Regrowth by Swards of Subterranean Clover after Defoliation. 2. Carbon Exchange in Shoot, Root and Nodule

The carbon economy of subterranean clover swards subjected tothree defoliation treatments (removal of 30, 70 and 80% of shootdry weight) was compared with that of uncut swards. Carbon dioxideexchange in shoots and roots was measured independently 0, 4,8 and 12 d after defoliation. The respiration linked to nitrogenaseactivity was estimated by comparing root respiration measuredin an atmosphere containing 3% oxygen with the respiration in21% oxygen. Net photosynthesis fell by up to 100% immediately after defoliation.There was a decline of over 60percnt; in root respiration bythe end of the first light period, composed of a rapid declineof 70% in nitrogenase-linked respiration in all treatments anda slower decline of nearly 40% in root plus nodule growth andmaintenance respiration in the more severe treatments. Recoveryof net photosynthesis to rates achieved by uncut swards occurredover 4 d in the 30% cut treatment and at least 12 d in moresevere treatments. Whilst recovery of photosynthesis was theprinciple determinant of recovery of net positive carbon balance,the early reduction in respiration facilitated this outcome.After the immediate decline in nitrogenase-linked respiration,recovery in this component of respiration appeared to be linkedwith the recovery in net photosynthesis (approximately 10% ofnet photosynthesis). Carbon budgets revealed priorities in allocation towards leafin the first 5 d and later also towards root growth in severelydefoliated swards. Root respiration comprised a large respiratorycost (up to 75% of net photosynthesis) during early regrowth. Carbon budget, defoliation, N₂ fixation, photosynthesis, regrowth, respiration, subterranean clover, Trifolium subterraneum L     

Effects of Temperature on Growth and Nitrogen Fixation by Swards of Subterranean Clover

Subterranean clover (Trifolium subterraneum L.) cv. ‘Woogenellup’ swards were grown at 10, 15, 20 and 25 Cwith a 12 h photoperiod of 500 or 1000 µmol m^–2s^–1 [low and high photosynthetic photon flux density (PPFD)].Nitrogen-fixing swards received nutrient solution lacking combinednitrogen while control swards received a complete nutrient solution.Growth was measured by infra-red analysis of carbon dioxideexchange and by accumulation of dry matter. Swards were harvestedat intervals between 95 and 570 g d. wt m^–2 for estimationof nitrogenase activity by acetylene reduction and hydrogenevolution assays. Nitrogen fixation was also measured by increasein organic nitrogen. The growth rate was highest at 10 C at low PPFD, and at 10–15C at high PPFD. Nitrogen-fixing swards grew slower than thosereceiving combined nitrogen. Nitrogen fixation measured by increasein organic nitrogen responded similarly to the growth rate,as did acetylene reduction between 10 and 20 C. At 25 C therelationship between acetylene reduction and nitrogen fixationwas distrupted. The difference between the rates of acetylenereduction and hydrogen evolution, theoretically proportionalto nitrogen fixation, was not a reliable indicator of nitrogenfixation because hydrogen uptake developed. Trifolium subterraneum L, subterranean clover, growth, nitrogen fixation, temperature, acetylene reduction     

Effect of Previous Defoliation Regime and Mineral Nitrogen on Regrowth in White Clover Swards: Photosynthesis, Respiration, Nitrogenase Activity and Growth

Small swards of white clover (Trifolium repens L.) cv. Haifawere grown in solution culture in a controlled environment at24 °C day/18 °C night and receiving 500 µE m^-2S^–1 PAR during a 14-h photoperiod. The swards were cuteither frequently (10-d regrowth periods) or infrequently (40-dregrowth) over 40 d before being cut to 2 cm in height. Halfof the swards received high levels of nitrate (2–6 mMN in solution every 2 d) after defoliation while the othersreceived none. Changes in d. wt, leaf area and growing pointnumbers were recorded over the following 10 d. CO₂ exchangewas measured independently on shoots and roots and nitrogenase-linkedrespiration was estimated by measuring nodulated root respirationat 21% and 3% oxygen in the root atmosphere. There was a general pattern in all treatments consisting ofan initial d. wt loss from roots and stubble and reallocationto new leaves, followed by a period of total d. wt gain andrecovery, to a greater or lesser extent, of weight in non-photosyntheticparts. Frequently cut swards had a smaller proportion of theirshoot d. wt. removed by cutting and had a greater shoot d. wt,growing point number and leaf area at the start of the regrowthperiod. As a result of these differences, and also because ofdifferences in relative growth rates, frequently cut swardsmade more regrowth than infrequently cut. Initial photosyntheticrates were higher in frequently cut swards, although the laminaarea index was very low, and it was concluded that stolons andcut petioles made a significant contribution to carbon uptakeduring the first few d. Infrequently cut swards continued toallocate carbon to new and thinner leaves at the expense ofroots and stubble for longer than frequently cut swards andas a result achieved a similar lamina area index after 10 d. Nitrogenase-linked respiration was low in all treatments immediatelyafter cutting: frequently cut swards receiving no nitrate maintainedhigh nitrogenase activity, whereas recovery took at least 5d in infrequently cut swards. Swards which received nitrateafter cutting maintained only low rates of nitrogenase-linkedrespiration and their total nodulated root respiration overthe period was lower than those receiving no nitrogen: greaterregrowth in nitrate fed swards over the 10 d compared to N₂-fixingswards was in proportion to this lower respiratory burden. White clover (Trifolium repens L.), defoliation, regrowth, nitrogen, photosynthesis, respiration, nitrogenase-activity     

The Effect of Defoliation on Carbohydrate, Protein and Leghaemoglobin Content of White Clover Nodules

Single white clover plants grown in pots of Perlite in a controlledenvironment and completely dependent on N₂ fixation were defoliatedto various degrees (46–85 per cent of shoot weight removed).The soluble protein content of nodules declined by about 20per cent and leghaemoglobin content by 50 per cent in the first4–7 d after defoliation but increased again to controllevels as new leaf tissue appeared. In the short term (2–3h) carbohydrate content of nodules declined to different extentsdepending on the severity of defoliation. The initial declinein N₂ fixation and the respiration associated with it, appearednot to be related to the instantaneous carbohydrate contentof nodules but rather to the supply of current photosynthatefrom the shoot. After 24–48 h, however, the carbohydratecontent of nodules had declined to low levels, regardless ofthe severity (46 or 71 per cent shoot removed) of defoliation.As new leaf tissue appeared carbohydrate levels in all partsof the plant gradually recovered towards control levels. Microscopic examination of nodule sections indicated that onlyafter very severe defoliation (80–85 per cent shoot removed)was nodule deterioration evident. Even here, as the plant establishednew leaves, the damage to nodules was repaired and no noduleloss was apparent. Trifolium repens, white clover, defoliation, carbohydrate, protein, leghaemoglobin     

Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation

Single plants of white clover, grown in a controlled environmentand dependent for nitrogen on fixation in their root nodules,were defoliated once by removing approximately half their shoottissue. Their regrowth was compared with the growth of comparableundefoliated plants. Two similar experiments were carried out:in the first, plants were defoliated at 2.5 g, and in the secondat 1.2 g total plant d. wt. Defoliation reduced rate of N₂ fixation by > 70 per cent,rate of photosynthesis by 83–96 per cent, and rate ofplant respiration by 30–40 per cent. Nodule weights initiallydeclined following defoliation as a result of loss of carbohydratesand other unidentified components. No immediate shedding ofnodules was observed but nodules on the most severely defoliatedplants exhibited accelerated senescence. The original rates of N₂ fixation were re-attained after 5–6or 9 d regrowth, with increase in plant size at defoliation.In general, the rate of recovery of N₂ fixation was relatedto the re-establishment and increase of the plant's photosyntheticcapacity. Throughout the growth of both defoliated and undefoliatedplants nodule respiration (metabolism) accounted for at least23 ± 2 per cent of gross photosynthesis. The unit ‘cost’of fixing N₂ in root nodules, in terms of photosynthate, appearedto be unaffected by defoliation, except perhaps for plants veryrecently defoliated. Similarly, the percentage nitrogen contentsof shoot, root and nodules of defoliated plants became adaptedwithin a few days to those characteristic of undefoliated plants. Trifolium repens, white clover, N₂ fixation, defoliation, photosynthesis, respiration     

Defoliation in White Clover: Nodule Metabolism, Nodule Growth and Maintenance, and Nitrogenase Functioning During Growth and Regrowth

In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO₂ g^–1h^–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N₂ fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N₂,fixation: 9.1 µmol CO₂ µmol^–1N₂. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO₂ µmol^–1 C₂H₄;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N₂ fixation increased with plant growthfrom 1 to 22 µmol N₂ per plant h^–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N₂ fixation indicated a value of 2.9 µmolC₂H₄ µmol^–1 N₂, for rates of N₂ fixation between1 and 22 µmol N₂ per plant h^–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN₂ fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N₂, fixation, nitrogenase     

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     

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     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 1. Carbon and Nitrogen Economies of Mixed Swards at Low Temperature

Simulated mixed swards of perennial ryegrass (Lolium perenneL. cv. S23) and white clover (Trifolium repens L. cv. S100)were grown from seed under a constant 10°C day/8°C nighttemperature regime and their growth, and carbon and nitrogeneconomies examined. The swards received a nutrient solution,every second day, which contained either high (220 µgg^–1) or low (40 µg g^–1) nitrate N. The High-N swards had rates of canopy photosynthesis and drymatter production (over the linear phase of growth) similarto those previously shown by mixed swards at high temperature.The Low-N swards grew more slowly; canopy photosynthesis, ata given LAI, was similar to that at High-N but lower LAI's weresustained. Clover increased its contribution to total carbonuptake and total dry weight throughout the period in the Low-Ntreatment and, despite the fact that grass took up most of theavailable nitrate, clover maintained a consistently higher Ncontent by virtue of N₂-fixation. At High-N, grass dominated throughout the measurement period.Earlier, when plants grew as spaced individuals, clover grewless well than grass, but once the canopy was closed it hada similar relative growth rate and thus maintained a steadyproportion of total sward dry weight. It is proposed that earlyin the development of the crop, leaf area production is thelimiting factor for growth, and that in this respect cloveris adversely affected by low temperature relative to grass.Later, as the LAI of the crop builds up, and the canopy becomesfully light intercepting, net canopy photosynthesis plays amore dominant role and here the higher photosynthetic rate perunit leaf area of the clover is crucial. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, low temperature, nitrogen, photosynthesis     

Influence of Carbon Dioxide Concentration on Growth, Carbohydrate Content, Translocation and Photosynthesis of White Clover

White clover ramets were grown at various carbon dioxide concentrations(200, 350 and 1000 µl 1^–1), defoliated and regrownat the same concentrations. Morphological characteristics, dryweights and non-structural carbohydrate contents of plant organs,diurnal variation of sugar and starch content of leaves, translocationof assimilates and photosynthesis were determined. Carbon dioxide concentration influenced the dry weights, butnot the number and size of the plant organs. However, defoliationof plants at low carbon dioxide concentration resulted in decreasedleaf size and stolon length. Carbon dioxide concentration influencedthe content and diurnal variation of starch and sugar in theleaves. Starch was accumulated at medium carbon dioxide concentrationand sugar at a higher concentration when the storage capacityfor starch seemed to be exceeded. Starch was preferentiallyaccumulated in the first and sugar in the second half of thelight period. Translocation was decreased during the periodsof accumulation. Sugar accumulation in the leaves seemed tobe a consequence of the imbalance between sink and source, whereasstarch accumulation seemed to follow an in-built diurnal pattern.Accumulation of both starch and sugar during the photoperiodwas followed by degradation and export during the dark period.Decreased dark export occurred at low carbon dioxide concentrationwhen neither starch nor sugar was accumulated during the photoperiod. Carbon dioxide, white clover, Trifolium repens L., growth, carbohydrates, starch, sugar, translocation, photosynthesis     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 2. A Comparison of Monocultures and Mixed Swards

White clover (Trifolium repens L.) and Perennial ryegrass (Loliumperenne L.) plants were grown, in Perlite, in simulated swardsas either monocultures or mixtures of equal plant numbers. Theywere supplied with a nutrient solution either high (220 µgg^–1) or low (40 µg g^–1) in ¹⁵N-labelled nitrateand grown to ceiling yield at either high (20°C day/15°Cnight) or low (10°C day/8°C night) temperature. Temperature had little effect on the maximum rates of grosscanopy photosynthesis which were similar in High-N grass andHigh-N and Low-N clover monocultures. However these maxima werereached more slowly in clover than grass, and more slowly atlow rather than high temperature. Nitrogen supply increasedphotosynthesis in grass but not in clover. Clover had higherN contents than grass in all four treatments, although in anygiven treatment its N content was lower, and contribution ofN₂-fixation relative to nitrate uptake higher, in mixture thanin monoculture. Conversely, grass had higher N contents in mixturethan monoculture, because more nitrate was available per plantand not because of transfer of biologically fixed N from clover. Under Low-N, clover outyielded grass in mixture, particularlyat high temperature. The grass plants in the Low-N mixtureshad higher N contents and higher SLA, LAR and shoot: root ratiosthan those in monoculture. It is proposed that competition forlight is the cause of the low relative yield and negative aggressivityof grass in these swards. Under High-N, grass outyielded cloverin monoculture and mixture, at both temperatures but particularlyat low temperature when grass had a high aggressivity. Nitrogenand yield component analyses shed no light on clover's apparentlylow competitive ability and evidence is drawn from the previouspaper to demonstrate that grass grew faster than clover onlyas spaced individuals during non-com petitive growth. The relativemerits of measures of competitive ability based on final harvestdata and physiological data taken over a growth period are discussed. Trifolium repens L., white clover, Lolium perenne, perennial ryegrass, competition, temperature, nitrogen     

平茬高度对香椿生长及其光合特性和非结构性碳水化合物含量的影响

该研究以4年生香椿为试验材料,设置平茬20 cm(T₁)、50 cm(T₂)、80 cm(T₃)和不平茬(CK)4种处理,观测其萌枝和叶片生长情况,以及叶片的气体交换参数、光合色素含量、非结构性碳水化合物(NSC)含量的变化,分析不同平茬高度的生长生理响应差异,以明确平茬措施下香椿植株更新复壮再生的生理机制。结果表明：(1)平茬能够显著提高香椿的萌枝能力,促进侧枝和叶片生长,其萌枝数、侧枝长度在T₃处理下最高,成枝数、叶长、叶宽、叶面积及侧枝粗度在T₂处理下达到最大值。(2)随着平茬高度的增加,香椿叶片净光合速率、蒸腾速率、气孔导度和水分利用效率均先升后降,并在T₂处理达到最大,较CK分别显著提高了17.33%、10.00%、13.51%和6.98%;平茬也提高了叶片光合色素含量,叶绿素a、叶绿素b、总叶绿素含量和类胡萝卜素含量在T₂处理下分别比CK显著提高了18.34%、27.07%、21.11%和23.05%。(3)不同平茬高度处...     

Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: II. EFFICIENCY OF NITROGENASE FUNCTIONING

Changes in nitrogenase activity (C₂H₂ reduction and H₂ production),nodulated root respiration and the efficiency of nitrogenasefunctioning were measured in response to progressive dehydrationof nodules on intact well-watered plants of subterranean clover(Trifolium subterraneum L.) cv. Seaton Park. The nodulated rootsof vegetative plants grown to the 14-leaf stage were incubatedin a gas exchange system through which a continuous dry airstreamwas passed over an 8 d period. The root tips were immersed inan N-free nutrient solution during this time so that water andion uptake was unimpeded. The decline in nodulated root respirationresulting from nodule drying was associated with a continualreduction in respiration coupled to nitrogenase activity. Asnodule water potential (_nod) decreased, the proportion of totalnodulated root respiration which was nitrogenase-linked declinedfrom 50% (day 1) to 33% (day 8). This was accompanied by a 79%reduction in specific nitrogenase activity (from 3.79 to 0.81umol C₂H₄ g^–1 nodule dry weight min^–1). Nodule dehydrationalso induced a decline in hydrogen (H₂) production in air. Therelative decline in hydrogen production exceeded that of acetylenereduction activity and this resulted in an increase in the relativeefficiency of nitrogenase functioning. However, the carbon costof nitrogenase activity progressively increased above 2.0 molCO₂ respired per mol C₂H₄ reduced as rood decreased below –0.4to –0.5 MPa. Consecutive measurements of the rates ofhydrogen evolution, ¹⁵N₂ fixation and acetylene reduction activityon intact unstressed plants resulted in a C₂H₄/N₂ conversionfactor of 4.08 and an electron balance of 1.08. These resultsindicated that the pre-decline rate of acetylene reduction activitymeasured in a flow-through system provided a valid measure ofthe total electron flux through nitrogenase. Key words: Subterranean clover, dehydration, efficiency, nitrogenase activity     

Effect of Carbohydrate Demand on the Remobilization of Starch in Stolons and Roots of White Clover (Trifolium repens L.) after Defoliation

White clover plants were grown from stolon tips in growth cabinetsand then defoliated. Thereafter, changes in the contents ofnon-structural carbohydrates such as starch, sucrose, glucose,fructose, maltose, and pinitol in stolons and roots were monitored.Initial contents of carbohydrate reserves, photosynthetic supplyof new carbohydrates and carbohydrate demand after defoliationwere varied by growing the plants at various CO₂ partial pressures,by varying the extent of defoliation and by removing eitherroots or stolon tips at the time of defoliation. Remobilization of carbohydrate reserves in stolons increasedproportionally to their initial contents and was greater whenplants had been severely defoliated, suggesting that carbohydrateswere remobilized according to availability and demand. Starchwas the predominant reserve carbohydrate. Starch degradationwas associated with decreased contents of sucrose, glucose andfructose in young stolon parts and roots but not in old stolonparts suggesting that starch degradation was not strictly controlledby the contents of these sugars. A decrease in the demand forcarbohydrates by removal of roots did not decrease starch degradationbut increased the contents of sucrose, glucose, and fructose.Removal of stolon tips decreased starch degradation and contentsof sucrose, glucose, and fructose. The results suggest thatstarch degradation was controlled by a factor other than sucrose,glucose, and fructose which was exported from stolon tips, e.g.gibberellin. Key words: White clover, storage carbohydrates, remobilization, regrowth     

The Effect of Nitrogenous Fertilizer on the Photosynthesis and Growth of White Clover/Perennial Ryegrass Swards

The application of nitrogenous fertilizer in March to a whiteclover (cv. Blanca) and perennial ryegrass (cv. S23) sward resultedin a rapid suppression of the clover, relative to clover ina treatment given no added nitrogen. Thereafter, the cloverin both treatments grew more rapidly than the grass and itsproportion of the total leaf area in the mixture increased,as the leaf area index rose to 8. After a second applicationof N in early July, clover was not suppressed to the same extentas in the first growth period. Overall, the photosynthetic capacities of newly expanded cloverlaminae were similar in the two treatments. Clover laminae hadhigher photosynthetic capacities than grass, even in the grass-dominant+ N treatment. Lamina area, petiole length, and the number of live leaves perstolon were similar in the two treatments, indicating that thedifferences in total leaf area were due to the presence of fewerstolon growing points in the + N treatment. Trifolium repens L., white clover, Lolium perenne L., perennial ryegrass, nitrogen, leaf area index, photosynthesis, growth     

遮荫对七子花幼苗光合特性和非结构性碳水化合物含量的影响

为探讨不同光环境下濒危植物七子花(Heptacodium miconioides)的生长及适应规律,以2年生七子花幼苗为研究对象,设置全光照、60%遮荫和90%遮荫3种光照强度,研究不同光环境对七子花幼苗的光合特性和非结构性碳水化合物(NSC)含量的影响。结果表明：(1)3种光照处理下,七子花幼苗的净光合速率(P_n)日变化均呈“单峰型”曲线,P_n日均值为全光照>60%遮荫>90%遮荫,90%遮荫下水分利用效率(W_UE)日均值显著低于全光照和60%遮荫。(2)与全光照相比,60%遮荫下七子花幼苗的光补偿点(L_CP)和暗呼吸速率(R_d)显著下降,而光饱和点(L_SP)、最大净光合速率(P_nmax)、最大羧化速率(V_cmax)、最大电子传递速率(J_max)和磷酸丙糖利用率(T_PU)无显著差异;重度遮荫(90%遮荫)导致植株的P_nmax、L     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions II. Nitrogen Fixation and Nitrate Uptake

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and harvested at intervals over and88 d growht period. The swards received a nutrient solutiondaily, which was either High (220 mg l^–1) or Low (10 mgl^–1) in nitrate N. The nitrate was labelled with the ¹⁵Nisotope. An acetylene reduction assay was carried out on eachsward just prior to harvest. Rates of acetylene reduction agreed qualitatively with the ^l5Nanalyses but absolute values did not match (assuming a 4:1 C₂H₄:N₂ratio) and errors in the acetylene assay are discussed. In theLow-N swards clover relied almost entirely on symbioticallyfixed N₂, fixing more than ten times as much as the High-N cloverplants. In the Low-N treatment the grass was N-deficient despiteobtaining much more nitrate per unit root dry weight than clover.In the High-N swards, however, clover took up more nitrate perunit root weight than grass. The High-N clover plants also fixedsome N₂ and maintained a higher total-N content than grass throughoutthe period. There was no evidence of transfer of symbioticallyfixed N from the clover to the grass in either treatment. Trifolium repens, Lolium perenne, nitrate, nitrogen fixation, ¹⁵N, acetylene reduction     

Some Effects of Potassium and Magnesium on the Growth of Subterranean Clover (Trifolium subterraneum)

Subterranean clover plants (Trifolium subterraneum L., cv. Mt.Barker) were grown in culture solutions at optimum nutrientlevels and on the 23rd day after sowing transferred to solutionswithout magnesium or potassium. A third group remained at thesame nutrient levels as before (controls). Magnesium deficiency caused a particularly rapid decline inroot growth, followed later by a net loss of root dry matter.This, and the fact that leaf expansion declined relatively morethan the increase in laminae dry matter, resulted in an accumulationof assimilates per unit leaf area. Transfer of magnesium-deficientplants to complete solutions on day 35 caused a preferentialdistribution of dry matter to the roots, then to petioles andrelatively less to the laminae. These changes caused a pronouncedfall in the root: shoot ratio as the deficiency became moresevere and a rise in the ratio during recovery. Plants in solutions without potassium showed no marked shiftsin dry matter distribution between plant parts. The root: shootratio remained close to that for control plants, except duringthe recovery, when there was a decrease in the ratio. Net rates of CO₂ uptake by laminae from potassium-deficientplants showed little change during the first 10 days of thedeficiency although values were somewhat lower than those forcorresponding control laminae. After transfer to complete solutionsthere was a marked response in photosynthesis, rising to a finalvalue close to that for control laminae. Laminae of plants placedin solutions without magnesium showed a rapid decline in photosynthesisonly 4 days later; there was little response when plants weretransferred to complete solutions a week later. Trifolium subterraneum L., subterranean clover, growth, root: shoot ratio, potassium deficiency, magnesium deficiencies     

Carbon and Nitrogen Yield, and N2 Fixation in White Clover Plants Receiving Simulated Continuous Defoliation in Controlled Environments

Single plants of white clover grown in controlled environments,and dependent for nitrogen on N, fixation, were defoliated at1 or 2 d intervals to 3, 2 and 1 expanded leaves per stolon(Expt 1), and to 1,0.5 (1 leaf on every alternate stolon) and0 expanded leaves per stolon (Expt 2), for 43–50 days Plants adapted to severe defoliation by developing much smallerleaves with a slightly reduced specific leaf area, more stolons,a smaller proportion of weight in leaf, root and nodules anda greater proportion of weight in stolons. The daily yield (materialremoved by defoliation) of d. wt and nitrogen generally decreasedwith severity of defoliation, as did the residual plant weight.However, the ‘efficiency’ of yield (daily yield/residualweight x 100) of dry matter and nitrogen was greater in themost severely defoliated treatments, attaining a maximum of5–6 % All plants adapted to the imposed defoliation regimes, howeversevere, with the result that even plants maintained withoutany fully expanded leaves invested a similar fraction of theirmetabolic resources in shoot and root as less severely defoliatedplants, and continued to grow and fix N₂, albeit at a very reducedrate of 1–2 mg Nd^–11. The energetic cost of N₂ fixation(acetylene reduction) remained constant in all treatments at31 mole CO₂ mole C₂H₄^–1, but there was some evidence thatrate of N₂ fixation per unit of nodule weight declined in themost harshly defoliated treatment. Trifolium repens, white clover, continous defolation, growth, N₂ fixation     

Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: I. CARBOHYDRATE STATUS AND SHORT-TERM RECOVERY OF NODULATED ROOT RESPIRATION

The nodule water potential (_nod) of subterranean clover (Trifoliumsubterraneum L.) cv. Seaton Park incubated in a flow-throughgas-exchange system was induced to decline independently ofleaf water potential (₁) by passing a continuous dry airstreamover the nodulated roots of intact well-watered plants. Reducedtranspiration by plants whose nodules had become dehydratedwas hypothesized to be related to the decline in nitrogen fixation.Whole-plant and nodule soluble carbohydrates increased as _noddeclined. Throughout an 8 d period of continual nodule dehydration,the gaseous diffusion resistance of nodules increased and theoptimum pO₂ for nitrogenase activity declined from 52 to 28kPa. Following rehydration of the nodulated roots between days4 and 5 and between days 7 and 8, nodulated root respirationincreased to or above pre-stress levels whereas nitrogenaseactivity did not recover. Re-establishment of initial ratesof nodulated root respiration was due to the stimulation ofgrowth and maintenance respiration, not to the respiration coupledto nitrogenase activity. Although no recovery of nitrogenaseactivity occurred, the elapsed time from the introduction ofacetylene into the gas stream flowing past the nodules untilmeasurement of the acetylene-induced decline in nitrogenaseactivity, decreased substantially. This was characteristic ofan increase in the permeability of the nodules to gaseous diffusionupon rehydration. However, calculated values of nodule diffusionresistance after the 24 h periods of rehydration did not indicateany recovery of gaseous diffusion resistance based on measurementsof the respiration coupled to nitrogenase activity. Hence, useof a diffusion analogue (i.e. Fick's Law) in conjunction withnodule respiratory CO₂ efflux was unable to predict changesin permeability of the variable barrier of legume nodules followingnodule dehydration and recovery. Key words: Subterranean clover, gaseous diffusion, respiration, carbohydrates, drought