The Effect of Phloem Ringing on Root Growth and Potassium Uptake by Helianthus annuus

The effect of phloem ringing on the uptake and transport ofpotassium by the roots of 4 week old sunflower plants has beeninvestigated. Ringing caused a rapid decline in both K⁺ uptakeand its transport (⁸⁶Rb tracer) to the shoot. The rate of rootelongation and the levels of sucrose in the root showed paralleldecreases after ringing. Measurement on isolated roots indicatedthat the effect of ringing the stem on K⁺ uptake by the rootswas confined to the apical 10 mm that included the extensionzone. It is postulated that the decline in potassium uptakeand transport, brought about by ringing, is due to the severanceof the sucrose supply which stops root growth. Key words: Roots, Growth, Salt uptake     

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

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

Uptake and distribution of 65Zn and 54Mn in wheat grownat sufficient and deficient levels of Zn and Mn: I. During vegetative growth

The uptake and distribution of ⁶⁵Zn and ⁵⁴Mn by wheat (Triticumaestivum cv. Aroona) was investigated. Plantswere grown in achelate-buffered nutrient solution with either sufficient Znand Mn, low Zn or low Mn. A single representative seminal rootfrom 14-d-old and 42-d-old plants was dual-labelled with ⁶⁵Znand ⁵⁴Mn. The 14-d-old plants were harvested every 10 min from10–140 min of labelling, whilst the 42-d-old plants wereharvested after 2 h of labelling. At harvest, each plant wasseparated into leaves, main stem, unexposedroots, and tillers.In addition, the crown was separatedfrom the stem in the 14-d-oldplants In the control plants labelled at 14 d, ⁶⁵Zn was firstdetectedand accumulated in the crown of the roots after 40–60min. Labelled Zn was then detected in the stem, followed bythe leaves. The oldest and youngest leaves received less ⁶⁵Znthan the second and third oldest leaves. The plants grown underlow Zn conditions accumulated more ⁶⁵Zn in their older leavesand transferred ⁶³Zn to the unexposed roots. Distribution of⁵⁴Mn was similar in the controls to that of ⁶⁵Zn, except theolder leaves received no HMn, At the second harvest, a similardistribution pattern of ⁶⁵Zn and ⁵⁴Mn was observed with regardto leaf age. Large amounts of ⁶⁵Zn and ⁵⁴Mn were detected withinthe unexposed roots of all treatments. It is suggested thatthe distribution of root-supplied Zn and Mn may be determinedby micronutrient status and its relationship with leaf transpirationrates. Key words: Distribution, manganese, vegetative growth, wheat, zinc     

Cycling of Amino-Nitrogen and other Nutrients between Shoots and Roots in Cereals--A Possible Mechanism Integrating Shoot and Root in the Regulation of Nutrient Uptake

A split root system was used to investigate the cycling of nitrogenbetween shoots and roots in young wheat and rye plants. ¹⁵N-nitratewas supplied to one part of the root system for various periods,at the end of which these roots were excised. Xylem sap wasthen collected from the other roots which had not been supplieddirectly with ¹⁵N-nitrate. ¹⁵N detected in the xylem sap indicatedcycling of nitrogen between shoots and roots. Calculations showedthat over 60% of the amino-N flux in the xylem was cycling.Thus nitrate assimilation in the root could account for onlya minor part of amino-N in the xylem sap. The specific activity of ¹⁵N in the total N of xylem sap washigher than in the total N of roots and shoots through whichit had cycled. This is because exchange between amino-N in thetransport pools and bulk tissue N is limited. It is proposedthat there is, in effect, a single regulatory pool of amino-N,common to shoots and roots, and that this pool may be a keyelement in the control of N uptake at the level of the wholeplant. The likely energy costs of cycling and implications for thepartitioning of N between shoots and roots are discussed. Infurther investigations the cycling of ⁴²K-potassium and ³²S-sulphurwas demonstrated. Key words: Potassium, sulphur, transport, xylem     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

Impairment of sucrose utilization for cell wall formation in the roots of aluminum-damaged cotton seedlings

Shoots of normal cotton seedlings rapidly fixed a pulse of ¹⁴CO₂from the ambient atmosphere and translocated some of the resultinglabeled sucrose to the roots. Roots of these plants assimilatedmost of the radioactivity from a 10-min labeling pulse intoinsoluble cell wall materials and other stable metabolites within4 to 6 hr after the pulse. However, roots of cotton seedlingswhich had been exposed to 1 ppm of Al^3$ for 24 hr before labelingtended to accumulate the ¹⁴C-label as free sucrose. Histologicand microautoradiographic evidence suggested that Al^3$ impairedthe root's capacity to utilize sucrose in further metabolicproducts so that ¹⁴C-labeled sucrose was not polymerized intocell wall materials as it was in the roots of control plants. (Received July 7, 1971; )     

Hypoxia affects root sodium and chloride concentrations and alters water conductance in salt-treated jack pine (<Emphasis Type="Italic">Pinus banksiana</Emphasis>) seedlings

The effects of NaCl were studied in 6-month-old jack pine (Pinus banksiana Lamb.) seedlings growing in solution culture under hypoxic (approximately 2 mg l^у O₂) and well-aerated (approximately 8 mg l^у O₂) conditions. The results showed that hypoxia led to further reduction of stomatal conductance (g_s) in plants treated with 45 mM NaCl. This effect was likely due to a reduction in root hydraulic conductance by both stresses. When applied individually or together, neither 45 mM NaCl nor hypoxia affected cell membrane integrity of needles as measured by tissue electrolyte leakage. Hypoxia did not alter shoot Na⁺ and Cl^m concentrations in NaCl-treated plants. However, root Na⁺ concentrations were lower in NaCl-treated hypoxic plants, suggesting that hypoxia affected the ability of roots to store Na⁺. Hypoxia also induced root electrolyte leakage from NaCl-treated and control plants. The higher root Cl^m concentrations compared with Na⁺ and the positive correlation between root Cl^m concentrations and electrolyte leakage suggest that Cl^m played a major role in salt injury observed in jack pine seedlings. Roots of well-aerated plants treated for 1 week with NaCl contained almost two-fold higher concentration of total non-structural carbohydrates compared with plants from other experimental treatments and these concentrations decreased in subsequent weeks. We suggest that under prolonged hypoxic conditions, roots lose the ability to prevent Cl^m uptake resulting in the increase in root Cl^m concentration, which has damaging effects on root cell membranes.     

The Translocation of Sulphonamides in Higher Plants: III. ACETYLATION AND DEACETYLATION OF SULPHANILAMIDE IN BROAD BEANS AND WHEAT

The acetylation of both amino groups on sulphanilaniide hasbeen demon strated by paper chromatography of sap expressedfrom broad beans treated with suiphanilamide through the roots.Deacetylation of N¹- and N⁴ mides occurred in the leaves oftreated plants and the N⁴ compound was also acetylated to yieldthe N¹: N⁴-diacetylsulphanilamide. About 30 per cent. of the suiphanilamide entering broad-beanroots was acetylated in the roots, the maximum proportion ofthe N⁴-acetyl compound being reached after 7 days. Deacetylationis a more limited process but reaches a steady state after thesame time. Low levels of acetylation and deacetylation wererecorded in wheat. The amounts of suiphanilamide and N⁴ entering and accumulatingin the various tissues were calculated on a water-uptake basisarid these data show that the main site of acetylation is inthe roots and that deacetyla tion occurs to a limited extentin the roots but predominates in stems and leaves.     

Hypoxia Induces Membrane Depolarization and Potassium Loss from Wheat Roots but does not Increase their Permeability to Sorbitol

This paper deals with the responses of roots of wheat {Triticumaestivum L.) to hypoxia with special emphasis on the effectsof severe O2 deficiency on membrane integrity, loss of K+ fromthe root and root membrane potentials. Seminal and crown roots of 26-d-old plants exposed to severehypoxia (0.003 mol O₂ m^–3) for 3 h or 10 d prior to excisionand subsequently exposed to hypoxic solutions, had slightlylower rates of sorbitol influx and a slightly smaller apparentfree space than roots in aerated solutions. These results indicatethat neither a few hours nor a 10-d exposure to hypoxia hadadverse effects on the membrane integrity of the bulk of thecells in the roots. However, both 6-d-old seedlings and 26-d-oldplants lost K⁺ from the roots following their transfer fromaerated to hypoxic nutrient solutions. In the 26-d-old plants,which were of high nutritional status, there was a net K⁺ effluxfrom the roots to the external solution. In contrast, with the6-d-old seedlings, which were of low nutritional status, thedecrease in the K⁺ content of the roots was smaller than thenet K⁺ uptake to the shoots. Exposure of excised roots to 0.008 mol O₂ m^–3caused arapid and reversible membrane depolarization from –120to ––80 mV. These data and the magnitude of thenet effluxes strongly suggest that K⁺ losses during the earlystages of hypoxia are due to membrane depolarization ratherthan to increases in the permeability of membranes to K ⁺. Key words: Hypoxia, membrane integrity, membrane potentials, seminal and crown roots     

The Distribution and Identity of Assimilates in Tomato with Special Reference to Stem Reserves

Much of the work on the distribution of ¹⁴C-labelled assimilatesin tomato has been done in winter under low light intensities,and consequently the reported distribution patterns of ¹⁴C maynot be representative of plants growing in high light. Further,there are several somewhat conflicting reports on patterns ofdistribution of ¹⁴C-assimilates in young tomato plants. We soughtto clarify the situation by studying the distribution of ¹⁴C-assimilatesin tomato plants of various ages grown in summer when the lightintensity was high. In addition, the role of the stem as a storageorgan for carbon was assessed by (a) identifying the chemicalfractions in the stem internode below a fed leaf and monitoring¹⁴ C activity in these fractions over a period of 49 d, and(b) measuring concentrations of unlabelled carbohydrates inthe stem over the life of the plant. The patterns of distribution of ¹⁴C-assimilates we found fortomato grown under high light intensity confirmed some of thosedescribed for plants grown under low light, but export of ¹⁴Cby fed leaves was generally higher than reported for much ofthe earlier work. Lower leaves of young plants exported over50% of the ¹⁴C they fixed, although export fell sharply as theplants aged. Initially, the roots and apical tuft were strongsinks for assimilates, but they had declined in importance bythe time plants reached the nine-leaf stage. On the other hand,the stem became progressively more important as a sink for ¹⁴C-assimilates.Older, lower leaves exported more of their ¹⁴C-assimilates tothe upper part of the plant than to the roots, whereas youngleaves near the top of the plant exported more of their assimilatesto the roots. The stem internode immediately below a fed leafhad about twice the ¹⁴C activity of the internode above theleaf. Mature leaves above and below a fed leaf rarely importedmuch ¹⁴C, even when in the correct phyllotactic relationshipto the fed leaf. In the first 3 d after feeding leaf 5 of nine-leaf plants, theorganic and amino acid pools and the neutral fraction of theinternode below the fed leaf had most of the ¹⁴C activity, butby 49 d after feeding, the ethanolic-insoluble, starch and lipidfractions had most of the ¹⁴C activity. Glucose, fructose andsucrose were the main sugars in the stem. Although concentrationsof these sugars and starch declined in the stem as the plantsmatured, there was little evidence to indicate their use infruit production. Stems of plants defoliated at the 44-leafstage had lower concentrations of sugars and starch at maturity,and produced less fruit than the controls. It was concludedthat tomato is sink rather than source limited with respectto carbon assimilates, and that the storage of carbon in thestem for a long period is possibly a residual perennial traitin tomato.Copyright 1994, 1999 Academic Press Lycopersicon esculentum, tomato, assimilate distribution, ¹⁴C, internode storage, sink-source relationships, starch, stem reserves, sugars     

The Redistribution of Labelled Sulphate Ions in Sunflower Plants subjected to Different Treatments

The effect of 0.5 and 5.0 mM sulphate concentrations and 15,25, and 35°C root temperatures on the redistribution of³⁵SO₄ in sunflower plants was studied at two rates of transpiration.It is concluded from the results that ³⁵SO₄ was lost rapidlyfrom the free space of the roots to the external solution andthat this losa was independent of the transpiration rates androot temperatures. The fact there was no marked decrease inthe ³⁵SO₄ content of the shoots during the ‘washing’process indicates that the free space is likely to be confinedto the cortical region of the roots, and there seems to be adiffusion barrier between the cortex and xylem preventing arapid movement of ³⁵SO₄ from the cortex to the xylem.     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjectedto 15 d of nitrogen deprivation were resupplied for 10 d with1.0 mol m^–3 nitrogen provided as NO₃^–, NH₄⁺, orNH₄⁺+NO₃^– in flowing hydroponic culture. Plants in a fourthhydroponic system received 1.0 mol m^–3 NO₃^– duringboth stress and resupply periods. Concentrations of solublecarbohydrates and organic acids in roots increased 210 and 370%,respectively, during stress. For the first day of resupply,however, specific uptake rates of nitrogen, determined by ionchromatography as depletion from solution, were lower for stressedthan for non-stressed plants by 43% for NO₃^- resupply, by 32%for NH₄⁺ + NO₃^– resupply, and 86% for NH₄⁺ resupply. Whenspecific uptake of nitrogen for stressed plants recovered torates for non-stressed plants at 6 to 8 d after nitrogen resupply,carbohydrates and organic acids in their roots had declinedto concentrations lower than those of non-stressed plants. Recoveryof nitrogen uptake capacity of roots thus does not appear tobe regulated simply by the content of soluble carbon compoundswithin roots. Solution concentrations of NH₄⁺ and NO₃^– were monitoredat 62.5 min intervals during the first 3 d of resupply. Intermittent‘hourly’ intervals of net influx and net effluxoccurred. Rates of uptake during influx intervals were greaterfor the NH₄⁺ -resupplied than for the NO₃^– -resuppliedplants. For NH₄⁺ -resupplied plants, however, the hourly intervalsof efflux were more numerous than for NO₃^– -resuppliedplants. It thus is possible that, instead of repressing NH₄⁺influx, increased accumulation of amino acids and NH₄⁺ in NH₄⁺-resupplled plants inhibited net uptake by stimulation of effluxof NH₄⁺ absorbed in excess of availability of carbon skeletonsfor assimilation. Entry of NH₄⁺ into root cytoplasm appearedto be less restricted than translocation of amino acids fromthe cytoplasm into the xylem. Key words: Ammonium, nitrate, nitrogen-nutrition, nitrogen-stress, soybean     

Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

Influence of the Form of Nitrogen Supply on Root Uptake and Translocation of Cations in the Xylem Exudate of Maize (Zea mays L)

Concentrations of inorganic cations are often lower in plantssupplied with NH₄⁺ as compared with NO₃^–. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m^–3 NO₃^–or NH₄⁺ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO₃^– and NH₄⁺ declined by more than 50%Compared with NO₃^– supply, NH₄⁺ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH₄⁺- than in NO₃^–-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH₄⁺-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH₄⁺ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH₄⁺-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation     

Regulation of Sulphate Transport in a Tropical Legume, Macroptilium atropurpureum, cv. Siratro

When young plants of Macroptilium atropurpureum, cv. Siratrowere deprived of external sulphate (-S plants) growth of shootsand roots continued at rates comparable to those in plants wellsupplied with sulphate (control) for 3 d and 5 d respectively.Dilution of internal sulphur therefore took place and redistributionof sulphur occurred between inorganic and organic forms andbetween roots and younger leaves. Even when S-deficiency limitedgrowth, plants contained 16% of their total sulphur as sulphate,but most of this was retained in old leaves and redistributedslowly to growing zones. The capacity for sulphate uptake increased in roots of –Splants very soon after they were deprived of external sulphate;within 24 h the absorption from 0.25 mol m^–3 SO₄^2–was more than five times that of control roots. Maximum increasedcapacity was reached after 2–3 d stress when the V_maxof system 1 was 1948 nmol h^–1g^–1root fr. wt. in–S plants and 337 nmol h^–1g^–1root fr. wt.in controls. The K^mfor system 1 did not change significantlywith S-stress being between 5–8 µM in both setsof plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growthrate and the rate at which sulphate uptake capacity was enhancedby withholding sulphate from culture solutions. When –S plants were replaced in sulphate-containing solutiontheir capacity for SO₄^2– declined to the control levelwithin 24 h. Very marked repression of capacity was also foundwhen –S plants were treated with L-cysteine, but therewas no immediate effect with methionine. Roots of this species appear to have a very active system fordegrading L-cysteine to sulphate, 30% of the label in ³⁵S-cysteineabsorbed by roots was recovered in ³⁵SO₄^2– after 20 minor 2 h incubation. By contrast, roots had a very weak abilityto reduce sulphate. When part of the root system was in solution lacking sulphatethere was enhanced uptake of sulphate by other parts which themselveswere amply supplied with sulphate. This is seen as an exampleof compensatory absorption. The response to S-stress is specific and there were no positiveinteractions between S-stress and the absorption of phosphate,or P-stress and the uptake of sulphate. The results are discussed in relation to the close control ofsulphate uptake by internal sulphate concentration, redistributionof forms of sulphur during stress and mobility of sulphate inthe phloem. Key words: Kinetics, Amino-S, Sulpholipid, Repression;, Deficiency     

The Movement of 150 through Barley and Rice Plants

Air labelled with ¹⁵O moves from the shoot to the root in riceand barley plants. The kinetics of the movement are consistentwith gaseous diffusion of ¹⁵O through continuous gas spaces.The size of the gas spaces in barley roots is considerably smallerthan in rice roots.     

The Translocation and Redistribution of Manganese in Avena

⁵⁴Mn present in the first two leaves of oat seedlings subsequentlydeprived of manganese was later redistributed to leaves 4 and5. ⁵⁴Mn was found in leaves 3 and 4 even when the roots of seedlingswere excised immediately after exposure to ⁵⁴Mn, but more wasdetected if the roots were left intact. ⁵⁴Mn applied as a drop to the 4th leaf of manganese-deficientoat plants was concentrated in the stem and translocated primarilyto the youngest developing leaf or to the grain if present.⁵⁴Mn was readily detected in the roots but almost none was translocatedto the first three leaves. More ⁵⁴Mn was translocated in 96hrs. than 24, but little or no more was translocated in 192hrs. Plants which were given 0.5 p.p.m. stable manganese until theyreached the 4th leaf stage, and were then exposed to ⁵⁴Mn, showeda fairly uniform distribution of ⁵⁴Mn throughout the plant.There was relatively slight concentration at active growth centres. It is concluded that physiologically significant redistributionof manganese occurs in the oat plant.     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.