Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

During vegetative growth in controlled environments, the patternof distribution of ¹⁴C-labelled assimilates to shoot and root,and to the meristems of the shoot, was measured in red and whiteclover plants either wholly dependent on N₂ fixation in rootnodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot wereexposed to ¹⁴CO₂, nodulated plants of both clovers generallyexported more of their labelled assimilates to root (+nodules),than equivalent plants utilizing nitrate nitrogen, and thiswas offset by reduced export to branches (red clover) or stolons(white clover). The intensity of these effects varied with experiment.The export of labelled assimilate to growing leaves at the terminalmeristem of the donor shoot was not influenced by source ofnitrogen. Internode elongation in the donor shoot utilized nolabelled assimilate. Whole plants of white clover exposed to ¹⁴CO₂ on seven occasionsover 32 days exhibited the same effect on export to root (+nodules),which increased slightly in intensity with increasing plantage. Nodulated plants had larger root: shoot ratios than theirequivalents utilizing nitrate nitrogen. Trifolium repens, Trifolium pratense, red clover, white clover, nitrogen fixation, nitrate utilization, assimilate partitioning     

Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

Growth and N-accumulation rates in leaves, stolons and rootsof individual white clover plants were studied in three experimentsusing two methods. In a growth chamber experiment, the relativedifferences between tissues were found to be almost constantfor a wide range of clover plant sizes. The stolon dry matter(DM) production was 56% and the root DM production 40% of theDM production in leaves. The N yield of stolons was 30% whileN yield in roots was 34% of N yield in leaves. The effect ofN application on these relations was investigated in a glasshouseexperiment. Application of N reduced the root:shoot N ratiofrom 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e.for abovevs.below cutting-height tissues) was only reduced from0.97 to 0.80. In a field trial with two contrasting N regimes,growth and N accumulation were measured on individual cloverplants. Dinitrogen fixation was estimated by¹⁵N isotope dilutionbased on analysis of leaves-only or by including stolons. Usingleaves-only did not affect the calculation of percentage ofclover N derived from N₂fixation (% Ndfa) since the¹⁵N enrichmentwas found to be uniform in all parts of the clover. A correctionfactor of 1.7 to account for N in below cutting-height tissueis suggested when N₂fixation in white clover is estimated byharvesting the leaves only.Copyright 1997 Annals of Botany Company Leaves; N accumulation; N₂fixation; ¹⁵N isotope dilution; pastures; roots; root/shoot ratio; stolons; Trifolium repensL.; white clover     

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     

The Influence of Leaf Position and Defoliation on the Assimilation and Translocation of Carbon in White Clover (Trifolium repens L.) 1. Carbon Distribution Patterns

The assimilation of carbon (C) by, and distribution of ¹⁴C from,leaves at each end of an unbroken sequence of ten mature leaveson the main stolon of clonal plants of white clover (Trifoliumrepens L.) were measured to identify intra-plant factors determiningthe direction of C movement from leaves. Leaves at two intermediatepositions were also measured. Localized movement of ¹⁴C to sinks at the same node as, or atthe one to two nodes immediately behind, the fed leaf accountedfor 40–50% of the total ¹⁴C exported by all measured leaves.A further 50–60% of exported ¹⁴C was therefore availablefor more-distant sinks, and the direction of translocation ofthis C was determined by the relative total strength or demand(number x size x rate of activity or growth) of sinks forwardof, or behind, the leaf in question. Thus 85% of the ¹⁴C exportedfrom the youngest measured leaf moved toward the base of thestolon, while about 60% of the ¹⁴C exported from the oldestleaf moved acropetally. Defoliating plants to leave just one mature leaf on the mainstolon (at any one of the same four positions studied in undefoliatedplants), and no leaves on branches, resulted in: (1) increasednet photosynthetic rate in all residual leaves: (2) increased%export of fixed C from one of the four leaves; (3) increasedexport to the main stolon apex from all except the eldest leaf;(4) increased export to branches from three of the four leaves;and (5) decreased export to stolon tissue and roots from allleaves, within 3 d of defoliation. These responses would seemto ensure the fastest possible replacement of lost leaf areaand, thus, restoration of homeostatic growth. The observed patternsof C assimilation and distribution in both undefoliated anddefoliated white clover plants are consistent with the generalrules of source-sink theory; the distance between sources andcompeting sinks, and relative sink strength, emerge as the mostimportant intra-plant factors governing C movement. These resultsemphasize the need to consider plant morphology, and the modularnature of plant growth, when interpreting patterns of resourceallocation in clonal plants, or plant responses to stressessuch as partial defoliation. Trifolium repens L, white clover, photosynthesis, assimilate translocation, defoliation     

The Effect of Shade During Leaf Expansion on Photosynthesis by White Clover Leaves

In three experiments measurements of photosynthesis were madeon single leaves of white clover (Trifolium repens L.) on threecultivars grown in a controlled environment. Plants which had grown under an irradiance of 30 J m^–2s^–1, or in shade within a simulated mixed sward, producedleaves with photosynthetic capacities some 30 per cent lowerthan did plants grown at 120 J m^–2 s^–1 without shade.There were no differences between treatments either in photosynthesismeasured at 30 J m^–2 s^–1, or in respiration ratesper unit leaf dry weight. Respiration per unit leaf area washigher in the plants grown at 120 J m^–2 s^–1, reflectingthe lower specific leaf area of these leaves. There were nodifferences between the three cultivars examined. Leaves which were removed from the shade of a simulated swardshortly after becoming half expanded achieved photosyntheticcapacities as high as those which were in full light throughouttheir development. It is suggested that it is this characteristicwhich enables clover plants growing in an increasingly densemixed sward to produce a succession of leaves of high photosyntheticcapacity, even though each lamina only reaches the top of thesward at a relatively late stage in its development. Trifolium repens L., white clover, photosynthesis, leaf expansion, shade, specific leaf area, stomatal conductance     

Mobilization of nitrogen reserves during regrowth of defoliated Trifolium repens L. and identification of potential vegetative storage proteins

Although it is well established that carbon reserves contributeto shoot regrowth of leguminous forage species, little informationis available on nitrogen reserves except in Medicaqo sativaL. and Trifolium subterraneum L. In this study, reserves werelabelled with ¹⁵N to demonstrate the mobilization of endogenousnitrogen from roots and stolons to regrowing leaves and newstolons during 24 d of regrowth in white clover (Thfolium repensL.). About 55% and 70%, respectively, of the nitrogen contentsof these organs were mobilized to support the regrowth of leaves.During the first 6 d, nitrogen in regrowing leaves came mainlyfrom N reserves of organs remaining after defoliation. Afterthese first 6 d of regrowth, most of the shoot nitrogen wasderived from exogenous nitrogen taken up while the contributionof nitrogen reserves decreased. After defoliation, the buffer-solubleprotein content of roots and stolons decreased by 32% duringthe first 6 d of regrowth. To identify putative vegetative storageproteins, soluble proteins were separated using SDS-PAGE ortwo-dimensional electrophoresis. One protein of 17.3 kDa instolons and two proteins of 15 kDa in roots seemed to behaveas vegetative storage proteins. These three polypeptides, initiallyfound at high concentrations, decreased in relative abundanceto a large extent during early regrowth and then were accumulatedagain in roots and stolons once normal growth was re-established. Key words: White clover, regrowth, ¹⁵N-labelled, vegetative storage proteins, electrophoresis     

Rates of Hydrogen Ion Efflux by Nodulated Legumes Grown in Flowing Solution Culture with Continuous pH Monitoring and Adjustment

The net efflux of H⁺ from lucerne (Medicago saliva L.), redclover (Trifolium pratense L.) and white clover (Trifolium repensL.) growing in flowing solution culture and dependent upon symbioticfixation of atmospheric N, was measured over a 75 d experimentalperiod. Considerable and rapid increases in acidity of the nutrientsolution of up to 1.45 pH units were recorded when the pH wasriot held constant over a 30 h period. There was little differencein H⁺ efflux when solution pH was held constant at 4.75, 5.75or 6.75, but there was an immediate cessation when it was adjustedto 3.75. Differences in the daily net efflux of H⁺ closely followedthe pattern of daily differences in incoming radiation, andthere was also evidence of a diurnal pattern of H⁺ efflux. Althoughthere were initially distinct differences between the speciesin the calculated rate of net H⁺ efflux (µg H⁺ g^–1dry shoot d^–), by day 75 these had diminished. In allspecies, however, the maximum rate of efflux per unit of shootsoccurred during the earlier rapid phases of growth. The measuredefflux of H⁺ was well equated with the plant content of excesscations (as measured by ash alkalinity) and, on average, theratio of acidity produced to N assimilated (expressed as anequivalent) was 0-24. Medicago sativa L., Trifolium pratense L., Trifolium repens L., lucerne, red clover, white clover, acidification, cation/anion balance, flowing solution culture, H⁺ efflux, nitrogen fixation     

A Quantitative Analysis of the Uptake of Carbon and of the Supply of 14C-labelled Assimilates to Areas of Meristematic Growth in Lolium temulentum

A quantitative analysis of the ¹⁴C-labelled assimilate suppliedby leaves on the main shoot to terminal meristem, stem, tillers,and roots was conducted during parallel periods of reproductiveand vegetative development in Lolium temulentum. The initial rate of entry of carbon into the shoot varied withthe area and photosynthetic efficiency of the assimilating leaf.Subsequently, respiratory losses of carbon during translocationand incorporation of assimilate at the site of utilization alsovaried. The combined effect of these differences resulted inthe supply of recently assimilated carbon being twofold greaterin reproductive shoots than in vegetative shoots, while withinshoots the carbon supply of the youngest fully expanded leafranged from four-or five-fold greater than the oldest leaf inyoung shoots, to two-or three-fold greater in older shoots.In both reproductive and vegetative shoots, the two or threeyoungest leaves thus dominated the supply of carbon for meristematicgrowth. Meristematic tissue in expanding leaves and leaf primordia atthe terminal meristem of the vegetative shoot received 18–27per cent of the total shoot carbon. This meristem utilized aboutthe same proportion of shoot carbon when it developed into aninflorescence, indicating no major change in the level of meristematicactivity. The proportion of shoot carbon utilized in stem growthincreased as both reproductive and vegetative shoots aged; thisincreased meristematic activity in stem internodes was accompaniedby reduced export of carbon to roots, which received less than10 per cent of the shoot carbon when the experiments ended.The main shoot translocated 20–30 per cent of its recentlyassimilated carbon to developing and rooted tillers, which assinks for carbon were thus as important as the terminal meristemand stem. This outward flow of carbon continued relatively uncheckedwhen donor and receptor shoots developed inflorescences.     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

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 their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

Phosphate Uptake and Transport by Roots and Stolons of Intact White Clover (Trifolium repens L.)

The extent to which phosphate can be absorbed directly fromthe outer medium by stolon internodes and contribute to thetotal accumulation of phosphate by intact plants of white clover(Trifolium repens L. cv. Blanca) was assessed in hydroponicexperiments in a controlled environment room. The uptake ofphosphate by intact roots or stolons was measured by sealinga segment (6-0 mm long) across a flow-cell in which ³²P-labellednutrient solution was circulated for 24 h, the rest of the rootsystem receiving unlabelled nutrient solution. The rate of uptakeof phosphate (µmol g^–1 d^–1 dry wt. basis)by roots was more than 300 times that by intact stolons. Pretreatmentof stolons by gentle abrasion to remove cuticle, so as to simulatethe condition of stolons in the field, increased the uptakeof phosphate 7-fold compared with that of intact stolons. However,the potential of stolons to contribute to the P status of whitedover in the field was calculated to be small (5%). When an incision was made through the hypodermal layer of stolons,the rate of phosphate uptake greatly increased, attaining 71%of that by root segments. This increase, which was greater athigher phosphate concentrations, indicates that the suberi.zedhypodermis constitutes a major barrier to the influx of phosphatein the stolon. After withholding phosphate for different time intervals, thesubsequent rate of phosphate uptake by roots was increased 2-3-foldafter 2 d phosphate deprivation and 3-4-fold after 6 d or 13d phosphate deprivation. A higher proportion of absorbed phosphatewas transported to shoots in phosphate-deprived plants. After1 d of uptake following restoration of the phosphate supply,the concentrations of labelled phosphate in shoots were greaterthan in control plants, although the concentrations of labelin roots was less. However, the rate of uptake of phosphateby stolons, following deprivation, was not significantly increased.These results suggest that the mechanism regulating the enhancedrate of phosphate loading into the xylem, initiated by a periodof phosphate deprivation, is specific to roots and is not inducedin stolons. The results are discussed in relation to the growth and acquisitionof phosphate by white clover in the field. Key words: Nutrient deficiency, phosphate, stolons, transport (ions), Trifolium repens     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Root-Tiller Interrelationships in Spring Barley (Hordeum distichum (L.) Lam.)

Root-tiller relations were investigated in spring barley grownin soil in deep pots. The total dry wt of the root system reachedits maximum 6 weeks from sowing, when the shoot weight was only50 per cent of its value at maturity. Seminal and nodal rootscomprised 40 and 60 per cent, respectively, of the total rootdry wt at maturity; the majority of the nodal root weight wasassociated with the main shoot. The main shoot had approximatelytwice as many nodal roots as either of the first two primarytillers (T1 and T2), and the primary and secondary tillers appearinglater were very poorly rooted. Some tillers, especially secondarytillers that died prematurely, produced no nodal roots. Theweight of the seminal roots and nodal roots attached to themain shoot continued to increase up to maturity but the drywt of nodal roots on tillers declined with time. This patternof growth was closely related to the pattern of ¹⁴C assimilateddistribution within the root system. A very small proportionof ¹⁴C assimilated by the main shoot and T1 and T2 was exported.The majority of the exported assimilate went to the seminalroot system and to nodal roots attached to the main shoot. Individualnodal and seminal roots seemed to have different roles in supplyingnutrients to the shoot system, with the former mainly providing³²P-phosphate to its tiller of origin and the latter generallysupplying the main shoot and primary tillers. Hordeum distichum. (L.) Lam., barley, root growth, nodal roots, seminal roots, tillering, assimilate distribution, ³²P-distribution     

Effects of Blue Light on the Vertical Colonization of Space by White Clover and their Consequences for Dry Matter Distribution

The morphology of white clover is very sensitive to the lightenvironment, especially to the ratio of red:far-red light andto photon irradiance. However, less is known about the effectsof blue light on clover morphogenesis. Cuttings of white cloverwere grown for 56 d in two controlled chambers receiving lightwith similar photosynthetic efficiency and phytochrome photoequilibriumstate but different levels of blue light: some plants were grownunder orange light (very low blue light, 0.02 µmol m^-2s^-1)or under white light containing blue light (83 µmol m^-2s^-1).Other plants were switched from white light to orange lightorvice versa,after 30 d. The absence of blue light modifiedthe growth habit of clover and raised the laminae in the upperlayer of the canopy by increasing petiole length, and petioleangle from the horizontal, and by raising stolons above theground surface. Moreover, the absence of blue light had no effecton total leaf area and total dry weight per plant, but increasedthe leaf area and biomass of petioles of the main axis. Largerpetioles and laminae were associated with the allocation ofmore dry weight to the petiole at the same petiole thicknessbut with thinner laminae. These results indicate that a decreasein blue light is involved in the perception of, and adaptationto, shading by the plant.Copyright 1997 Annals of Botany Company Biomass allocation; blue light; growth habit; leaf area; light quality; photomorphogenesis; Trifolium repensL.; white clover     

The Growth and Carbon Allocation Patterns of White Clover (Trifolium repens L.) Plants of Contrasting Branching Structure

The growth, morphology and carbon allocation patterns of F1progeny white clover (Trifolium repens L.) plants selected foreither low (‘LBF’) or high (‘HBF’) frequencyof stolon branching were compared in two controlled-environmentexperiments. Selections from within both a small-leaved (‘GrasslandsTahora’) and a large-leaved (‘Grasslands Kopu’)clover cultivar were compared, and plants were grown under arelatively lenient defoliation treatment (expt 1) or under threelevels of defoliation seventy (expt 2). Carbon allocation patternswere measured by ¹⁴CO₂ pulse-chase labelling using fully unfoldedleaves on the main (parent) stolon. LBF and HBF displayed consistent differences in the selectedcharacter though, within cultivars, the difference between selectionswas most pronounced for Kopu. The selections developed fundamentallydifferent branching structures resulting from differences inbranching frequency, with total branch weight per plant averaging122 mg for LBF and 399 mg for HBF (mean of both experiments).More C moved from parent stolon leaves to branches in HBF thanin LBF (mean 22.6% vs. 15.1% respectively of the ¹⁴C exportedfrom source leaves). More C also moved to stolon tissue in HBF,but, counterbalancing this and the difference in allocationto branches, less moved to developing leaves and roots on theparent stolon itself compared to LBF. However, the total weightof developing leaves and roots per parent stolon was generallygreater in HBF than in LBF, probably reflecting greater C importby these sinks from the higher number of branches present perplant in the former selection. HBF plants were consistentlylarger at harvest than LBF plants. There were no defoliationtreatment x selection interactions in C allocation patternsin expt 2. The implications of the results for plant performancein grazed pastures are discussed. Branching, carbon translocation, defoliation, growth, morphology, Trifolium repens, white clover     

The Export and Distribution of 14C-labelled Assimilates from each Leaf on the Shoot of Lolium temulentum during Reproductive and Vegetative Growth

In both reproductive and vegetative plants of Lolium temulentumL., the export of ¹⁴C-labelled assimilates from each healthyleaf on the main shoot to terminal meristem, stem, tillers,and roots was measured each time a new leaf was expanded, fora period of 5 to 6 weeks. Some labelled assimilates moved fromeach leaf on the main shoot to every meristem in the same shoot,as well as to the tops and roots of adjacent organically attachedtillers. The terminal meristem of the reproductive shoot, which includedthe developing inflorescence, received 70–80 per centof the carbon assimilated by the emerged portion of the growingleaf, 15–25 per cent of the carbon assimilated by thetwo youngest expanded leaves, and 5–10 per cent of thatfrom each of the older leaves. A similar pattern of carbon supplyto the terminal meristem was found in vegetative shoots, exceptthat older leaves on young vegetative shoots supplied even lessof their carbon to the terminal meristem. The general conclusionis that developing leaves at the tip of the shoot receive aboutthe same proportion of carbon from each leaf as does a developinginflorescence. Young expanded leaves provided most labelled assimilates forstem growth; during both reproductive and vegetative growth,expanded leaves increased their export of labelled carbon tostem, and exported less of their ¹⁴C to roots and sometimesto tillers. In these reproductive and vegetative shoots, grown in a constantexternal environment, the major changes in the pattern of distributionof labelled assimilates appeared to be the result of increasedmeristematic activity in stem internodes; the development ofan inflorescence had no obvious direct effect on the carboneconomy of shoots.     

Photosynthesis of White Clover Leaves as Influenced by Canopy Position, Leaf Age, and Temperature

Microswards of white clover (Trifolium repens L.) were grownin controlled environments at 10/7, 18/13 and 26/21 °C day/nighttemperatures. The vertical distribution of leaves of differentages and their rates of ¹⁴CO₂-uptake in situ were studied. Extending petioles carried the laminae of young leaves throughthe existing foliage. A final position was reached within 1/4to 1/3 of the time between unfolding and death. Newly unfoldedleaves had higher rates of ¹⁴CO₂-uptake per leaf area than olderones at the same height in the canopy. At higher temperatures,the decrease with age was faster. However, the light-photosynthesisresponse of leaves which were removed from different heightsin the canopy varied much less with leaf age than did the ratesof ¹⁴CO₂-uptake in situ. The comparison of the rates of ¹⁴CO₂-uptake in situ with thelight-photosynthesis response curves suggests that young leavesreceive more light than older ones at the same height in thecanopy. This would imply that young white clover leaves havethe ability to reach canopy positions having a favourable lightenvironment. This ability may improve the chances of survivalof white clover in competition with other species. Trifolium repens L., white clover, photosynthesis, canopy, leaf age, ¹⁴CO₂-uptake, ecotypes, temperature     

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     

Flowering of Contrasting White Clover Varieties in Relation to Temperature in Controlled Environments

Effects of temperature on floral initiation of ten white clovervarieties growing in controlled environments are described.Plants grown under long days (16 h) were subjected to constanttemperatures of 26, 18 and 10 ^°C. Relationships betweenmorphological and physiological traits and flowering were examined. Most plants flowered at the two higher temperatures but only10 per cent of plants flowered at 10 ^°C. Larger leaved typestended to produce more reproductive buds per stolon at the highertemperatures than did smaller leaved varieties. Of the floral characters studied, floret number was least affectedby temperature. Ovule number and peduncle length were greatestat 18 ^°C. Variation in, and absolute level of nectar secretionwas greatest at the highest temperature. Trifolium repens, white clover, flowering, temperature     

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