Plant litter feedback and population dynamics in an annual plant,<Emphasis Type="Italic"> Cardamine pensylvanica</Emphasis>

The presence of litter has the potential to alter the population dynamics of plants. In this paper, we explore the effects of litter on population dynamics using a simple experimental laboratory system with populations of the annual crucifer, Cardamine pensylvanica. Using a factorial experiment with four densities and three litter levels, we determined the effect of litter on biomass and plant fecundity, and the life stages responsible for these changes in yield. Although litter had significant effects on seed germination and on seedling survivorship, we show, using a population dynamics model, that these effects were not demographically significant. Rather, the potential effect of litter on population dynamics resulted almost entirely from its effect on biomass. Persistent litter suppressed plant biomass and apparently removed the direct density effect present in the absence of litter. Thus, litter changed the shape of the recruitment curve from slightly humped to asymptotic. In addition to changing the shape of the recruitment curve, litter reduced the carrying capacity of the populations. Thus, the population dynamics model indicated that not all statistically significant responses were dynamically significant. Given the potential complexity of litter effects, simple population models provide a powerful tool for understanding the potential consequences of short-term responses. Received: 8 September 1999 / Accepted: 5 April 2000     

Effect of nutrient level on competition intensity in the field for three coexisting grass species

A field experiment encompassing both neighbour- and nutrient-manipulations was conducted in a nutrient-impoverished old-field habitat to investigate how the intensity of plant competition was affected by soil nutrient level. Three perennial grasses were used as target species: Agropyron repens, Poa pratensis and Phleum pratense. Neighbour manipulations involved the removal (through herbicide application) of all neighbouring vegetation within a 20 cm or 40 cm radius around target plants. Target performance was measured under five levels of added nutrients (N-P-K) in both the neighbour-removal plots and in non-removal (control) plots. Both neighbour and nutrient manipulations had a highly significant effect on both biomass and tiller production but the interaction between these treatments was generally insignificant. Below-ground/above-ground biomass quotient was affected only by neighbour manipulations and was greatest in the control plots (with no neighbours removed) for all three species. The suppressive effect of neighbours was not markedly affected by nutrient level. However, yield suppression showed a significant decreasing trend with increasing nutrient level for biomass production in Agropyron and an increasing trend for tiller production in Phleum. For Poa, there was no trend in the intensity of competition across nutrient level. The results suggest that the general intensity of competition within this community neither increases nor decreases with increasing nutrient level. Rather, coexisting species appear to respond individually in terms of the intensity of competition that they experience. These results conflict with predictions from the triangular C-S-R model of plant strategies. However, they are consistent with a recently modified ‘habitat templet’ model for vegetation.     

UV-microscopic studies on the vacuolar changes caused by the flavone “aglycone” isovitexin inSilene pratensis plants

Summary UV-microscopic and chromatographic studies have been performed on the variation in contents and configuration of the flavones present in epidermal cells of the petals, stem leaves, rosette leaves and cotyledons ofSilene pratensis plants. Most of the flavone contents is located in the vacuole of the upper epidermis cells, the concentration depending on the light intensity at which the plants were grown. In plants able to glycosylate isovitexin in the petals (genotypegG/. gl/gl fg/fg, accumulating isovitexin 7-O-glucoside) the vacuole is completely filled with the UV absorbing flavone. In plants which are unable to glycosylate isovitexin in their petals (genotypeg/g gl/gl fg/fg, accumulating only isovitexin) the upper epidermal cells of stem leaves and petals contain droplet like structures in their vacuoles. At high light intensities these structures increase in mass and become detectable in the visible light. These denser structures often condense to structures with radiating threads.As compared with the accumulation of isovitexin in upper epidermal cells of stem leaves and petals in genotypeg/g gl/gl fg/fg, the cotyledons and the rosette leaves contain two isovitexin glycosides. In the latter organs the upper epidermal cells are very similar to the upper epidermal cells fromgG/. gl/gl fg/fg plants, having a vacuole filled with UV absorbing material. It appears therefore that isovitexin itself causes the formation of the structurés in the cells. It was shown by varying the light intensity that a relative high concentration of isovitexin is necessary for the droplet like structures to appear. Still higher concentrations are needed for the formation of the structures with radiating threads. It is hypothesized that isovitexin interferes with the energy supply of the cells, which therefore are not able to maintain their turgor.     

Effects of Different Photoperiods and Chilling Treatments on Growth and Reproduction of Cardamine flexuosa With. and Its Ecological Implications

Abstract Phenotypic variability of Cardamine flexuosa (Cruciferae) was examined in response to different lengths of exposure to low temperature (5°C) at the juvenile stage, and to two photoperiod regimes (8 and 16 hrs day-lengths) in the subsequent growth period. The results indicated that this species had a facultative chilling or long-day requirement for flowering. The long-day and chilling treatments both caused an earlier onset of stem internode elongation. Longer chilling treatments reduced the number of nodes and increased internode length under both long and short photoperiod regimes. Prolonged chilling treatments followed by long-day photoperiod remarkably induced more numerous basal branches and inflorescences, as a result producing more siliques. Remarkable rosette leaves Were formed at the base of the main stem when partial chilling and the short-day treatment suppressed stem internode elongation. The responses to chilling and long-day treatment closely resembled the synchronized flowering of this species in spring in the field. Delayed flowering under the short-day treatment resembled size- or age-dependent flowering in late summer to autumn in the field populations.     

Impact of the field slug Deroceras reticulatum on establishment of ryegrass and white clover in mixed swards

The relationship between numbers of Deroceras reticulatum and the establishment of ryegrass cv. Melle and white clover cv. Milkanova was studied on a mini-plot experiment. Assessments 12 and 33 days after sowing showed that Deroceras reticulatum had a proportionally greater impact on survival and growth of clover than on ryegrass. Slugs had no apparent effect on numbers or growth of three species of broad-leaved weeds (Senecio vulgaris, Cardamine hirsuta and Epilobium sp.) which occurred in the field experiment. In pot experiments where seedlings of ryegrass together with one of three clover cultivars, with or without chickweed (Stellaria media), were exposed to Deroceras reticulatum, seedlings of white clover cv. Aran were less affected by slug grazing than cvs S184 or Milkanova. The presence of different clover cultivars did not significantly influence the impact of slugs on growth of ryegrass or chickweed, nor did the presence or absence of chickweed influence the effect of slugs on ryegrass and clover seedlings. Slugs had less impact on the growth of chickweed than on ryegrass or clover. Thus slug grazing would make clover less able to compete with chickweed as well as the other weeds found in the mini-plot experiment.     

Hybridization between tetraploid species of Alopecurus L. (Poaceae): chromosome behaviour of natural and artificial hybrids

The meiotic behaviour of plants from mixed populations of Alopecurus pratensis × A. geniculatus and of A. pratensis × A. arundinaceus ( 2n = 4×(= 28) is reported together with observations on some artificially produced hybrids. This meiotic behaviour is correlated with the degree of hybridity as shown by the hybrid index values of the plant. In A. pratensis × A. geniculatus hybrids there are marked differences between populations in the degree of meiotic disturbance and in one population there was an almost complete breakdown of meiosis. Alopecurus pratensis × A. arundinaceus hybrids show fairly regular meiotic pairing but in most plants there are a few univalent chromosomes at metaphase I. Artificially produced hybrids of A. pratensis × A. aequalis gave meiotic configurations that suggest that the genomes of the parent species are very similar. Taken together with the results from artificially produced hybrids of A. pratensis × A. geniculatus , it is suggested that bivalent-promoting mechanisms reduce multivalent formation both in the species and the hybrids. The interaction of different genotypes involved in the control of meiosis may account for the variation in meiotic behaviour in the different populations of hybrids. Pollen fertility is reduced in most populations of hybrids and likely to be an important factor permitting introgressive hybridization.     

新疆驼绒藜属（藜科）一些新分类群

报道了产于新疆的驼绒藜(Krascheninnikovia ceratoides(L) Guedenst)2个新变种及昆仑山驼绒藜(K.compacta(Losinsk.)Grub.)1个新变种：叶城驼绒藜（K.compacta var.yechengensis A L Fu.f.nov.）。每一新分类群均有插图。荒漠驼绒藜(Krascheninnikovia ceraroides var. deserticola(Losinsk.)G.Yang comb.nova)主要生于平原荒漠或低山区,常在下部分枝,形成垫状灌丛,叶片狭窄,披针形、狭椭圆形、狭长圆形,两面均被细绒毛。因而两面同色;草原驼绒藜(K.ceratoides var. pratensis(Losinsk.)Z Li comb.nova)主要生在山地草原,分枝也多在上部,叶上面无毛,下面疏毛,因而两面不同色;叶城驼绒藜(K.compacta var. yechengensis A L Fu var.nov.)的叶片跟博乐驼绒藜（变型）很近似,但雌花苞片为淡绿色,分离部分远长于连合部分,而甚易区别,也仅见于叶城昆仑山。     

农杆菌介导的单子叶植物早熟禾的转化

利用种子和胚分别在两种培养基K3和K5诱导产生了早熟禾(Poa pratensis L.)一个品种Mado的胚性愈伤组织.K3培养基含有10.0μmol/L的二氯苯氧乙酸(2,4-D)、0.5μmol/L的苄氨基嘌呤(BAP).K5培养基是K3另加0.5μmol/L的硫酸铜.光照条件为20～30 μmol.m-2.s、16 h光照、8 h黑暗.温度保持在24℃.用携有bar基因和gus基因的pDM805质粒转化的农杆菌AGL1对胚性愈伤组织进行转化.共得到4个转基因株系.影响转基因效率的主要因素有愈伤组织的胚性、光照条件、共转化时间、抗生素浓度、选择压力.本研究建立了单子叶早熟禾农杆菌介导的转基因方案.     

白三叶入侵对草地早熟禾功能性状及草坪质量的影响

【目的】白三叶入侵会挤占草坪草的生存空间,严重威胁草坪的质量和功能。明晰白三叶（Trifolium repens）入侵对草坪植物草地早熟禾（Poa pratensis）功能性状及草坪质量的影响,可深入了解白三叶入侵机制,为禾本科草坪的建植养护提供理论依据。【方法】以白三叶入侵草地早熟禾草坪为研究对象,采用样地调查方法,考察不同白三叶入侵程度下（对照及轻度、中度、重度入侵）草地早熟禾功能性状及草坪质量的差异特征,并通过隶属函数法综合评价各入侵程度下草坪质量。【结果】（1）随白三叶入侵程度的增加,草地早熟禾各器官碳含量、叶片建成成本及碳氮比、碳磷比和氮磷比逐渐降低,而比叶面积、各器官氮和磷含量增大。（2）同一入侵程度下,草地早熟禾各器官碳、氮和磷含量均表现为叶大于根和茎,碳氮比和碳磷比表现为根和茎大于叶,而其氮磷比表现为茎和叶大于根。（3）随白三叶入侵程度的增加,草地早熟禾草坪密度、质地、绿度指数和均一性降低,而地上生物量和地下生物量增大。（4）隶属函数法综合评价结果显示,草坪质量以白三叶未入侵时最优,重度入侵时最差,且叶碳含量对草坪质量的影响最大。【结论】白三叶入侵改变了草地早熟禾功能性状,降低了入侵区草坪质量,严重影响了草坪景观和功能。     

Flowering requirements of Scandinavian Festuca pratensis

Flowering requirements of three Scandinavian cultivars of Festuca pratensis Huds, have been studied in controlled environments. At 3 and 6°C, primary induction was independent of photoperiod, while short days (8 h) were more effective than long days (24 h) at higher temperatures. The critical temperature for induction was about 15°C in short days and about 12°C in long days. Saturation of induction required 18–20 weeks of exposure to optimal conditions. At temperatures below 12°C both induction and initiation of inflorescence primordia took place in long days, while a transition to long days was required for inflorescence initiation after primary induction in short days. A minimum of 8 long-day cycles were required for flowering of plants primary induced in short days and saturation of flowering required more than 16 cycles. The critical photoperiod for secondary induction was about 13 h. High temperature (21°C) had some devernalization effect in primary induced plants, suppressing flowering compared with 15°C.