifty milligrams of Mn²⁺ dm^–3 reduced the growth of thedry soil clone but increased the growth of the wet soil clonewith Fe²⁺ at 5 mg dm^–2 (P < 0.05). Iron at 0.5 mg dm^–2was suboptimal for shoot growth of both clones at any levelof Mn²⁺ and caused more severe leaf chlorosis in the wet soilclone. Leaf tissue of clone L contained more iron than thatof clone A after waterlogging (P < 0.01) but in solutionculture, though increasing iron from 0.5 to 50 mg dm^–3almost doubled leaf iron content (P < 0.001), the interactionClones x Mn x Fe just failed to reach significance at P <0.05. The manganese content of leaf tissue from the two clonesvaried differently in response to solution manganese (Clonesx Mn P < 0.01), clone A showing a slightly greater increaseof manganese content at high solution concentration. Iron at50 mg dm^–3 suppressed Mn uptake (Mn x Fe, P < 0.001)in both clones. The two clones thus show marked environmentaladaptation to the chemistry of wet and dry soils. Dactylis glomerata, Cocksfoot grass, Orchard grass, waterlogging, iron, manganese, toxicity, deficiency, ecotypes     

Reduction of root iron in Plantago lanceolata during recovery from Fe deficiency

Changes in root-associated Fe(III) reductase activity and Fe concentration during recovery from temporary iron starvation were investigated in hydroponically grown Plantago lanceolata L. Within two days, interruption of the Fe supply resulted in enhanced rates of reduction by intact plant roots. Transfer of iron deficient plants to a solution containing various amounts of chelated Fe caused a transient increase in reduction activity before the rates declined to a level determined by the amount of Fe added. Repression of root-associated redox activity was independent of the Fe concentration in the preculture. When iron deficient plants were submitted to a supply of Fe localized to a part of the root system (split-root plants), the decrease in reduction rates was much more pronounced in the Fe-deprived portion of the roots than in the Fe-supplied one. No correlation was observed between root Fe concentration and Fe(III) reductase activity. Continued growth of split-root plants in the +Fe/-Fe regime increased the reduction rates of the +Fe-grown portion of the root system over the rates in iron sufficient plants with non-divided roots. The results are discussed in relation to putative factors mediating intra- and interorgan regulation of iron nutrition.     

Reduction of root iron in Plantago lanceolata during recovery from Fe deficiency

Changes in root-associated Fe(III) reductase activity and Fe concentration during recovery from temporary iron starvation were investigated in hydroponically grown Plantago lanceolata L. Within two days, interruption of the Fe supply resulted in enhanced rates of reduction by intact plant roots. Transfer of iron deficient plants to a solution containing various amounts of chelated Fe caused a transient increase in reduction activity before the rates declined to a level determined by the amount of Fe added. Repression of root-associated redox activity was independent of the Fe concentration in the preculture. When iron deficient plants were submitted to a supply of Fe localized to a part of the root system (split-root plants), the decrease in reduction rates was much more pronounced in the Fe-deprived portion of the roots than in the Fe-supplied one. No correlation was observed between root Fe concentration and Fe(III) reductase activity. Continued growth of split-root plants in the +Fe/-Fe regime increased the reduction rates of the +Fe-grown portion of the root system over the rates in iron sufficient plants with non-divided roots. The results are discussed in relation to putative factors mediating intra- and interorgan regulation of iron nutrition.     

Breaking of Seed Dormancy by Nitrate as a Gap Detection Mechanism

Germination of Planlago lanceolata seeds buried in a chalk grasslandwas higher in bare soil than in vegetated soil, and measurementof soil nitrate concentrations showed that they were high enoughto account for this stimulation. When seeds of P. lanceolatawere sown in pots of soil with or without plants, and wateredwith nutrient solution containing either no nitrate, or 14 mMnitrate (sufficiently high that not all nitrate was absorbedby the plants), the presence of plants inhibited germinationonly when the nutrient solution contained no nitrate. It wasconcluded that breaking of seed dormancy by nitrate can functionas a gap detection mechanism if nitrate concentrations in baresoil are high enough to break seed dormancy, but are too lowto break dormancy when vegetation is present Plantago lanceolata, seed, dormancy, germination, nitrate, gap detection     

Differences in Iron Nutrition Strategies of Two Calcifuges,Carex piluliferaL. andVeronica officinalisL.

Veronica officinalisandCarex pilulifera, widespread calcifugeplants in Europe, were cultivated in acid and calcareous soilsto study differences in Fe aquisition strategies indicated inprevious studies. The experiments were performed in a computer-controlledglasshouse at a soil solution moisture content of 50–60%water holding capacity; additional light was supplied at 70W m^-2if ambient light was <100 W m^-2between 0600 and 1800h.Both species developed chlorosis when grown in the calcareoussoil.C. piluliferaproved unable to translocate sufficient amountsof Fe to the leaves when cultivated in calcareous soil, butmuch Fe accumulated in, and especially as a precipitate on thesurface of roots. In contrast,V. officinalistended to increaseFe taken up into the leaves of plants grown on calcareous soil,but a much greater proportion of the leaf tissue Fe was accumulatedas less active forms not extracted by Fe complexing agents,e.g. 1,10-phenanthroline, than was the case in acid-soil grownplants. Considerably less Fe was accumulated in the root biomassofV. officinaliscompared toC. pilulifera.It is concluded thatchlorosis inC. piluliferais related to insufficient Fe uptakein the leaves, whereas leaf immobilization of Fe in physiologicallyless active forms is the problem inV. officinalis. Iron; chlorosis; calcifuge; iron immobilization; leaf tissue; fractionation; Carex pilulifera; Veronica officinalis     

Induction of flowering by cytokinins in a short-day plant, Lemna paucicostata

Lemna paucicostata, normally a short-day plant, can be inducedto flower under long-day conditions by providing a cytokininin a medium containing a high level of ferric citrate (5 x 10^–4M).Interestingly, when a cytokinin and EDDHA are present togetherin the medium, flowering is induced even at low levels of iron(10^–5 and 5 x 10^–5M ferric citrate). However, inthe absence of a cytokinin, flowering takes place only undershort days. (Received September 30, 1968; )     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.