Correlations between Growth and Flowering in Chenopodium amaranticolor: I. Initiation of Leaf and Bud Primordia

Vegetative plants of Chenopodium amaranticolor were inducedto flower by exposure to 2, 6 or continuous short days (SDs)and the effect of such treatments on organogenesis at the apexof the main stem followed by means of dissections. The mostoutstanding responses to SD treatment were (I) an immediateelongation of the apex, (2) a stimulation of the rate of initiationof leaf primordia, and (3) a promotion of the rate of initiationof axillary bud primordia. In response to as few as 2 SDs, therate of initiation of leaf primordia increased from 0.47 toa maximum of 3.70 per day and the rate of initiation of axillarybud primordia immediately increased from 0.47 to 1.35 per day. Precocious initiation of axillary bud primordia led to the formationof double ridges. The results indicate double ridges to be homologouswith vegetative axillary buds; although they normally developedinto reproductive tissues, they passed through a period of vegetativegrowth following minimal induction to flowering by exposureto 2 SDs. The rate and degree of flowering were highest in plants whichreceived the longest period of SDs, but the differences in finalflowering response were greater than the differences betweenthe initial responses at the apices. The effect of SDs was thusnot confined to an initial stimulation of organogenesis; a prolongedexposure to SDs must have enhanced the subsequent developmentof double ridges into flower primordia. The results are discussed in relation to previous findings andthe general conclusion drawn that the initiation of double ridgesis very widely accompanied by a stimulation of apical growth.It is suggested that inductive conditions remove a general growthinhibition and that the resultant stimulation of apical growthmight lead to the initiation of double ridges.     

Gibberellins and Inhibitors in Relation to Flowering of White Clovers (Trifolium repens L.)

Flower primordia appeared sooner and flowering intensity increasedas day length was extended beyond 8 h (short day) in two whiteclovers: cv. Tamar and cv. Tammisto, originating from Israeland Finland, respectively. Application of GA₃ under short dayscaused Tamar to flower more intensively as much as the doseof GA₃ was increased, but it did not affect the flowering ofTammisto. The level of gibberellin-like substances in stolon apices washigher under long (16 h) than under short days in both cultivars.The level of inhibitors extracted from leaves was not affectedby day length in Tamar. However, in Tammisto this level wasalmost twice as high under short than under long days. It issuggested that the presence of a relatively high level of inhibitorsin Tammisto when grown under short days interfered with theactivity of gibberellins in its effect on flowering. ABA wasamong the inhibitors: its application under long days had noeffect on flowering, but it increased the number of stolons.Possibly inhibitors other than ABA are involved in the preventionof flowering. Leaf expansion responded to day length and GA₃ and ABA applicationin a similar way as the response of flowering, i.e. towardsflowering the size of the leaves increased.     

Floral Initiation of White Clover (Trifolium repens L.) in Response to Daylength and Temperature

The response to different combinations of daylength (14, 15and 16 h) and day/night temperature (20/10°C, 30/15°C)of four white clover (Trifolium repens) varieties in terms offloral initiation and development was studied. More plants flowered under long days (16 h) at 30°C thanin any other combination. Varietal differences in response todaylength were large but temperature was the factor controllinginitiation. Daylength was more important in regulating peduncle length,floret and ovule numbers than was temperature. The amount ofnectar produced was lowest at the higher temperature althoughnectar concentration was unaffected by temperature. Nectar concentrationwas lowest in the shortest daylength (14 h). White clover, floral initiation, temperature, daylength     

The inheritance of cyanoglucoside content in Trifolium repens L.

A four generation backcross breeding program was undertaken. Analysis of the levels of cyanoglucoside in Acac progeny shows that the level of cyanoglucoside (linamarin and lotaustralin) is inherited and that part of the inherited variation in cyanoglucoside levels is attributable to the existence of different Ac alleles in the parent plant. In vitro microsomal cyanoglucoside biosynthetic activity was measured in a high-level and a low-level parent plant. There was no evidence for the presence of microsomes with different qualitative properties in the two plants. The Ac locus was shown to segregate independently of the S incompatibility locus.This research was supported in part by SERC Grant GRA 95550.     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

The Carbon Economy of Clonal Plants of Trifolium repens L.

Fluxes of carbon between sources and sinks were quantified forclonal plants of Trifolium repens L. (cv. Blanca) in two glasshouseexperiments. Carbon sources were (a) leaves on the parent (=main)stolon apex, or (b) leaves on either young or old branches,and the major sinks of interest were the parent stolon apex,branches, and the adventitious root arising at the same parentstolon node as a young source branch. Defoliation treatmentswere applied to the parent stolon and/or branches (excludingsource branches). Carbon moved freely from the parent stolon to branches and vice-versa;these bidirectional exchanges of C provided important supplementarysources of carbohydrate for the sinks and buffered them againstthe effects of defoliation. Young branches exported more C tothe parent plant (mean=6.3µmol d^–1) than they importedfrom leaves on the parent stolon (5·2µmol d^–1)which, in turn, exceeded the amount fixed by leaves on the branchand utilized within the branch itself (2·7µmold^–1). In contrast, the C economy of old branches was largelyself-contained with, on average, 25·4µmol d^–1exported to the parent plant, 1·8µmol d^–1imported from the parent, and 63·0µmol d^–1fixed and utilized by the branch itself. Thus the growth ofyoung branches was immediately reduced by removal of parentstolon leaves, but old branches were unaffected. An estimated 42% of the C utilized by the main stolon apex originatedfrom branches, while by far the largest proportion (84%) ofthe C used for growth of young nodal roots originated from theassociated branch and not from leaves on the parent stolon towhich the root was directly attached. Key words: Trifolium repens, clonal growth, carbon economy, physiological integration, defoliation     

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     

The Initiation, Growth, and Emergence of Leaf Primordia in Fragaria

The leaf initiation rate in Fragaria vesca (var. ‘RoyalSovereign’) has been compared with the elongation rateof the leaf primordia at different seasons. Certain conceptionsof growth correlation within the bud are presented. Experimentson the nature of elongation and emergence of primordia are described,and the causes of emergence are discussed.     

Rates of Leaf Initiation and Leaf Growth in Agropyron repens (L.) Beauv.

Rates of leaf initiation, emergence, and growth have been measuredduring the period between the production of one and of ten matureleaves on the primary shoot of Agropyron repens. There is aprogressive decline in the growth rate of successively formedleaves so that at the time of formation of the next leaf primordiumeach primordium is smaller than its predecessor at a comparablestage of development. There is also a trend towards a diauxicpattern of growth with a lag phase apparently coinciding withthe transition from apical to intercalary growth of the youngleaf. Up to the six-leaf stage the rate of leaf formation exceedsthe rate of emergence and leaf primordia accumulate on the shootapex. Thereafter the rate of emergence exceeds the rate of formation.These changes in rates of leaf formation and growth are interpretedin terms of competition for assimilates between expanding leavesand leaf primordia, and between the primary and axillary shootapices.     

Functional plasticity of Trifolium repens L. in response to sulphur and nitrogen availability

Recent control of atmospheric SO₂ pollution is leading to important soil sulphur impoverishment. Plasticity could be a mechanism allowing species to adapt to this rapid global change. Trifolium repens L. is a key grassland species whose performances in community are strongly linked to nitrogen availability. Plasticity of three white clover lines contrasting in their ability to use atmospheric N₂ or soil N was assessed by analysing a set of functional traits along a gradient of nitrogen and sulphur fertilisation applied on a poor soil. White clover traits showed high morphological and physiological plasticity. Nitrogen appeared to be the most limiting factor for the VLF (Very Low Fixation) line. S was the element that modulated the most traits for the nitrogen fixing lines NNU (Normal Nitrate Uptake) and LNU (Low Nitrate Uptake). As expected, N fertilisation inhibited white clover fixation, but we also observed that N₂ fixation was enhanced when S was added. S fertilisation increased nodule length as well as the proportion of nodules containing leghaemoglobin. S fertilisation, with a direct effect and an indirect effect through N₂ fixation, increases white clover performances particularly with regards to photosynthesis and potential vegetative reproduction. The important plasticity in response to S availability should allow it to adapt to a large range of abiotic conditions, but its sensitivity to S nutrition would be a disadvantage for competition in a situation of soil sulphur impoverishment. In contrast, S fertilisation could help maintain this species when nitrogen status is against it.     

Accumulation of Metabolites during Seed Development in Trifolium repens L.

Metabolite deposition during seed development was examined histochemicallyin Trifolium repens by light- and fluorescence microscopy. Allendosperm haustorium at the chalazal pole of the embryo sacand wall protrusions in cell walls of the suspensor and theembryo sac suggest that transfer of metabolites from maternalto offspring tissue takes place primarily at these sites. Thisis further supported by prominent cutinization of the interpolarregion of the embryo sac wall, accumulation of starch in integumentaltissue at the embryo sac poles, and breakdown of interpolarendothelial cells. Decomposition of osteosclereid starch isfollowed by accumulation in the cellular endosperm and subsequentlyin the embryo parallel to endosperm degradation. The starchaccumulates gradually inward from the subepidermal cells ofthe embryo to the stele. Protein bodies are formed in the vacuolesalong the tonoplast, later to be cut off in vesicles releasedinto the cytoplasm. At maturity the embryo is packed with proteinand starch, but without lipid reserves. Phytin is observed inthe protein bodies. The mature embryo is surrounded by a proteinand starch containing aleurone layer which originates from theendosperm.Copyright 1994, 1999 Academic Press White clover, protein, starch, cuticle, embryo sac wall     

Genetic analysis of shikimate dehydrogenase allozymes in Trifolium repens L.

 A genetic analysis was carried out on progeny families from pair crosses among plants polymorphic for shikimate dehydrogenase (SDH) isozymes in white clover (Trifolium repens L.). SDH was controlled by two independently assorting disomic loci. This result is consistent with the presence of a single gene (Sdh) represented in this putative allotetraploid by one locus in each of the genomes. One of the Sdh loci is linked (6.0±2.0 cM) to the linamarase (Li) locus. There was no evidence for differentiation of the duplicate Sdh loci (both carry common alleles). White clover behaves genetically as a diploidised allotetraploid but the possibility of a low frequency of multivalent formation and homoeologous pairing has not been ruled out. The SDH locus is likely to be useful for the marker-assisted selection of agronomic traits. Received: 5 June 1997 / Accepted: 19 November 1997     

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.     

Differences between Natural Populations of Trifolium repens L. in Response to Mineral Nutrients

The response to phosphate of samples of natural local populationsof T. repens, collected from soils of different phosphate contents,was determined over a wide range of phosphate levels in sandculture. A marked correlation (r = +0.96) was found betweenresponse of populations to phosphate at low phosphate levelsand the amount of phosphate in the native soil of the population;populations from soils high in phosphate showed larger reductionsin growth at low phosphate levels than did those from soilslow in phosphate. It is concluded that response to phosphateis one aspect of the physiological adaptation of the naturalpopulations to particular soil conditions. Populations alsodiffered in response to very high levels of phosphate, and inthe range of phosphate levels tolerated; the possible adaptivesignificance of this is discussed in relation to the phosphatebuffering effects of the native soils. Populations tolerantof low phosphate levels had higher concentrations of phosphorusin the shoot material, and a higher uptake of phosphorus perunit weight of roots, than did intolerant populations. The differencesbetween populations in response to phosphate are therefore believedto be due to difference in uptake rather than to differencein metabolic requirement of phosphate.     

Axillary Bud Development in White Clover in Relation to Defoliation and Shading Treatments

A period of growth under shade netting in the glasshouse allowedthe cultivation of white clover stolons with an accumulationof undeveloped axillary buds similar to that often found onstolons from grass/clover swards. The subsequent capacity ofthese nodes to develop branches under different circumstanceswas investigated in three experiments. Removal of the laminaeand petioles subtending sets of four buds along a stolon reducedthe rate at which branches were initiated from the buds. Treatmentsin which petioles, or petioles plus laminae, were retained initiatedbranches more quickly. Shading the stolons reduced both therate of initiation and the percentage of buds which developed,unless both petioles and laminae were retained. There was someevidence that conditions applied to individual buds may actin the same way as the same conditions applied to sets of fourbuds and that illuminated nodes may depress the performanceof neighbouring shaded notes. Fewer buds developed at older nodes than at younger nodes duringthe summer, but during the autumn younger buds initially developedmore slowly than older buds. This suggests that buds can developat a younger nodal age in summer than in winter. When leafless stolons were cut up into component internodesbuds developed faster than on intact stolons, provided the budwas located at the end of the internode nearest the main stolongrowing point. If the bud was at the other end, branch developmentwas slower than on intact stolons. The results are discussedin relation to clover growth in sward conditions. White clover, Trifolium repens, axillary bud development, branching, growing points, defoliation, shading     

On the origin of the cyanogenic polymorphism in Trifolium repens L.

Trifolium repens L. and Trifolium nigrescens Viv. are two of the approximately six cyanogenic species known in the genus Trifolium. The two species are closely related: T. nigrescens is considered to be one of the diploid ancestors of the amphidiploid T. repens . We studied morphology, meiosis and the cyanogenic system in T. repens (amphidiploid), T. nigrescens (diploid) and their reciprocal hybrids. A comparison of the enzyme linamarase in the species and hybrids shows that there is a general resemblance between their linamarases. Immunological studies indicate that the linamarases must have a somewhat different three-dimensional structure. These facts are consistent with the view that T. nigrescens (or an ancestral form of this species) has donated the Li gene of T. repens . The other putative parent, T. occidentale has probably not donated an active Li gene. The hypothesis of the origin of the Li gene does not explain its polymorphism in European populations of T. repens , as T. nigrescens is monomorphic for cyanogenesis and amphidiploids do not segregate for genes which are homozygous dominant in one of the parents. Segregation for Li could be caused by a gene mutation or a small exchange between homeologous chromosomes. The latter event is more probable. A nigrescens-repens exchange would give rise to a chromosomal region with reduced homology to both parental chromosomes. The genes in the region of exchange will be tightly linked due to diminished cross-over frequency. It has been known for years that Li has effects on the vegetative and reproductive characters in T. repens and we have recently shown that these effects must be the result of genes linked to Li . As the associated characters influence the fitness of the cyanotypes, not only the origin but also the maintenance of the cyanogenic polymorphism is closely related to the evolutionary history of T. repens .     

Influence of Temperature and Floret Age on Nectar Secretion in Trifolium repens L.

The influence of temperature on nectar secretion in non-pollinatedflorets of Trifolium repens was investigated in growth chambersat 10, 14, 18 and 22°C. The effect of temperature on therate of nectar secretion was significant in all clones. Theoptimum temperature for secretion in three clones varied from10°C for a clone of Icelandic origin, to 18°C in a cloneselected from a Danish variety. Similarly, the average nectaryield varied significantly among clones of different geographicalorigin. One clone secreted two to four times more than othersat 10°C. The optimum day temperature for nectar secretionwas higher when the plants were exposed to low night temperature,presumably a result of decreased night respiration. Nectar accumulatedat the floret base until senescence. Evidence for reabsorptionof nectar was obtained in four clones. Sucrose, fructose andglucose were identified as the major sugars in the nectar. Highnight temperatures led to decreased sucrose percentage in favourof glucose and fructose. The frequency of new florets openingper day was not influenced by temperatures between 10 and 22°Cin one clone, whereas low temperatures significantly decreasedthe number of new florets in another. Few or no modified stomatawere observed in the epidermis of the nectary. The high variationwith respect to nectar secretion at low temperatures, alongwith the high heritability of this quality, suggests that breedingfor high nectar production at low temperature is plausible.The significance of nectar yield in pollination biology is discussed.Copyright1994, 1999 Academic Press Trifolium repens, white clover, nectar, temperature, floret age, flowering, nectary     

Ovule abortion in white clover (Trifolium repens L.)

Plants of white clover ( Trifolium repens L.) cv. Olwen were grown in an open glasshouse maintained at a mean temperature of 20^oC and ovule growth and seed production measured. Differences in the rate of growth of ovules within ovaries were observed as early as 2 days after pollination. Ovules reached a maximum size after 8 days with the smallest only half the size of the largest. After 8 days, the smallest ovules became flaccid and shrivelled. Ovule position within the ovary had little effect on the frequency of seed set and although there was an apparently higher probability that central ovules produced a seed than those nearer the peduncle or style this was not statistically significant. Inflorescence position and floret position on the inflorescence had a significant effect on the number of seeds per floret and seed weight; the first formed inflorescences and the first florets to be pollinated on each inflorescence had more seeds per floret and heavier seeds and fewer florets with no seed than later pollinated florets. There were also differences between florets within the same whorl. The role of a number of factors which may influence floret site utilisation are discussed.