Cumulative activation of axillary buds by nodal roots in Trifolium repens L

In Trifolium repens the rate of outgrowth of an axillary bud was closely correlated with its duration of exposure to a nearby nodal root. The dose-dependent nature of this relationship, over 0-22 d, is consistent with the concept that axillary buds are cumulatively activated by a root signal (RS) such that the longer they receive the signal the higher is their level of activation and hence their rate of outgrowth. Furthermore, the activation level attained by a bud was subsequently retained following the excision of the nodal root providing the source of its activation: its rate of growth 3-6 weeks after root excision still reflected the initial level of activation of the bud. Thus, once activated, a bud required relatively little RS to maintain its rate of outgrowth, implying that activation involves the establishment of an autonomous control mechanism within the bud itself. This provides an explanation of how a strongly activated apical bud can continue growth at relatively low RS levels when it is distanced from its nearest root system, while at the same time the prevailing low RS environment leads to weak activation of the axillary buds emerging from it.     

Translocation of phosphorus from nodal roots in two contrasting genotypes of white clover (Trifolium repens)

Patterns of translocation of recently-assimilated phosphorus (P) exported from'young' source roots (located 3–4 nodes from the stolon apex) and 'old' source roots (located near the base of the stolon) on the primary stolon of clonal plants of the forage legume white clover ( Trifolium repens L.) were determined using ³²P. Plants of a small-leaved genotype and of a large-leaved genotype were grown in sand culture at two notionally limiting or near-limiting rates of P supply and one non-limiting rate of supply. The small-leaved genotype showed little response in growth rate to the full range of P treatments whereas growth of the large-leaved genotype at the non-limiting rate of P supply was 2. 4 times greater than at the two low rates of P supply. Source roots of both genotypes exported only 26–30% of the P they acquired to the shoot within 24 h when P supply was limited whereas at the high-P rate 54% of recently-assimilated P was exported. Patterns of translocation of exported P to specific sinks differed little between the genotypes and the P treatments; branches were the main sink, accounting for nearly 80% of the estimated amounts of P (μg day⁻¹) exported from young and old roots combined. Translocation patterns from individual roots were determined largely by the modular structure of plants and by the location of the root relative to the major sinks, and were therefore consistent with the same source-sink principles which govern carbohydrate translocation in clonally-growing species. There were strong suggestions that storage of P in stolons and roots played a much greater role in the growth of the small-leaved plants than of the large-leaved plants.     

Relative importance of nodal roots and apical buds in the control of branching in Trifolium repens L.

Clonal species are characterised by having a growth form in which roots and shoots originate from the same meristem so that adventitious nodal roots form close to the terminal apical bud of stems. The nature of the relationship between nodal roots and axillary bud growth was investigated in three manipulative experiments on cuttings of a single genotype of Trifolium repens. In the absence of locally positioned nodal roots axillary bud development within the apical bud proceeded normally until it slowed once the subtending leaf had matured to be the second expanded leaf on the stem. Excision of apical tissues indicated that while there was no apical dominance apparent within fully rooted stems and very little in stems with 15 or more unrooted nodes, the outgrowth of the two most distal axillary buds was stimulated by decapitation in stems with intermediate numbers of unrooted nodes. Excision of the basal branches from stems growing without local nodal roots markedly increased the length and/or number of leaves on 14 distally positioned branches. The presence of basal branches therefore prevented the translocation of root-supplied resources (nutrients, water, phytohormones) to the more distally located nodes and this caused the retardation in the outgrowth of their axillary buds. Based on all three experiments we conclude that the primary control of bud outgrowth is exerted by roots via the acropetal transport of root-supplied resources necessary for axillary bud outgrowth and that apical dominance plays a very minor role in the regulation of axillary bud outgrowth in T. repens.     

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.     

A comparison of ovule abortion in self-incompatible and self-fertile lines of white clover (Trifolium repens L.)

Plants of white clover (Trifolium repens L.) cv. Olwen and three genotypes of each of three self-fertile inbred lines of white clover were grown from seed in a bee-proof glasshouse. When in flower, a series of crosses within and between the self-incompatible (SI) and self-fertile (SF) lines was made and ovule growth and seed production measured 7 and 28 days after pollination (DAP) respectively. At harvest, the SF plants had a lower proportion of ovules aborting and more seeds per floret than the SI plants, though the mean seed weight of the SI plants was greater. Ovule abortion was observed in both the SF and SI plants though the number of aborted ovules was greater in the SI plants. The stage when abortion occurred also differed. All of the ovule abortion in the SI plants occurred within a 7-day period after pollination. In the SF plants, some abortion was observed in this period but also between 7 and 28 DAP, when the seed was harvested. This suggested that abortion, up to 7 DAP, is due primarily to genetic factors whilst subsequent abortion, between 7 and 28 DAP, is due to non-genetic factors.     

Studies on the feeding of Sitona lineatus L. (Coleoptera: Curculionidae) on white clover (Trifolium repens L.) seedlings

Measurements of feeding damage by sitona weevil (Sitona lineatus L.) adults on differing numbers of seedlings of white clover (Trifolium repens L.) at the first and fourth trifoliate leaf stage were made in the glasshouse at 20°C. S. lineatus consumed more of the trifoliate component of the seedling. Sitona adults caused significant yield reduction at all levels of plant population. Total clover consumption increased with increasing size of sitona population, but consumption per adult weevil was reduced.     

利用P5CS基因转化白三叶的研究

为了培育抗旱白三叶,构建了植物表达载体pBPC-P5CS,利用基因枪法转化白三叶的愈伤组织,PCR检测和Southern blot鉴定证实白三叶中已导入P5CS基因。对转P5CS基因白三叶植株的不同抗旱指标进行了分析。结果表明,与对照相比,转P5CS基因株系的抗旱能力得到了较大的提高。干旱胁迫下,与对照相比,转P5CS基因植株的脯氨酸含量和相对含水量分别比对照高20.0%-21.2%和5.6%-8.5%。     

Effects of VA mycorrhizal colonization on photosynthesis and biomass production of Trifolium repens L.

The influence of vesicular–arbuscular mycorrhizal (M) colonization on biomass production and photosynthesis of Trifolium repens L. was investigated in two experiments in which the foliar nitrogen and phosphorus contents of non-mycorrhizal (NM) plants were manipulated to be no lower than that of M plants. Throughout both experiments there was a stimulation in the rate of CO₂ assimilation of the youngest, fully expanded leaf of M compared with NM plants. In addition, M plants exhibited a higher specific leaf area compared with NM plants, a response that maximized the area available for CO₂ assimilation per unit of carbon (C) invested. Despite the increased rate of photosynthesis in M plants there was no evidence that the additional C gained was converted to biomass production of M plants. It is suggested that this additional C gained by colonized plants was allocated to the mycorrhizal fungus and that it is the fungus, by acting as a sink for assimilates, that facilitated the stimulation in the rate of photosynthesis of the plant partner.     

Mycorrhizal sink strength influences whole plant carbon balance of Trifolium repens L.

A comparative analysis of daily carbon (C) budgets and aspects of the C physiology of clover ( Trifolium repens L.) colonized by vesicular-arbuscular (VA) mycorrhizal fungi was carried out over a 70 d growth period under conditions designed to ensure that shoots of mycorrhizal (M) and non-mycorrhizal (NM) plants were of similar nutrient status. C budgets did not differ on day 24 but by day 42 M plants had a significantly higher rate of photosynthesis than their NM counterparts when expressed on a whole shoot basis or unit dry weight basis. As both sets of plants were of the same size it was concluded that this greater C gain was the result of increased sink strength provided by the mycorrhizal fungus. By day 53 M plants had become larger than their uncolonized counterparts and a sink-induced stimulation in the rate of photosynthesis was no longer apparent. M plants had higher root sucrose, glucose and fructose pools from day 24. Analyses suggested that these sugars were utilized for trehalose and lipid synthesis, for the production of the large extramatrical mycelium and for the support of the respiratory demands of the M root system. Increased C allocation to roots of M plants was associated with a stimulation of the activities of cell wall and cytoplasmic invertases and of sucrose synthase in roots colonized by VA fungi. Such increases in enzyme activity may provide the mechanism enabling increased partitioning of carbohydrate both to the M root system and the fungal symbiont.     

Longevity of white clover (Trifolium repens) leaves, stolons and roots, and consequences for nitrogen dynamics under northern temperate climatic conditions

BACKGROUND AND AIMS: White clover (Trifolium repens) is, due to nitrogen (N) fixation, important to the N dynamics of several northern temperate agroecosystems. This study aimed at monitoring growth and death of major white clover plant organs to assess their potential contribution to within-season N input and risk of off-season N losses. METHODS: White clover ('Snowy') was studied in a plot and root window experiment in southeast Norway (60 degrees 42'N, 10 degrees 51'E). Leaves, stolons and roots were tagged for lifespan measurement in harvested and unharvested stands during two experimental years. The availability of soil inorganic N was measured by plant root simulator (PRS) probes. KEY RESULTS: The longevity of leaves and petioles ranged from 21 to 86 d (mean = 59 d), of main stolon sections from 111 to over 677 d (mean = 411 d) and of roots from 27 to 621 d (mean = 290 d). About 60 % of the leaves produced had turned over by the end of the growing season and another 30 % had died or disappeared by the subsequent spring. Harvesting reduced the longevity of stolons and increased plant fragmentation, but did not decrease leaf or root lifespan or increase soil N availability. From the plant organ turnover data, it was estimated that the gross N input to the soil-plant system from white clover in pure stand during two growing seasons corresponded to a 2.5-fold increase over the total N in harvestable shoots. CONCLUSIONS: The short lifespan and poor over-wintering of leaves showed their potential importance as a nitrogen source in the soil-plant ecosystem but also their potential contribution to the risk of off-season N losses.     

Distribution of mineral nutrient from nodal roots of Trifolium repens: Genotypic variation in intra-plant allocation of 32P and 45Ca

To assess genotypic variability in nutrient supply of shoot branches, the distribution of ³²P and ⁴⁵Ca exported from a source nodal root (24-h uptake period) was measured within a genotype of a large-leaved (Kopu) and a small-leaved (Tahora) cultivar of Trifolium repens. Source-sink relationships of plants were modified by root severance, defoliation, and shade treatments. In control plants of both genotypes distribution of ³²P and ⁴⁵Ca closely followed the pathways that could be predicted from the known phyllotactic constraints on the vascular system. As such there was little allocation of radioisotopes (3.1% and 2.5% of exported ³²P and ⁴⁵Ca, respectively) from the source root to branches on the apposite side of the parent axis (far-side branches). However, genotypic differences in nutrient allocation were apparent, when treatments were imposed to alter intra-plant source-sink relationships. In the large-leaved genotype, the imposed treatments had minor effects on the allocation to far-side branches: whereas, in the small-leaved genotype, root severance and defoliation treatments increased lateral transport to far-side branches to 30% (³²P) and 10% (⁴⁵Ca) of exported radioisotopes. Genotypes with low (8–9) and high (12–13) numbers of vascular bundles were selected from within the large-leaved cultivar. Distribution of ³²P was then measured after plants had been pre-treated by removal of all far-side roots two days prior to labelling. Genotypes with low vascular bundle number allocated 20% and those with high vascular bundle number 3.2% of exported ³²P to far-side branches. It was concluded (1) that genotypic variation exists within T. repens for potential to alter intra-plant allocation of mineral nutrients, in response to treatments that modify source-sink relationships within plants; and (2) that this variation is correlated with differences among genotypes in the organisation of the vasculature of their stolons.     

温度胁迫对白车轴草水浸提液化感作用的影响

采用蚕豆根尖细胞微核试验和染色体畸变试验,研究了温度胁迫下白车轴草水浸提液对蚕豆根尖细胞的化感作用。结果表明:不同温度(5℃、25℃和35℃)处理后的白车轴草的水浸提液对蚕豆根尖细胞有丝分裂指数的影响为低浓度促进,高浓度抑制。在白车轴草水浸提液的作用下,蚕豆根尖细胞内出现了多种染色体畸变,如微核、染色体断片、染色体桥、落后染色体等,对微核率和染色体畸变率的诱导效应表现为随水浸提液浓度的增大而增大。温度胁迫后的化感作用明显大于胁迫前,其化感效应表现为低温(5℃)下白车轴草水浸提液高温(35℃)常温(25℃)白车轴草水浸提液。推测原因可能是温度胁迫改变和增加了化感物质的释放。     

Short-term effects of manipulating the source:sink ratio of white clover (Trifolium repens) plants on export of carbon from, and morphology of, developing leaves

The source:sink ratio of clonal white clover ( Trifolium repens L.) plants was manipulated by shading or removing leaves, and the consequences for carbon export from, and for the weight, area and net photosynthesis of, developing leaves were determined. When treatments were imposed just before young leaves usually change from C sinks to C sources, no effect on the point at which the sink-to-source transition occurred was observed, Leaves exported a similar proportion of the C they fixed, irrespective of stresses imposed upon the rest of the plant. However, differences in the destination of exported C were observed. More C moved to the stolon apex, and less to the stolon tissue itself, from leaves at Carlson stage 0.8 (leaflets about 60% unfolded) when mature leaves were removed or shaded. When 2 out of 3 mature leaves were removed from a stolon, short-term responses such as a 10% increase in net photosynthesis of the residual mature leaf, and greater export of C from this leaf to the apex, partially compensated very young leaves for loss of C supply. The result was that, when these young leaves were fully unfolded, they had similar surface area to those which had developed on undefoliated plants, but weighed nearly 20% less. Thus the immediate response to defoliation was an increase in the speeific leaf area [cm² (g of dry weight)⁻¹] of new leaves, and the assimilation rate (net photosynthesis×area) of these leaves remained unchanged.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

Response to soil aluminium of two white clover (Trifolium repens L.) genotypes

An aluminium (Al) tolerant genotype of white clover was compared with an Al susceptible genotype in artificial soil profiles in which exchangeable Al increased with depth. The tolerant genotype had a greater proportion of its root mass deeper in the soil than the susceptible genotype. Nitrogenase activity showed a similar pattern. Shoot Al concentration did not vary between the genotypes but root Al in the susceptible line was twice that in the tolerant genotype. Plant potassium content in the susceptible line was relatively less, probably in response to higher aluminium content.