The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

Nitrogen remobilization response to current supply in young citrus trees

Internal nitrogen (N) storage and remobilization processes support seasonal growth (flowering/fructification and subsequent leaf development) in particular in early spring, when soil temperatures are unfavourable for adequate N uptake. Storage nitrogen mobilization in young citrus trees was studied under two contrasting N supplies; high N (HN) and low N dose (LN) in the critical period of flowering and fruit set. ¹⁵N labelling technique was used to distinguish N derived from internal remobilization from that taken up by the roots. Regardless N supply, the greatest N remobilization took place from the beginning of the vegetative activity until flowering. Low N availability significantly increased (+14%) N retranslocation at the end of June drop agreeing with the hypothesis that reserve mobilization depends on soil N availability during flowering and fruit set. At the end of fruit drop, N remobilization contributed up to 70% and 61% of total N of young organs for LN and HN, respectively. Remobilized N was mainly recovered in abscised organs of both HN and LN trees and to a lesser extent in new flush leaves; however a greater percentage partitioned to abscised organs of LN as a consequence of the greater remobilization rate and the increased fruit abscission. Old leaves of LN remobilized significantly higher N, while woody organs and root system did not show differences between HN and LN supplied trees. The results presented in this paper demonstrate that the amount of N remobilized by young citrus plants depends on external N availability. Thus, low N application rates in early stages (flowering and fruit set) lead to higher translocation of N stored during the previous cycle to developing new organs.     

The Effect of Defoliation on the Nitrogen Economy of White Clover: Regrowth and the Remobilization of Plant Organic Nitrogen

Single plants of white clover (Trifolium repens) were establishedfrom stolon cuttings rooted in acid-washed silver sand. Allplants were inoculated with Rhizobium trifolii, and receivednutrient solution containing 0·5 mg ¹⁵N as either ammoniumor nitrate weekly for 12 weeks (i.e. 6 mg ¹⁵N in total). Plantswere then leniently defoliated or left intact, and the labelledN supply was replaced with unlabelled N. Lenient defoliationremoved fully expanded leaves only, leaving immature leaveswhich accounted for 50–55% of the total; growing pointnumbers were not reduced. Nodules, leaves and growing pointswere counted over the following 21 d period, and d. wts, N contents,and ¹⁵N enrichments of individual plant organs were determined. Defoliated plants had fewer nodules, but numbers of growingpoints were unaffected by defoliation. The rates of both leafemergence and expansion were accelerated in defoliated plants;in consequence the number of young leaves remained less thanin intact plants until day 21. Total dry matter (DM) and N accumulationwere less in defoliated plants, and a greater proportion oftotal plant DM was invested in roots. About 97 % of plant totalN was derived from fixed atmospheric N, but there was incompletemixing of fixed and mineral N within the plant. Relatively moremineral N was incorporated into roots, whereas there was relativelymore fixed N in nodules. There was isotopic evidence that Nwas remobilized from root and stolon tissue for leaf regrowthafter defoliation; approximately 2 % of plant N turned overdaily in the 7-d period after defoliation, and this contributedabout 50% of the N increment in leaf tissue. White clover, Trifolium repens L. cv. SI84, lenient defoliation, N economy, regrowth, N remobilization     

Nitrogen Supply Affects the Remobilization of Nitrogen for the Regrowth of Defoliated Lolium perenne L.

Nitrogen remobilization from roots and pseudostems during regrowthof Lolium perenne L. was studied in miniswards grown with contrastinglevels of (NH₄)₂SO₄ in solution culture. Growth with a highN supply (5.0 mol m^–3) increased theweight of leaf laminae recovered at each of five weekly clippings,and decreased the proportion of photosynthate used for rootgrowth. Clipped plants growing in a steady-state were suppliedwith ¹⁵N for 48 h and the recovery of labelled N in laminaemeasured after five weekly cuts. Recovery of labelled N in thelaminae from the second clipping onwards was derived only fromremobilization of N from roots and pseudostem. Miniswards grownwith low N (0.5 mol m^–3) relied moreupon remobilization of N for lamina growth than did high N plants.Thus after 14 d 20% of lamina N was labelled in low N plantsbut only 3% was labelled in the high N treatment. Thereafter,N remobilization declined until at the final clipping after35 d, labelled N represented only 4% and 1 % of the lamina Nin the low and high N plants. When plants were not clipped beforethe labelling period, they took up more ¹⁵N if grown with highN than cut plants. Thereafter, the remobilization of N followeda similar pattern as in the cut plants. Exponential models were used to calculate the rate of N transferfrom roots and pseudostem to laminae. When grown with low N,the half-life of remobilization was 1.56 weeks. High N miniswardshad an initial rapid remobilization with a half-life of 0.66weeks, and a slower phase with a half-life of 2.98 weeks. Key words: Lolium perenne L., nitrogen supply, regrowth, remobilization, internal cycling     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Economy of Symbiotically Fixed Nitrogen in Red Clover (Trifolium pratenseL.)

The seasonal dynamics of symbiotic fixation, distribution andfate of nitrogen (N) were studied on two successive crops ofred clover (Trifolium pratenseL.) grown outdoors in soil containersunder the Mediterranean climate of southern France. Nitrogenaseactivity was followed throughout the growing season using acetylene(C₂H₂) reduction assays. The distribution and transfer of symbioticallyfixed N were followed by periodic measurements of¹⁵N distributionin plants after exposure of the root systems to labelled dinitrogen(¹⁵N₂). In both years there were two peaks of nitrogenase activity,one in spring and one in late summer, separated by a sharp decreaseduring the flowering period. Over the entire growth cycle, symbioticallyfixed N accounted for 61 to 96% of the total plant N. Once weekafter incorporation, 60 to 90% of N derived from the atmospherewas recovered in the aerial parts of the plants. More than 50%of this was in the leaves, but there were differences in distributionaccording to the stage of development. The maximum percentage(20–28%) recovered from nodulated roots occurred in May–June,during maximum growth of the vegetative organs, and in September.Above-ground symbiotically fixed N was highly mobile with time,moving from the rosette leaves to the leaves attached to theelongated stems and then to the seeds, where 25 to 50% of Nfixed in May and June was recovered in September. Because of:(1) the high turnover rate of leaves; and (2) the relativelyhigh N content of dead leaves, as much as 50% of the symbioticallyfixed N in a year was potentially available to the soil micro-organismsas litter. The maximum transfer was in spring and winter. Ofthe remainder, 20 to 35% was recovered in living plant partsduring regrowth in March of the second year. Transfers to andfrom the root system were less pronounced, but significant decreasesin N content of the roots occurred early in the second yearjust after foliage regrowth was initiated. It is concluded that,because of its high foliage productivity and turnover rate,and high yield of symbiotically fixed N, red clover is a goodcandidate to provide substantial amounts of N to the soil throughoutthe year and therefore restore N fertility. Red clover; Trifolium pratenseL.; forage legumes; labelled dinitrogen (¹⁵N₂) reduction; acetylene reduction; nitrogen fixation; nitrogen distribution; nitrogen transfer     

The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

M26 apple rootstocks were grown in sand culture and suppliedwith three rates of nitrogen (N) with the irrigation: none,0·8 mol N m^–2 or 8·0 mol N m^–2. Allthe N supplied to the trees was labelled with ¹⁵N at 5·0atom percent enrichment. The effect of N supply on tree growth,N uptake and the remobilization of N from stems for the annualgrowth of the trees was measured. Increasing the N supply increasedleaf growth, but had no effect upon root mass and so alteredthe root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon storedreserves of N for their seasonal growth. Initially this N wasused for leaf growth, which stopped after a few weeks. Thereafterthe N-deficient plants retranslocated some of the N from theirleaves to support root growth. Increasing the N supply had littleeffect upon the amount of N remobilized for growth, althoughwell-fertilized plants accumulated N in their leaves and didnot retranslocate any to support root growth. The partitioningof N between roots and shoots was, therefore, altered by increasingthe N supply. Amino acid analysis of stems showed that the majorforms of N remobilized during growth were protein rich in asparagineand arginine The results show the importance of internal N cycling for thegrowth of young apple trees, and are discussed in relation toother studies of N cycling in deciduous trees Malus domestica Borkh., nitrogen, remobilization, growth, partitioning, storage     

The Effect of the Autumn Senescence of Leaves on the Internal Cycling of Nitrogen for the Spring Growth of Apple Trees

The internal cycling of nitrogen (N) has been studied in applerootstocks grown in sand culture and subjected to a constantN supply, or defoliation, or withholding the N supply in theautumn in order to manipulate the amount of N stored over thewinter. The trees subsequently received either no N or 8–0mol N m^–3 (labelled with ¹⁵N to 498 atom%) with the irrigationthe following spring in order to determine the effect of thecurrent N supply on the remobilization of N for leaf growth. Provision of an autumnal N supply delayed leaf senescence andreduced the amount of N withdrawn from leaves from 156 mg Nplant^–1 to 91 mg N plant^–1. Loss of protein ribulose1,5-bisphosphate carboxylase/oxygenase (RUBISCO) accounted for83–87% of the soluble protein N lost during leaf senescence,there being a preferential loss of RUBISCO compared with othersoluble leaf proteins. Remobilization of N from perennial woody tissues (stems androots) in the spring was used predominantly for leaf growth.The amount of N remobilized depended upon the size of the Nstore, but was unaffected by the current N supply, demonstratingthat fertilization of trees does not alter the efficiency withwhich they cycle N. Degradation of RUBISCO in the autumn accountedfor between 32% and 48% of the N subsequently remobilized forleaf growth the following spring, suggesting that RUBISCO hasa role as a summer store for N. Key words: Malus domestica, Borkh, nitrogen, senescence, ribulose 1, 5-bisphosphate carboxylase, oxygenase, storage, remobilization     

The Effect of Lenient Defoliation on the Nitrogen Economy of White Clover: The Contribution of Mineral Nitrogen to Plant Nitrogen Accumulation During Regrowth

Single plants of white clover (Trifolium repens L.) were grownfrom stolon cuttings rooted in sand. All plants were inoculatedwith Rhizobium trifolii, and for 14 weeks received nutrientsolution containing 0.5 mg N each week, as either ammonium ornitrate. Plants were then leniently defoliated or were leftintact and a ¹⁵N-labelled N source was applied at intervalsof 4 d to replace the unlabelled N. Lement defoliation removedfully expanded leaves only; the remaining immature leaves accountedfor 39–44% of the total. At harvests over the following21 d, leaf numbers were counted and dry matter (DM), N contentsand ¹⁵N enrichments of individual plant organs were determined. Rates of leaf emergence and expansion were accelerated in defoliatedplants; numbers of young leaves were similar in defoliated andintact plants. Total DM and N content were less in defoliatedthan intact plants and were not affected by form of N supplied.DM of young leaves, growing points and stolons and N contentof young leaves were, however, greater when ammonium ratherthan nitrate N was supplied. Rates of increase in the contentof plant total N were 8.2 ± 1.36 mg N d^-1 and 10.2±1.82 mg N d^-1 in defoliated and intact plants respectively.The increases were predominantly due to N₂ fixation, since recoveryof ¹⁵N showed that less than 1% of the increment in plant totalN was assimilated mineral N. Nevertheless, the contributionof mineral N to plant total N was 50% more in defoliated thanin intact plants; higher amounts of mineral N were found particularlyin young leaves and growing points. Partitioning of mineralN to nodulated roots increased over time and was greater whenammonium rather than nitrate N was present. White clover, Trifolium repens L. cv. S184, lenient defoliation, N accumulation, N₂ fixation     

Nitrogen and carbon storage in alpine plants

Alpine plants offer unique opportunities to study the processesand economics of nutrient storage. The short alpine growingseason forces rapid completion of plant growth cycles, whichin turn causes competition between vegetative and reproductivegrowth sinks during the early part of the growing season. Mobilizationof stored nitrogen and carbon reserves facilitates competingsinks and permits successful completion of reproduction beforethe onset of winter stress. We discuss the theoretical frameworkfor assessing the costs and benefits of nutrient storage inalpine plants in order to lay the foundation for interpretationof observations. A principal point that has emerged from pasttheoretical treatments is the distinction between reserve storage,defined as storage that occurs with a cost to growth, and resourceaccumulation, defined as storage that occurs when resource supplyexceeds demand, and thus when there is no cost to growth. Wethen discuss two case studies, one already published and onenot yet published, pertaining to the storage and utilizationof nitrogen and carbon compounds in alpine plants from NiwotRidge, Colorado. In the first case, we tested the hypothesisthat the seasonal accumulation of amino acids in the rhizomeof N-fertilized plants of Bistorta bistortoides provides anadvantage to the plant by not imposing a cost to growth at thetime of accumulation, but providing a benefit to growth whenthe accumulated N is remobilized. We show that, as predicted,there is no cost during N accumulation but, not as predicted,there is no benefit to future growth. In the presence of N accumulation,reliance on stored N for growth increases, but reliance on current-season,soil-derived N decreases; thus the utilization of availableN in this species is a ‘zero sum’ process. Inherentmeristematic constraints to growth cause negative feedback thatlimits the utilization of accumulated N and precludes long-termadvantages to this form of storage. In the second case study,we discuss new results showing high concentrations of cyclicpolyol (cyclitol) compounds in the leaves of many alpine speciesdominant in the dry fellfield habitat. In Artemisia scopulorum,cyclitols were induced as the growing season progressed, andreached highest concentrations during the dry, late-summer months.Leaf cyclitol concentrations were high in all four species ofthe Caryophyllaceae that we examined and appeared to be constitutivecomponents of the leaf carbohydrate pool as concentrations werehigh through the entire growing season. We observed correlationsamong seedling abundance, seeding survivorship and the presenceof high leaf cyclitol concentrations. We propose that the primaryfunction of cyclitols in the leaves of alpine, fellfield herbsis to promote drought tolerance through osmotic protection,and enhance fitness by improving seedling survival. We consideredthe possibility that cyclitols also function as carbon storagecompounds that are remobilized at the end of the growing seasonand used to support growth the following year. Our observationsdo not support this hypothesis in the Caryophyllaceae becausethe requirement for high constitutive concentrations year-after-yearprevents long-term advantages of storage and remobilization.However, in A. scopulorum, remobilization of cyclitols followingthe end of the growing season may provide storage substratesthat can be used for growth the following season. From our analysiswe conclude that it is difficult to use current theory thatis embedded in the economic concept of costs and benefits tointerpret observed dynamics in nitrogen and carbon allocation.Future theoretical developments that move away from an abstractfoundation embedded in cost-benefit tradeoffs and toward phenotypicintegration of source-sink relationships will improve our abilityto merge observations and theory.     

Growth Analysis of Solution Culture-grown Winter Rye, Wheat and Triticale at Different Relative Rates of Nitrogen Supply

At low nitrogen (N) supply, it is well known that rye has ahigher biomass production than wheat. This study investigateswhether these species differences can be explained by differencesin dry matter and nitrogen partitioning, specific leaf area,specific root length and net assimilation rate, which determineboth N acquisition and carbon assimilation during vegetativegrowth. Winter rye (Secale cereale L.), wheat (Triticum aestivumL.) and triticale (X Triticosecale) were grown in solution cultureat relative addition rates (R_N) of nitrate-N supply rangingfrom 0.03–0.18 d^-1and at non-limiting N supply under controlledconditions. The relative growth rate (R_W) was closely equalto R_Nin the range 0.03–0.15 d^-1. The maximalR_W at non-limitingnitrate nutrition was approx. 0.18 d^-1. The biomass allocationto the roots showed a considerable plasticity but did not differbetween species. There were no interspecific differences ineither net assimilation rate or specific leaf area. Higher accumulationof N in the plant, despite the same relative growth rate atnon-limiting N supplies, suggests that rye has a greater abilityto accumulate reserves of nitrogen. Rye had a higher specificroot length over a wide range of sub-optimal N rates than wheat,especially at extreme N deficiency (R_N=0.03–0.06 d^-1).Triticale had a similar specific root length as that of wheatbut had the ability to accumulate N to the same amount as ryeunder conditions of free N access. It is concluded that thebetter adaptation of rye to low N availability compared to wheatis related to higher specific root length in rye. Additionally,the greater ability to accumulate nitrogen under conditionsof free N access for rye and triticale compared to wheat maybe useful for subsequent N utilization during plant growth.In general, species differences are explained by growth componentsresponsible for nitrogen acquisition rather than carbon assimilation.Copyright 1999 Annals of Botany Company Growth analysis, nitrogen, nitrogen productivity, partitioning, specific root length, Secale cereale L.,Triticum aestivum L., X Triticosecale, winter rye, winter wheat, winter triticale.     

Nitrogen from senescing lower leaves of common bean is re-translocated to nodules and might be involved in a N-feedback regulation of nitrogen fixation

The objective of the present study was to elucidate whether remobilized N from lower leaves is involved in causing the drop in N(2) fixation during pod-filling in common bean (Phaseolus vulgaris L). Moreover, we addressed the question of whether remobilized N from lower leaves would reach the nodules. Nodulated common bean plants were grown in a growth chamber in quartz sand. During a 2-week period, at vegetative and at reproductive growth, 50% of the leaves (lower part) were either excised or individually darkened, thereby removing the same photosynthetic capacity yet allowing N to be remobilized from the darkened leaves. Moreover, at the vegetative growth period, three lower leaves per plants were (15)N labelled by applying (15)NH(4)NO(3) prior to imposing the darkening treatment. Leaf darkening at vegetative growth induced N remobilization as well as reduced N(2)-fixation rates and growth. Leaf excision at reproductive growth enhanced N(2) fixation. Changes in N(2)-fixation rates were in all cases the result of altered growth rates, while the % N in the whole plant and in various plant parts remained conserved. Directly after leaf labelling, but also at the end of the vegetative growth period, substantial amounts of (15)N from the leaves could be recovered in nodules in the control, and in higher amounts in the leaf-darkening treatment. It is proposed that nitrogen from leaves circulates within the plant via nodules, and that the strength or composition of this circular flow may be the signal for a putative N-feedback effect.     

Effect of Nitrogen Fertilizer on Photosynthesis of Several Varieties of Winter Wheat

The rates of gross photosynthesis of the flag leaf and the nextleaf below (second leaf) in crops of winter wheat were estimatedfrom the ¹⁴C uptake of the leaves after exposure to short pulsesof ¹⁴CO₂. The photosynthetic rates of both leaves during thegrain-filling period decreased with increase in nitrogen fertilizerbecause the intensity of photosynthetically active radiationwas less at the surface of the leaves in the dense crops withadditional nitrogen. In addition, the rate of photosynthesisat saturating light intensity was slightly decreased by nitrogen.The effects of nitrogen, in decreasing the rate of photosynthesisper unit area of leaf and in increasing the leaf-area indexof the top two leaves, were such that the photosynthetic productivityper unit area of land of the flag leaf was increased by nitrogenbut the productivity of the second leaf was unaffected. Applying180 kg N ha^–1 increased the productivity of the top twoleaves by a factor of 2.3 but increased grain yield by only1.8. The photosynthetic productivity of the second leaf duringthe grain-filling period was about half that of the flag leaf. There was no difference in photosynthetic rate per unit areaof leaves of Cappelle-Desprez and Maris Huntsman which couldaccount for the larger yield of the latter cultivar. There wasa slight indication that the leaves of the semi-dwarf cultivarsMaris Fundin and Hobbit photosynthesized faster than those ofMaris Huntsman. Triticum aestivum L., winter wheat, photosynthesis, nitrogen fertilizer     

Dynamics of exogenous nitrogen partitioning and nitrogen remobilization from vegetative organs in pea revealed by 15N in vivo labeling throughout seed filling

The fluxes of (1) exogenous nitrogen (N) assimilation and (2) remobilization of endogenous N from vegetative plant compartments were measured by 15N labeling during the seed-filling period in pea (Pisum sativum L. cv Cameor), to better understand the mechanism of N remobilization. While the majority (86%) of exogenous N was allocated to the vegetative organs before the beginning of seed filling, this fraction decreased to 45% at the onset of seed filling, the remainder being directed to seeds. Nitrogen remobilization from vegetative parts contributed to 71% of the total N in mature seeds borne on the first two nodes (first stratum). The contribution of remobilized N to total seed N varied, with the highest proportion at the beginning of filling; it was independent of the developmental stage of each stratum of seeds, suggesting that remobilized N forms a unique pool, managed at the whole-plant level and supplied to all filling seeds whatever their position on the plant. Once seed filling starts, N is remobilized from all vegetative organs: 30% of the total N accumulated in seeds was remobilized from leaves, 20% from pod walls, 11% from roots, and 10% from stems. The rate of N remobilization was maximal when seeds of all the different strata were filling, consistent with regulation according to the N demand of seeds. At later stages of seed filling, the rate of remobilization decreases and may become controlled by the amount of residual N in vegetative tissues.     

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N₂ fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with ¹⁵N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N₂ fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N₂fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N₂ fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of ¹⁵N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N₂ fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N₂ fixation, N remobilization, N₂ partitioning, regrowth     

Photosynthesis by Flag Leaves of Wheat in Relation to Protein, Ribulose Bisphosphate Carboxylase Activity and Nitrogen Supply

The effects of nitrate supply on the composition (cell numbers,protein and chlorophyll contents) of flag leaves of winter wheatgrown with two amounts of N fertilizer and of spring wheat grownin the glasshouse under controlled nitrate supply are describedand related to photosynthesis. Nitrogen deficiency decreasedthe size of leaves, mainly by reducing cell number and, to asmaller extent, by decreasing cell volume. Protein content perunit leaf area, per cell and per unit cell volume was largerwith abundant N. Total soluble protein, ribulose bisphosphatecarboxylase-oxygenase (RuBPc-o) protein and chlorophyll changedin proportion irrespective of nitrogen supply and leaf age.Photosynthesis per unit area of flag leaf and carboxylationefficiency in both winter and spring wheat were proportionalto the amount of total soluble protein up to 7.0 g m^–2and to the amount of RuBPc-o protein up to 4.0 g m^–2.However, photosynthesis did not increase in proportion to theamount of total soluble or RuBPc-o protein above these amounts.In young leaves with a high protein content the measured ratesof photosynthesis were lower than expected from the amount andactivity of RuBPc-o. Carboxylation per unit of RuBPc-o protein,measured in vitro, was slightly greater in N-deficient leavesof winter wheat but not of spring wheat. RuBPc-o activity perunit of RuBPc-o protein was similar in winter and spring wheatleaves and remained approximately constant with age, but increasedin leaves showing advanced senescence. RuBPc-o protein fromN-deficient leaves migrated faster on polyacrylamide gels thanprotein from leaves with high N content. Regulation of the rateof photosynthesis in leaves and chloroplasts with a high proteincontent is discussed. The conductance of the cell to the fluxof CO₂ from intercellular spaces to RuBPc-o active sites iscalculated, from cell surface areas and CO₂ fluxes, to decreasethe CO₂ partial pressure at the active site by less than 0.8Pa at an internal CO₂ partial pressure of 34 Pa. Thus the decreasein partial pressure of CO₂ is insufficient to account for theinefficiency of RuBPc-o in vivo at high protein contents. Otherlimitations to the rate of photosynthesis are considered. Key words: Wheat, photosynthesis, nitrogen, ribulose, bisphosphate carboxylase     

小麦籽粒充实期氮素的吸收和再分配及6－苄氨基嘌呤的调节作用

小麦从开花至成熟,其根、茎和叶中的Ｎ素不断地进行着分配和再分配,主要是向穗子输出。成熟时穗Ｎ含量占植株总Ｎ的８２％。在总Ｎ净输出的同时各营养器官也有相当程度的对当前土壤Ｎ的净吸收。在籽粒充实阶段用ＢＡ处理叶式穗均影响营养器官及穗子的Ｎ素吸收、分配和再分配。     

Long-term Partitioning, Storage and Remobilization of 14C Assimilated by Trifolium repens (cv. Blanca)

The fourth fully expanded leaf on the main stolon of white cloverplants was exposed to ¹⁴CO₂. Thereafter, quantitative and fractionalanalysis of the partitioning, storage and remobilization afterdefoliation of the ¹⁴C labelled assimilate was sequentiallyconducted over a 2- to 3-week period. In undefoliated plants, most ¹⁴C reached its final destinationwithin 24 h of feeding. Forty percent of assimilated ¹⁴C waslost through respiration, while the rest was exported, predominantlyto meristems, but also to roots, stolons and leaves. The ¹⁴Cinitially translocated to meristems was subsequently recoveredin stolon and leaf tissue as the plants matured. Approximately 10% of assimilated ¹⁴C was invested into long-termstorage in roots and stolons. These reserves were remobilizedafter both partial and total defoliation, and a portion of theremobilized ¹⁴C was incorporated into new growth, Partly defoliatedplants regrew more rapidly than totally defoliated plants, butmore ¹⁴C reserve depletion took place in the totally defoliatedtreatment. Reserve depletion took place from both stolons androots, but stolon reserves were preferentially utilized. Bothhigh and low molecular weight storage compounds were involved. Trifolium repens, white clover, assimilate partitioning, storage, remobilization, defoliation     

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

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

The Influence of Nitrogen (15NO3) Supply to Chicory (Cichorium intybus L.) Plants During Forcing on the Uptake and Remobilization of N Reserves for Chicon Growth

Tuberized tap roots of Witloof chicory (Cichorium intybus L.)were forced by placing in a dark chamber in a hydroponic systemunder high RH to produce an etiolated bud, the chicon. Plantswere fed nutrient solutions with two NO₃concentrations of 1·5or 18 mol m^–3 NO₃, or demineralized water. The nutrientsolutions were labelled with 2% atom excess ¹⁵N. Although thechicon biomass increased with increasing NO₃ concentration inthe nutrient solution, the chicon dry weight remained unchanged.The increased chicon biomass was, therefore, due to more waterin the chicon. The N in the chicon originated from either anendogenous source, the root, and/or an exogenous source, thenutrient solution. Organic N reserves remobilization and transferto the chicon were not been affected by NO₃ supply. At the endof the forcing period 75% of the root N had been remobilized.Differences in the amount of N in the chicons of the three treatmentswere due to the uptake of exogenous N. The flux of exogenousnitrogen to the chicon in high NO₃-plants was 2- to 6-fold higherthan in the low NO₃-plants and, at the end of the forcing period,exogenous nitrogen contributed 30% of total chicon N in highNO₃-plants and 10% in low NO₃-plants. Net uptake of NO₃ by chicory plants during the forcing processwas a function of N influx and N efflux. The increase in N influxwas accompanied by an increase in exogenous N flux to the chiconand probably a shift in root and/or chicon osmotic potentialwhich increased water flux to the chicon. Since NO₃ did notaccumulate in either the chicon or the root, it is proposedthat osmotic solutes, such as organic acids and amino acidsmay be involved in osmotic potential changes in chicory duringthe forcing process. Key words: Cichorium intybus L., efflux, influx, nitrogen (¹⁵NO₃) nutrition, remobilization