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

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

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napus L.) in Flowing Solution Culture: I. GROWTH UPTAKE OF NITROGEN

Oilseed rape plants (Brassica napus L. cv. Bien venu) were grownin flowing nutrient solution and acclimatized over 49 d to lowroot temperature (5 ?C) prior to treatment at root temperaturesof 3, 7, 11 or 17?C for 14 d, with common air temperature of20/15?C day/night and solution pH 6.0. The effects of root temperatureon growth, uptake and assimilation of N were compared with Nsupplied either as or at 10 mmol m^–3. Plant fresh weight, dry weight and leafarea increased with root temperature. Q₁₀ values for specificgrowth rates between 7?C and 17?C were 1.10 and 1.33, respectively,under and nutrition. Fresh weight and leaf area were favoured by nutrition, particularly at 11 ?C and 17 ?C. At each temperaturethe net uptake of over 14 d was similar to that of , increasing 4-fold between 3?C and 17 ?C. Unit absorption rates of increased over the first 3 or 4 d, to an extent that increased with roottemperature. Thereafter, near constant values were maintainedat 3, 7 and 11 ?C. Unit absorption rates of at 3 ?C and 7 ?C did not increase over the first 3 or 4 d. Plantsat 17?C responded differently from those at other temperatures,with steep increases in unit absorption rates of and after day 8. The tendency for plants to adjust unit absorption rates of N over 14 d to become independentof root temperature was limited to the range 7–11 ?C.Q¹⁰ values between 7 ?C and 17 ?C for unit absorption rateswere 1.7—1.8 and 1.4—1.5, respectively, for and . Plants at 7, 11 and 17?C absorbed more than was subsequently assimilated, but at 3 ?C the absorption of was insufficient to meet the assimilatory demand. The results are discussed comparativelyfor and and in terms of the factors that determine unit absorption rates of N inresponse to a change in root temperature. Key words: Brassica napus, oilseed rape, root temperature, nitrate, ammonium, ion uptake rate     

Effects of Nitrate Supply and Deprivation and/or Defoliation on Potassium Absorption and Distribution in Ryegrass

Absorption and distribution of K⁺ by two ryegrass genotypesfrom steady-state supplies at low external concentrations werefollowed through periods of NO₃^– deprivation and afterdefoliation. Electron microprobe analysis was also used to examineK⁺ concentrations across the root cortex and the effects onvacuolar/cytoplasmic K⁺ concentrations in the cortex. In manyrespects, the two genotypes behaved in a similar way. Totalquantities of K+ absorbed were less than 50% of those previouslyrecorded for NO₃^–, and NO₃^– deprivation slightlyreduced cumulative K+ uptake. Unit absorption rates (_K) declinedwith time in the control plants, and removal of NO₃^– resultedin a progressive decline in _K, with an initial rapid responsewhich was independent of any effects of growth. In one genotypeabsorption rates did not recover but gradually did so in theother. Defoliation reduced _K only slightly for 6 d, thereafter_K, increased almost linearly so that values for plants withsustained NO₃^– supply were 2 ? those of entire plants.There was no evidence of an oscillatory cycle of K⁺ unit absorptionrates of the kind previously shown for NO₃^– during therecovery periods. Concentrations of K⁺ on both a dry matterand tissue water basis remained within a narrow range irrespectiveof treatment or time. The immediate decline in K⁺ unit absorption rate after NO₃^–withdrawal indicated that at least a proportion of their uptakeswere coupled. Calculated values for the length of the diffusivepathway indicated that the primary site of absorption into thesymplasm was either the epidermis or the outermost corticalcell. This was supported by evidence from the electron microprobeanalysis which indicated a strong gradient of K⁺ concentrationswhich declined from outer to inner cortex and was much steeperin plants with a sustained NO₃^– supply. Concentrationsin the cytoplasm of most cortical cells were generally greaterthan in the vacuoles: this difference was greater in low N plants.The changes in K+ in the cortex were related to the role ofK+ in the transport of NO₃^– in the xylem and effects onrecycling to the roots in the phloem. Key words: Cytoplasm, defoliation, diffusion, electron probe X-ray microanalysis, Lolium, nitrate, potassium, S.E.M.,, unit absorption, vacuole     

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

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

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. in Flowing Solution Culture: I. GROWTH

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. in flowing solution culture. I. Growth.—J.exp. Bot. 38: 42–52 Oilseed rape (Brassica napus L. cv. Bien venu) was grown for49 d in flowing nutrient solution at pH 6?0 with root temperaturedecrementally reduced from 20?C to 5?C; and then exposed todifferent root temperatures (3, 5, 7, 9, 11, 13,17 or 25?C)held constant for 14 d. The air temperature was 20/15?C day/nightand nitrogen was supplied automatically to maintain 10 mmolm^–3 NH₄NO₃ in solution. Total dry matter production wasexponential with time and similar at all root temperatures givinga specific growth rate of 0?0784 g g^–1 d^–1. Partitioningof dry matter was influenced by root temperature; shoot: rootratios increased during treatment at 17?C and 25?C but decreasedafter 5 d at 3?C and 5?C. The ratio of shoot specific growthrate: root specific growth rate increased with the ratio ofwater soluble carbohydrates (shoot: root). Concentrations ofwater soluble carbohydrates in shoot and root were inverselyrelated to root temperature; at 3, 5 and 7?C they increasedin stem + petioles throughout treatment, coinciding with a decreasein the weight of tissue water per unit dry matter. These resultssuggest that the accumulation of soluble carbohydrates at lowtemperature is the result of metabolic imbalance and of osmoticadjustment to water stress. Key words: Brassica napus, oilseed rape, root temperature, specific growth rate     

White Clover N2-Fixation in Response to Root Temperature and Nitrate: I GROWTH AND UPTAKE OF NITRATE FROM FLOWING NUTRIENT SOLUTIONS

Established, nodulated white clover plants (Trifolium repensL. cv. Blanca) were transferred to eight plant culture unitsof a system of flowing solution culture. Solution temperatureswere 5, 11, 17, and 25 ?C (two units per temperature), withshoot temperature of 25/15 ?C day/night and light regime commonto all plants. After 7 d, was supplied at 10 mmol m^–3 to one of each pair of culture units and thenet uptake of was monitored over 14 d. The remaining four culture units served as controls in which plantswere entirely dependent on N₂ fixation, as estimated by sequentialharvesting and the change in the amount of N in plants. Totalfresh and dry weights increased exponentially with time andwith increasing root temperature, between 5–25 ?C in nitrateplants and 5–17 ?C in control plants, respectively, byfactors of 2.9 and 1.8. Nodule dry weight of nitrate plantsshowed little increase after 6 d compared with control plants.Dry weight shoot: root ratios increased with time in all treatmentsexcept 5 ?C control plants. Total net uptake of over 13 d was 0.41, 4.27, 11.92, and 12.77 mmol plant-1, respectively,at 5, 11, 17, and 25 ?C. At all temperatures except 5 ?C, plantsaccumulated high concentrations of (10–40 mol m^–3) in leaflets and roots, within 2 or 3 days ofsupplying . Daily mean unit absorption rates of increased 12-fold with increasing temperature between 5 and 25 ?C, and showed little change with time at 5,17, and 25 ?C. The total N₂ fixed by nitrate plants over 14d increased 5-fold with temperature between 5 and 17 ?C, butwas always less than the amount fixed by control plants. Thepercentage contributed by N2 fixation to total N acquisitionby nitrate plants over 14 d decreased with increasing temperature,from 77% at 5 ?C to 11 % at 25 ?C. Mean daily rates of N₂ fixationper unit dry weight of nodule were lower in nitrate plants thanin control plants throughout treatment at 5 ?C and 25 ?C, butat 11 ?C and 17 ?C the rates for nitrate plants increased progressivelywith time and exceeded the rates for control plants after 8d. In both nitrate and control plants the effect of temperatureon N₂ fixation per nodule dry weight was proportionately lessthan that on unit absorption rate of . The results are discussed in terms of the overall regulation ofN accumulation by white clover and the adaptive significanceof differences in the sensitivities of uptake and N₂ fixation to root temperature. Key words: Trifolium repens, white clover, root temperature, N₂ fixation, nitrate uptake     

Combined Effects of Partial Defoliation and Nutrient Availability on Cloned Betula pendula Saplings: I CHANGES IN GROWTH, PARTITIONING AND NITROGEN UPTAKE

The combined effects of partial defoliation and nutrient availabilityon dry matter accumulation and partitioning, and on nitrogenuptake and partitioning, were studied in cloned Betula pendulaRoth saplings. The saplings were randomly assigned to differentnutrient levels (5, 1·5 and 0·5 mol Nm^–3)in aerated nutrient culture and to the following defoliationtreatments: (1) control (no damage), (2)damage of the developingmain stem leaves (half of the leaf lamina removed), and (3)removalof the developing main stem leaves (entire leaf lamina removed).Measured in terms of cumulative whole-plant dry weight (includingremoved leaf tissue), the birch saplings were unable to compensatefor the loss of the developing leaves (treatment 3) during the14 d study period. In response to leaf removal (treatment 3)the mean final percentage reduction in whole-plant dry weightwas actually greater than the initial mean percentage reductioncaused by the removal itself; the magnitude of the final reductionwas independent of nutrient availability. After removal of thedeveloping leaves, branch growth was favoured at the expenseof the growth of the rest of the shoot; the relative branchgrowth was most pronounced at the highest nutrient level. Atthe two highest nutrient levels the nitrogen uptake of the saplingswith the developing leaves removed was less than that of undamagedsaplings. We suggest, however, that the incapacity of the saplingsfor compensatory growth after removal of the developing leaveswas primarily due to the decreased total carbon gain of thesaplings rather than to the decreased nitrogen uptake rate. Key words: Partial defoliation, nutrient availability, birch sapling, dry matter, nitrogen     

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

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

Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: I. GROWTH, O2 UPTAKE AND SYNTHESIS OF FERMENTATIVE END-PRODUCTS BY ROOT SEGMENTS

Root growth of 7-d-old wheat (Triticum aestivum cv. Gamenya)seedlings was impaired at dissolved O₂ concentrations of 0.01and 0.055 mol m^–3 O₂, while growth at 0.115 mol m^–3O₂ was the same as that in continuously aerated controls (0.26mol m^–3 O2). Oxygen uptake by apical (0–2 mm), expanding (2–4mm) and expanded (10–12 mm) tissues of the roots decreasedbelow 0.16, 0.09 and 0.05 mol m^–3 O₂, respectively. Thishierarchy is consistent with the metabolic rates of these tissues.There was a small (c. 9%) inhibition of O₂ uptake and some netsynthesis of ethanol and alanine in root apices at 0.115 molm^–3 O₂. Significant amounts of anaerobic end-productsaccumulated at 0.055 mol m^–3 O₂ and even more so at 0.01mol m^–3 O₂, indicating that oxidative phosphorylationwas strongly inhibited. Net alanine synthesis increased in fully expanded (10–16mm) tissues exposed to <0.003–0.01 mol m^–3 O₂,and this increase was accompanied either by a proportionallysmaller increase in the concentration of other free amino acidsor by a net decrease in free amino acid levels excluding alanine.This suggests that alanine was synthesized as an end-productof anaerobic catabolism and did not accumulate simply becauseof decreased net protein synthesis. Comparing the carbon flow to CO₂, ethanol, lactate and alaninein roots at 0.01 mol m^–3 O₂ with carbon loss as CO₂ inaerated roots suggests that carbon flow to products of metabolismwas not greatly enhanced due to O₂ deficiency. This infers,but does not prove that, in wheat, generation of energy duringperiods of O₂ deficiency is not enhanced due to a Pasteur effect. Key words: Anaerobic, fermentation, oxygen, wheat     

The Uptake of Nitrate by Lolium perenne from Flowing Nutrient Solution: II. EFFECT OF LIGHT, DEFOLIATION, AND RELATIONSHIP TO CO2 FLUX

Experiments with simulated swards of perennial ryegrass (Loliumperenne L.) grown in flowing nutrient solution with NO₃- heldat 0.1 mg N I^–1 show that the rate of NO₃- uptake wasrelated to diurnal, day-to-day, and seasonal changes in radiation.In summer the diurnal variation in NO₃-uptake ranged from 25to 50 mg N m^–2 h^–1 and the day-to-day variationranged from 500 to 1500 mg N m^–2 d^–1. Mean dailyrates of uptake over 12 d periods in summer and in winter averaged908 and 44 mg N m^–2, respectively. The pattern of NO₃-uptake followed that of CO₂ flux with the maximum rate of theformer occurring 5 or 6 h after the maximum CO₂ influx. Afterdefoliation, NO₃- uptake was severely curtailed for 2 d concomitantwith a very small influx of CO₂. Analysis of the changes thatoccurred in the rate of NO₃- uptake immediately after the switchingon or off of artificial light suggests that two reversible processesmay be involved in the relation between NO₃-uptake and radiation,one with a longer and the other with a shorter time constant.     

Chelation in Auxin Action: I. A STUDY OF THE INTERACTIONS OF 3-INDOLYLACETIC ACID AND SYNTHETIC CHELATING AGENTS AS AFFECTING THE GROWTH OF WHEAT ROOTS AND COLEOPTILE SECTIONS

Factorial experiments have been carried out on the effects,upon growth of roots of intact wheat seedlings and growth ofwheat coleoptile sections, of different concentrations of 3-indolylaceticacid (IAA) and various known chelating agents. These have demonstrateda similar mutual antagonism between pairs of agents whetherthese are IAA and a single known chelating agent or two knownchelating agents. This interaction takes the form that eitheragent alone in ‘high’ concentration severely inhibitsgrowth but this inhibitory effect is almost or entirely removedby the presence of one-millionth the concentration of the otheragent; when both agents are present in ‘high’ concentrationthe inhibition is again severe. The substitution of a non-chelatinganalogue for one of the agents either destroys the mutual characterof the antagonism or entirely prevents either agent at low concentrationfrom reducing measurably the inhibition caused by high concentrationof the other. The fact that IAA interacts with known chelatingagents, in controlling the growth both of roots and coleoptilesections, in the same unexpected and symmetrical way that theseinteract with each other, is held strongly to support the hypothesisthat it is here itself acting as a chelating or complexing agent;the absence of such interactions with a non-chelating analoguemakes this the more convincing. These results are concernedwith the removal of growth inhibition, due to supra-optimalconcentrations of one agent, by minute proportions of another;it cannot be regarded as proven that the promotion of growthby IAA in the absence of another agent is also due to chelationor complex formation. This seems probable, however, when thefindings here presented are taken in conjunction with the accumulatingevidence that IAA and other auxins can form complexes or chelateswith metals in vitro, and with the finding already publishedin detail that the eight chelating agents tried promoted growthin the wheat coleoptile test. The main criticisms to which this hypothesis has been subjectedhave been concerned with the relative magnitudes of effectsof IAA and chelating agents upon growth, with the low stabilityconstants of metal complexes with IAA and other auxins, withthe lack of parallelism between stability constants and growth-promotingactivity, and with the fact that one chelating agent (ethylenediamine-tetraaceticacid; EDTA) has been found inactive in certain growth tests.A series of factorial experiments comparing the authors' techniques(which are here described in detail), chemicals, and strainof wheat with those used by Fawcett et al. (1956) demonstratethat the discrepancies found, both as regards magnitudes ofeffects of IAA and EDTA and optimal concentrations, were partlydue to differences in strain but mainly to differences of technique.It is considered that ‘foreign’ molecules such asEDTA are likely to have side effects, which may well differin different strains or tests; competition with internal chelators(Burstrom and Tullin, 1957) is also likely to differ; differencesin rate of penetration and steric hindrance may also be involved.For these reasons effective chelating activity in vivo may bevery different from that in vitro and in the first instancethe magnitudes of growth-promoting effects of chelating agents(which may indeed be the net result of stimulatory and inhibitoryprocesses) seem less important than the fact that they are foundin so many instances. Possible ways in which IAA and other growth substances may regulategrowth by chelation or complex-formation are discussed.     

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

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

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: III. EFFECT OF CHANGED PHOSPHATE CONCENTRATIONS ON THE GROWTH AND DISTRIBUTION OF PHOSPHATE WITHIN PLANTS OF LOLIUM PERENNE L.

Perennial ryegrass (Lolium perenne L.) was grown from seed for29 d in flowing solution culture containing 0.1, 0.4 or 6.4mmol m^–3 P before the concentrations were changed (0.1and 0.4 raised to 6.4; 6.4 lowered to 0.4; controls unchanged)for an experimental period of two weeks to test the hypothesisthat after the seedling stage, the maximum rate of plant growthcould be sustained by a lower concentration of phosphate atthe root/solution interface than was necessary for the maximumrate of seedling growth. During the 29 d seedling period growthwas greatest on 6.4 mmol m^–3 P achieving 179 mg per plantdry weight compared with 122 and 26 mg on 0.4 and 0.1 mmol m^–3P respectively. During the experimental period growth on thetreatment 6.4 lowered to 0.4 mmol m^–3 P continued at thesame rate as the 6.4 control achieving 981 and 983 mg per plantdry weight respectively. Similarly growth of the treatment 0.4raised to 6.4 mol m^–3 P was unaffected by the change inconcentration and was comparable with the 0.4 control. Bothresults support the hypothesis for seedlings exceeding about100 mg per plant dry weight. In contrast the small plants ofthe treatment 0.1 raised to 6.4 mmol m^–3 P behaved similarlyto seedlings and responded rapidly to the increased concentrationof phosphate in solution, achieving high rates of phosphateuptake and increasing the growth of shoot more than the growthof root so that the ratio of root: shoot declined from 065 to0.34, a value similar to that for the seedlings grown on 6.4mmol m^–3 P. Key words: Lolium perenne L, Phosphate concentration, Seedling growth     

Studies on the Growth in Culture of Plant Cells: XV. UPTAKE AND UTILIZATION OF URIDINE DURING THE GROWTH OF ACER PSEUDOPLATANUS L. CELLS IN SUSPENSION CULTURE

[2-¹⁴C]-uridine is rapidly taken up by sycamore cells in suspensionculture. A proportion of the radioactivity enters RNA withoutmeasurable delay, whilst the remainder equilibrates with a largepool of phosphorylated compounds, the major radioactive componentof which is 5'-UMP. Both the uracil and cytosine residues ofRNA receive label from [¹⁴C]-uridine and, when the cells aresupplied with high concentrations of uridine, these bases arederived almost exclusively from the nucleoside. [¹⁴C]-uridine is incorporated into RNA at all stages of thegrowth cycle of batch cultures; its continuing incorporation,when the total RNA content of the cells is rapidly decreasing,indicates a high rate of turnover of the total RNA. Long-termlabelling experiments also indicate turnover of RNA during thephase of active cell division and suggest that a large proportionof the degradation products are not re-utilized for RNA synthesis. Sycamore cells degrade [2-¹⁴C]-uridine with release of ¹⁴CO₂.The proportion degraded increases from 25 per cent at an externaluridine concentration of 10^–6M to 75 per cent at 10^–3M. Despite this, nucleic acids are the only macromolecules thatreceive a significant amount of radioactivity from [2-¹⁴]C-uridine.     

Nitrogen Utilization in N-limited Barley during Vegetative and Generative Growth: I. GROWTH AND NITRATE UPTAKE KINETICS IN VEGETATIVE CULTURES GROWN AT DIFFERENT RELATIVE ADDITION RATES OF NITRATE-N

Growth and nitrate uptake kinetics in vegetatively growing barley(Hordeum vulgare L., cvs Laevigatum, Golf, and Mette) were investigatedin solution culture under long-term limitations of externalnitrogen availability. Nitrate was fed to the cultures at relativeaddition rates (RA) ranging from 0.02 to 0.2 d^–1. Therelative growth rate (RG, calculated for total plant dry weight)correlated well with RA in the range 0.02 to 0.07 d^–1.In the RA range from 0.07 to 0.2 d^–1 RG continued to increase,but an increasing fraction of nitrogen, added and absorbed,was apparently stored rather than used for structural growth.The RG of the roots was less affected by RA. V_max, for net nitrateuptake increased with RA up to 0.11 d^–1, but decreasedat higher RA. The decline in V_max coincided with a build-upof nitrate stores in both roots and shoots. V_max, expressedper unit nitrogen in the plants (the relative V_max, was higherthan required for maintenance of growth (up to 30-fold) at lowRA, whereas at higher RA the relative V_max decreased. Kineticpredictions of steady-state external nitrate concentrationsduring N-limited growth ranged from 0.2 to 5.0 mmol m^–3over the RG range 0.02 to 0.11 d_–1. It is suggested thatthe nitrate uptake system is not under specific regulation atlow RA, but co-ordinated with root protein synthesis and growthin general. At RA higher than 0.11 d_–1, however, specificregulation of nitrate uptake, possibly via root nitrate pools,become important. The three cultivars showed very similar growthand nitrate uptake characteristics. Key words: Barley, growth, nitrogen limitation, nitrate uptake, kinetics     

Growth and Development in the Young Tomato: I. THE EFFECT OF TEMPERATURE AND LIGHT INTENSITY ON GROWTH OF THE SHOOT APEX AND LEAF PRIMORDIA

Tomato seedlings were grown at constant temperatures of 25°and 15° C. in a 12-hour day at light intensities of 1,600,800, and 400 f.c. The rate of increase in size of the shootapex and the rates of formation and growth of leaf primordiaduring the vegetative phase were followed by dissecting samplesfrom the time of cotyledon emergence onwards. The rate of enlargement of the shoot apex increased with lightintensity, but apical enlargement was delayed at the highertemperature, the delay being longer the lower the light intensity.The rates of leaf formation and leaf growth increased with bothtemperature and light intensity. Temperature had a larger effecton leaf growth than on leaf formation. More leaves were formedbefore flowering at 25° C. than at 15° C., the increasein leaf number being greater the lower the light intensity. It is suggested that the delay in the enlargement of the apexat high temperature can be explained in terms of competitionfor assimilate, the competitive potential of the expanding leafprimordia exceeding that of the apex at higher temperatures.     

Growth and Dormancy in Lunularia cruciata (L.) Dum.: I. RESUMPTION OF GROWTH AND ITS CONTINUATION

Lunularia cruciata may become dormant at three stages in itslife history: mature thallus, gemma, and spore. The resumptionof growth and its continuation in various conditions have beenstudied in thalli and gemmae. Air-dry, mature thalli of theIsrael strain, planted on a suitable medium, produce adventitiousbranches ventrally from the region immediately posterior tothe existing meristem, which itself fails to resume growth.When dormant gemmae are taken from the gemma-cup, however, theexisting apical cells and meristems simply recommence growing,so that the new growth is continuous with the old. Except inthe case of mature thalli aroused from dormancy, apical dominanceis pronounced, and branching occurs only by bifurcation. Thisdominance can be broken by applying sucrose solution to thegrowing tips—possibly a plasmolytic effect. The growth in area of freshly planted gemmae accelerates forabout 25 days before its relative rate slows to any great extent.During the first half of this period, growth is due exclusivelyto the expansion of existing cells, but subsequently cell numbersincrease rapidly from the 8–10,000 present in the dormantgemma. Cell numbers were estimated by using a modified macerationtechnique, in which chelation followed prolonged fixation. Lunularia grows successfully at quite low light intensities.Of the mineral nutrient deficienccs investigated, lack of Plimits growth most severely, although N-deficiency also restrictsit to a very low level. Gemma-cup production appears to be unaffected by light intensity,at least within levels permitting growth. There is, however,a large temperature effect, cup production decreasing markedlyabove 12° C.     

Nutrient Supply and the Growth of the Seminal Root System in Barley: I. THE EFFECT OF NITRATE CONCENTRATION ON THE GROWTH OF AXES AND LATERALS

A method was devised by which different zones along a singleseminal axis of an intact plant could be exposed for extendedperiods to contrasting concentrations of nitrate (either 0.01or 1.0 mM) in continuous flow, the supply of all other nutrientsbeing favourable throughout. The concentration of nitrate wasfound to exert a direct and strictly localized effect upon thegrowth of lateral roots which, depending upon the supply ofassimilates from the shoot, resulted in marked modificationsto the form of the root system. Zones receiving 1.0 mM nitrateshowed an increase in the number and extension rate of bothfirst- and second-order laterals, associated with a preferentialaccumulation of dry matter, compared with zones in 0.01 mM nitrate.The average number of laterals (both first and second order)per cm of parent root was 4.4 in the presence of 1.0 mM nitrateand 2.2 in 0.01 mM. The average extension rates of first-orderlaterals were 0.61 and 0.26 cm d^–1 and second-order laterals,0.10 and 0.05 cm d^–1 for nitrate concentrations of 1.0and 0.01 mM respectively. The precise numbers and extensionrates of laterals in any one zone were affected, however, bythe rate of growth of laterals in other parts of the root system.In contrast, the extension rates of axes were little affectedby the concentration of nitrate to which their apical meristemswere exposed and approached 2.0 cm d^–1 provided the plantswere not nitrogen-starved. The significance of these resultsto the physiology of root growth and soil-plant relations isdiscussed.     

Nutrient Supply and the Growth of the Seminal Root System in Barley: III. COMPENSATORY INCREASES IN GROWTH OF LATERAL ROOTS, AND IN RATES OF PHOSPHATE UPTAKE, IN RESPONSE TO A LOCALIZED SUPPLY OF PHOSPHATE

Responses to a localized supply of phosphate were studied inbarley grown in continuous flow solution culture. Root systemswere either uniformly supplied with 50 µM phosphate (controls)or the same solution was supplied to only a 4 cm or 2 cm lengthof a seminal root (localized supply), the remainder of the rootsystem receiving a nutrient solution lacking phosphate. Little development of laterals occurred on those parts of theroot system receiving no phosphate from the external solution,while an increase in the number and extension of laterals tookplace in the 4 cm zone enriched with phosphate. Compared withsimilar zones on controls, the total length of laterals wasincreased 15-fold in 21 d plants. In addition, rates of ³²P-phosphateuptake and translocation to shoots per unit root weight werehigher than in controls by a factor of 2?5–5?0. Furtherincreases in the growth of lateral roots, and rates of phosphateuptake, were induced when the segment initially supplied withphosphate was restricted to only 2 cm. These localized modifications to root growth and uptake of phosphatelargely compensated for the deficient supply of phosphate tothe remainder of the root system. After an early period of retardedgrowth and phosphate stress, the relative growth rate of plantsand the concentration of phosphate in shoots were restored tolevels similar to that of the controls. The manner in which the supply of phosphate may control rootdevelopment, and the nature of the co-ordination between rootgrowth, phosphate uptake, and shoot growth, are discussed.     

Uptake, Translocation, and Metabolism of IAA in the Olive (Olea europea): I. UPTAKE AND TRANSLOCATION OF [1-14C] IAA IN DETACHED 'MANZANILLA' OLIVE LEAVES

Absorption, translocation, and decarboxylation of [1–¹⁴C]IAAby excised mature and young olive leaves were studied. The decarboxylationwas considerably more intense in mature leaves than in youngones, while the opposite was true for absorption. The rate ofdecarboxylation was dependent on the presence of peltate scalesof the leaves. The amount of non-biological decarboxylationand the possible effect of bacterial contamination on the systemwere studied and the rate of their involvement is discussed.