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 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     

Physiological integration in the clonal perennial herb Trifolium repens L.

Summary Translocation of ¹⁴C-labelled carbohydrates between the parent stolon and branches, and among branches, of Trifolium repens plants was investigated in two glasshouse experiments to determine patterns of physiological organisation in this clonal species. Differential defoliation treatments were applied to the parent stolon and/or branches to test the sensitivity of translocation to the short-term carbon needs of defoliated sinks. Strong reciprocal exchange of carbohydrate between the parent stolon and branches was observed, with 18 41% of the ¹⁴C exported from leaves on the parent stolon moving to branches, while branches simulta-neously exported 25% (for old source branches) to 54% (for young source branches) of the ¹⁴C they assimilated to the parent plant, including translocation to other branches. Branch-to-branch translocation occurred both acropetally and basipetally. Parent-to-branch, branch-to-parent and branch-to-branch carbon fluxes all increased in response to defoliation of the sink, at the expense of carbon supply to stolon tissue or roots of the source module. Reduced export to stolon tissue of the parent axis played a major role in facilitating C reallocation from leaves on the parent stolon to defoliated branches. The observed patterns of C allocation and translocation could be adequately explained by accepted source-sink theory, and are consistent with a high degree of intra-plant physiological integration in resource supply and utilisation. This information provides mechanistic explanations for aspects of the growth dynamics and ecological interactions of T. repens in the patchy environment of a grazed pasture.     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

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     

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     

Photoperiodically Induced Changes in Seed Growth Rate of Soybean as Related to Endogenous Concentrations of ABA and Sucrose in Seed Tissues

Short-day photoperiods can increase the partitioning of assimilatesto filling seeds of soybean (Glycine max L. Merr.), resultingin higher seed growth rates. The plant growth substance ABAhas been implicated in the regulation of assimilate transferwithin filling soybean seeds. Thus, we hypothesized that anincreased concentration of endogenous ABA in seeds may enhancesucrose accumulation and seed growth rate of soybeans exposedto short-day photoperiods. Plants of cv. Hood 75 were grownin a greenhouse under an 8-h short-day photoperiod (SD) until11 d after anthesis (DAA) of the first flower, when half ofthe plants were transferred to a night-interruption (NI) treatment(3 h of low-intensity light inserted into the middle of thedark period). Plants remaining in SD throughout seed developmenthad seed growth rates 43% higher than that of plants shiftedto NI (7·6 mg seed^–1 d^–1 vs. 5·3 mgseed^–1 d^–1). On a tissue-water basis, the concentrationof ABA in SD seeds increased rapidly from 7.6 µmol l^–1at 11 DAA to 65·2 µmol l^–1 at 18 DAA, butthen declined to 6·6 µmol l^–1 by 39 DAA.In contrast, the concentration of ABA increased more slowlyin NI seeds, reaching only 47·4 µmol l^–1by 18 DAA, peaking at 57·0 µmol l^–1 on 25DAA, and declining to 10·2 µmol l^–1 by 39DAA. The concentration of sucrose in SD embryos peaked at 73·5mmol l^–1 on 25 DAA and remained relatively constant forthe remainder of the seed-filling period. In NI, the concentrationof sucrose reached only 38·3 mmol 1^–1 by 25 DAA,and peaked at 61·5 µmol l^–1 on 32 DAA. Thusin both SD and NI, sucrose accumulated in embryos only afterthe peak in ABA concentration, suggesting that ABA may havestimulated sucrose movement to the seeds. The earlier accumulationof ABA and sucrose in SD suggests that ABA may have increasedassimilate availability during the critical cell-division period,thus regulating cotyledon cell number and subsequent seed growthrate for the remainder of the seed-filling period. Glycine max L. Merr. cv. Hood 75, soybean, assimilate partitioning, abscisic acid, photoperiod, source-sink     

Assimilate Partitioning in Three White Clover Cultivars in the Autumn, and the Effect of Defoliation

Partitioning of recently assimilated¹⁴C in three cloned whiteclover cultivars (Aberherald, Grasslands Huia and Sandra), grownoutdoors, was determined at the end of Aug., the end of Sep.and the end of Oct. to examine the relative strength of differentsinks within the plant when growing conditions increasinglyfavour carbohydrate accumulation in relation to growth. Also,the effect of removing two out of three expanded leaves on¹⁴Cpartitioning was studied. Export of¹⁴C from leaves increasedfrom late Aug. to late Oct. More¹⁴C was partitioned to the rootsand less to the apex at the Sep. and Oct. harvests than at theharvest in Aug., irrespective of cultivar. Severe defoliationresulted in more¹⁴C being partitioned to the apex and less tothe main stolon irrespective of cultivar and harvest occasion.Sandra (a cultivar of northern origin) generally partitionedmore¹⁴C to the stolon and less to the apex than did the othertwo cultivars. Trifolium repens L.; white clover; assimilate partitioning; defoliation; photosynthesis; autumn growth     

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     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

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     

Ascorbate enhancement of H1 histamine receptor sensitivity coincides with ascorbate oxidation inhibition by histamine receptors

Ascorbate has previously been shown to enhance both ₁- and ₂-adrenergic activity. This activity is mediated by ascorbate binding to the extracellular domain of the adrenergic receptor, which also decreases the oxidation rate of ascorbate. H1 histamine receptors have extracellular agonist or ascorbate binding sites with strong similarities to _1- and ₂-adrenergic receptors. Physiological concentrations of ascorbate (50 µM) significantly enhanced histamine contractions of rabbit aorta on the lower half of the histamine dose-response curve, increasing contractions of 0.1, 0.2, and 0.3 µM histamine by two- to threefold. Increases in ascorbate concentration significantly enhanced 0.2 µM histamine (5–500 µM ascorbate) and 0.3 µM histamine (15–500 µM ascorbate) in a dose-dependent manner. Histamine does not measurably oxidize over 20 h in oxygenated PSS at 37°C. Thus the ascorbate enhancement is independent of ascorbate's antioxidant effects. Ascorbate in solution oxidizes rapidly. Transfected histamine receptor membrane suspension with protein concentration at >3.1 µg/ml (56 nM maximum histamine receptor) decreases the oxidation rate of 392 µM ascorbate, and virtually no ascorbate oxidation occurs at >0.0004 mol histamine receptor/mol ascorbate. Histamine receptor membrane had an initial ascorbate oxidation inhibition rate of 0.094 min·µg protein^–1·ml^–1, compared with rates for transfected ANG II membrane (0.055 min·µg protein^–1·ml^–1), untransfected membrane (0.052 min·µg protein^–1·ml^–1), creatine kinase (0.0082 min·µg protein^–1·ml^–1), keyhole limpet hemocyanin (0.00092 min·µg protein^–1·ml^–1), and osmotically lysed aortic rings (0.00057 min·µg wet weight^–1·ml^–1). Ascorbate enhancement of seven-transmembrane-spanning membrane receptor activity occurs in both adrenergic and histaminergic receptors. These receptors may play a significant role in maintaining extracellular ascorbate in a reduced state. molecular complementarity; vitamin C; seven-transmembrane-spanning membrane receptors     

Growth of White Clover, Dependent on N2 Fixation, in Elevated CO2 and Temperature

Clonal plants of white clover (Trifolium repens L ), whollydependent on N₂ fixation, were grown for 6 weeks in controlledenvironments providing either (C680 regime) 23/18 °C day/nighttemperatures and a CO₂, concentration of 680 µmol mol^–1,or (C340 regime) 20/15 °C day/night temperatures and a CO₂,concentration of 340 µmol mol^–1 During the firsthalf of the experimental period the C680 plants grew fasterthan their C340 counterparts so that by week 3 they were twicethe weight this 2 1 superiority in weight persisted until theend of the experiment The faster initial growth of the C680plants was based on an approx 70 % increase in leaf numbersand an approx 30 % increase in their individual area Initially,specific leaf area (cm2 g^–1 leaf) was lower in C680 thanin C340 leaves but became similar in the latter half of theexperiment Shoot organ weights, including petioles and stolons,reflected the C680 plant's better growth in terms of photosyntheticsurface Throughout, C680 plants invested less of their weightin root than C340 plants and this disparity increased with timeAcetylene reduction assays showed that nitrogenase activityper unit nodule weight was the same in both C680 and C340 plantsBoth groups of plants invested about the same fraction of totalweight in nodules Nitrogen contents of plant tissues were similarirrespective of growth regime, but C680 expanded leaves containedslightly less nitrogen and their stolons slightly more nitrogenthan their C340 counterparts However, C680 leaves containedmore non-structural carbohydrate Young, unshaded C680 leavespossessed larger palisade cells, packed more tightly withinthe leaf, than equivalent C340 leaves The reason for the C680regime's loss of superiority in relative growth rate duringthe second half of the experiment was not clear, but more accumulationof non-structural carbohydrate, constriction of root growthand increased self-shading appear to be the most likely causes Trifolium repens, white clover, elevated CO₂, elevated temperature, growth, N₂ fixation, leaf structure     

Effect of Severity of Defoliation on the Viability of Reproductive and Vegetative Axillary Buds of Trifolium repens L.

This glasshouse experiment was performed to assess the effectsof a range of constant defoliation regimes applied to cuttingsof a single large-leaved genotype ofTrifolium repens L. on theviability of its axillary buds. Plants were established to comprisea single main stolon (axillary branches were removed) and defoliationtreatments were applied by removing the older (basal) leavesuntil leaf complements of 1·0, 1·5, 2·0,2·5, 3·0 or all leaves (control) remained. Basalleaves were subsequently removed as necessary to maintain thetarget leaf complements. Only severe defoliation (leaf complements of 1·0 and1·5) induced a loss of viability in axillary buds. Lossof viability was greatest in reproductive buds present withinthe apical bud when the treatments were first imposed. Althoughthe most severe treatment (leaf complement 1·0) resultedin death of half the plants, in plants surviving that treatment,death of vegetative axillary buds was restricted to 21% of thevegetative buds at the three youngest node positions withinthe apical bud at the time of treatment application. No othertreatment induced any loss of viability of vegetative buds.There was no loss of viability of axillary buds at nodes formedafter the treatments were imposed. The frequency of initiationof inflorescences at nodes formed after treatments were imposeddecreased as defoliation severity increased. Severe defoliation resulted in marked changes in plant morphologyindicative of a sharp decrease in availability of intraplantresources. It was concluded that under severe defoliation: (1)the potential for vegetative growth (as represented by viablevegetative axillary buds) was maintained at the expense of reproductivegrowth; and (2) that the loss of viability of axillary budswas associated with the sudden changes in physiological processesinduced by defoliation as there was no loss of viability inbuds formed after plants had adjusted their phenotype to oneof smaller size. Trifolium repens L.; white clover; defoliation; axillary buds; viability; inflorescences     

Patterns of 14C-labelled Assimilate Partitioning in Red and White Clover During Vegetative Growth

A quantitative analysis of the ¹⁴C-labelled assimilate suppliedby the expanded leaves on the primary shoot to growing leaves,stem, lateral shoots (branches or stolons) and roots in redand white clover was conducted during vegetative growth. Stem growth of the primary shoot was inhibited in both cloversand utilized no energy resources. The growing leaves at theprimary shoot apex of white clover imported 4 per cent of theshoot's assimilate compared with 10 per cent in red clover.At the basal end of the primary shoot, the tap root of whiteclover imported 16 per cent of the shoot's assimilate comparedwith 22 per cent in red clover. Branches in red clover and stolonsin white clover were by far the largest sinks for primary shootassimilate, importing 39 per cent and 63 per cent of the labelledassimilate, respectively. Analyses of the translocation of assimilate from individualprimary shoot leaves demonstrated that in both clovers olderleaves exported more of their assimilate to branches or stolons,whereas younger leaves exported more of their assimilate toroots, and possibly in white clover, to growing leaves at thetip of the shoot. Of the labelled assimilate exported to branchesor stolons, each primary shoot leaf exported preferentiallyto the branch or stolon in its own axil, but in addition exportedsubstantial quantities of assimilate to all other axillary shoots,particularly those arising from basal axils where the subtendingleaf had died. Trifolium repens, Trifolium pratense, red clover, white clover, assimilate partitioning, perennation     

The Influence of Seedling Size and Carbohydrate Status and of Photon Flux Density During Vernalization on Inflorescence Initiation in Onion (AIlium cepa L.)

Experiments were done in controlled conditions to investigatethe relationship of plant weight, leaf number and raising conditionsto inflorescence initiation in onion seedlings. Above a shootdry weight of 0.06 g the spring-sown cv. Rijnsburger could initiateinflorescences, whereas the autumn-sown cv. Senshyu semi-globeYellow needed to be heavier than 0.45 g. Plants raised at aphoton flux density of 200 µmol m^–2S^–1 anda temperature of 25 C required longer to initiate inflorescencesthan plants raised at 600 µmol m^–2S^–1 and17 C which had higher reserve carbohydrate content. The minimumleaf number for inflorescence initiation was larger for plantswith low reserve carbohydrate content. Photon flux densitiesof 50, 100, 200 and 400 µmol m^–2S^–1 duringvernalization at 9 C caused no differences in inflorescenceinitiation in plants previously raised at l7 C and 600 µmolm^–2S^–1 but the lowest photon flux density duringvernalization reduced initiation in plants previously raisedat 25 C and 200 µmol m^–2S^–1.     

Vascular Architecture of a Large-leafed Genotype ofTrifolium repens

The objectives of this study were to identify the vascular connectionsfrom roots to upper axial bundles in one genotype ofTrifoliumrepensL. ‘Grasslands Kopu’, identify pathways followedby the transpiration stream, and establish whether these pathwayscould account for previously-observed patterns of clonal integration.The study provides new information on vascular connections betweenroot and parent and branch stolons at nodes possessing botha root and a branch, and to the first two leaves on branch stolons.A nodal root is connected to the lower nearside axial bundleof the parent stolon but to both lower and upper nearside axialbundles of the branch. Upper sympodia provide a long-distancetransport pathway from a parent stolon to the apex of branchstolons. Lower sympodia are functionally different, providingshort-distance transport to structures in close proximity tothe source root. This is consistent with observed patterns ofclonal integration inT. repensand may provide a simple architecturalmechanism facilitating foraging.Copyright 1998 Annals of BotanyCompany Acid fuchsin, clonal integration, foraging, physiological integration, serial sections, white clover,Trifolium repens(L.), vascular architecture, xylem transport.     

Root Aeration and Respiration in Young Mangrove Plants (Avicennia marina (Forsk.) Vierh.)

The roots of young plants of Avicennia marina (Forsk.) Vierh.grown under simulated tidal conditions were harvested so asto obtain the entire root system. The roots were subdividedand weighed and subsamples taken for manometric determinationof respiration rates at different temperatures. The supply capacityof the above-ground portion of the root system was determinedand the results compared in terms of supply and demand. Theoxygen consumption rate of the roots at 15°C was found tobe 1·69±0·07 µmol kg^–1 s^–1for cable roots and 3·27±0·12 µmolkg^–1 s^–1 for fine roots. The Q₁₀ for respirationwas 2·55 for oxygen consumption in both fine and cableroots, and for carbon dioxide production was 2·66 forfine roots and 3·04 for cable roots. The respiratoryquotient varied with temperature but was less than unity. Concentrationdifferences of between 1·8 mol m^–3 and 3·4mol m^–3 between the inside of root and the air were sufficientto permit aeration of the root system by diffusion alone, andthe aerenchyma contained sufficient oxygen to maintain aerobicconditions while the roots were covered with water. The effectof tide and seasonal temperature change on gas exchange, togetherwith the possibility of some form of carbon dioxide fixationwithin the root, are examined and the implications of theseeffects on growth and development are discussed. Key words: Mangrove, root aeration, respiration, aerenchyma     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

The duration and rate of grain growth, and harvest index, of wheat (Triticum aestivum L.) in response to temperature and CO2

Winter wheat (Triticum aestivum L. cv. Hereward) was grown inthe field inside polyethylene-covered tunnels at a range oftemperatures at either 380 or 684 µmol mol^–1 CO₂.Serial harvests were taken from anthesis until harvest maturity.Grain yield was reduced by warmer temperatures, but increasedby CO₂ enrichment at all temperatures. During grain-filling,individual grain dry weight was a linear function of time fromanthesis until mass maturity (attainment of maximum grain dryweight) within each plot. The rate of progress to mass maturity(the reciprocal of time to mass maturity) was a positive linearfunction of mean temperature, but was not affected by CO₂ concentration.The rate of increase in grain dry weight per ear was 2.0 mgd^–1 greater per 1 C rise, and was 8.0 mg d^–1 greaterat 684 compared with 380 µmol mol^–1 CO₂ at a giventemperature. The rate of increase in harvest index was 1.0%d^–1 in most plots at 380 µmol mol^–1 CO₂ andin open field plots, compared with 1.18% d^–1 in all plotsat 684 µmol mol^–1 CO₂. Thus, the increased rateof grain growth observed at an elevated CO₂ concentration couldbe attributed partly to a change in the partitioning of assimilatesto the grain. In contrast, the primary effect of warmer temperatureswas to shorten the duration of grain-filling. The rate of graingrowth at a given temperature and the rate of increase in harvestindex were only independent of the number of grains per earabove a critical grain number of 23–24 grains per ear({small tilde}20 000 grains m^–2). Key words: Winter wheat, grain growth, temperature, CO₂, harvest index, critical grain number