增加水分与养分对克隆植物羊柴自然种群繁殖权衡的影响

 羊柴(Hedysarum laeve)是豆科多年生半灌木,在自然条件下可以同时进行有性繁殖和克隆繁殖。该文在野外条件下研究了不同水平的水分和养 分处理对羊柴种群的繁殖权衡的影响。结果表明,与对照相比,增加一定量的水分处理显著减少了花和荚果的生物量;显著增加了克隆分株枝 的生物量,显著减少了分株根茎的生物量, 但没有影响其它部分的生物量。增加一定量的水分会抑制有性繁殖,改变生物量对克隆繁殖分株各 部分的分配比例。与对照相比,增加一定量的养分能够促进有性繁殖,抑制克隆繁殖。     

Soil water content and patterns of allocation to below- and above-ground biomass in the sexes of the subdioecious plant Honckenya peploides

Background and aims Dioecious plants often show sex-specific differences in growth and biomass allocation. These differences have been explained as a consequence of the different reproductive functions performed by the sexes. Empirical evidence strongly supports a greater reproductive investment in females. Sex differences in allocation may determine the performance of each sex in different habitats and therefore might explain the spatial segregation of the sexes described in many dimorphic plants. Here, an investigation was made of the sexual dimorphism in seasonal patterns of biomass allocation in the subdioecious perennial herb Honckenya peploides, a species that grows in embryo dunes (i.e. the youngest coastal dune formation) and displays spatial segregation of the sexes at the studied site. The water content in the soil of the male- and female-plant habitats at different times throughout the season was also examined. Methods The seasonal patterns of soil-water availability and biomass allocation were compared in two consecutive years in male and female H. peploides plants by collecting soil and plant samples in natural populations. Vertical profiles of below-ground biomass and water content were studied by sampling soil in male- and female-plant habitats at different soil depths. Key Results The sexes of H. peploides differed in their seasonal patterns of biomass allocation to reproduction. Males invested twice as much in reproduction than females early in the season, but sexual differences became reversed as the season progressed. No differences were found in above-ground biomass between the sexes, but the allocation of biomass to below-ground structures varied differently in depth for males and females, with females usually having greater below-ground biomass than males. In addition, male and female plants of H. peploides had different water-content profiles in the soil where they were growing and, when differences existed (usually in the upper layers of the soil), the water content of the soil was higher for the female plants had than for the male plants. Conclusions Sex-differential timing of investment in reproduction and differential availability and use of resources from the soil (particularly water) are factors that probably offset the costs of reproduction in the above-ground growth in males and females of H. peploides. The results suggest that the patterns of spatial segregation of the sexes observed in H. peploides may contribute to maximize each sex's growth and reproduction.     

水深梯度对钝脊眼子菜生长和繁殖的影响

水深是影响湿地植物生长和分布的一个重要限制性因子,该研究以具有典型异型叶性的钝脊眼子菜(Potamogeton octandrus)为对象,通过分析浅水处理(10 cm和30 cm)和深水处理(50 cm和70 cm) 4个水深梯度下幼苗生长、生物量及繁殖策略等,探讨钝脊眼子菜在不同水深条件下的适应机制和表型可塑性。结果表明,钝脊眼子菜植株到达水面后出现异型叶,相对生长率显著降低,且与水深梯度呈正相关。钝脊眼子菜的株高随着水深的增加呈现爆发式的增长,10 cm水深的总茎长显著低于其他水深处理。水深对节间数也有显著性影响,其中,30 cm组处理节间数最多;而深水处理组的节间长和生物量均显著高于浅水处理组。分蘖数在4组处理之间均表现出显著性差异,随着水深的增加呈现显著性递减。生物量和地上生物量分配则随着水深增加而明显增加。水深处理对有性生殖指标有显著性影响,水深的增加抑制其有性繁殖。其中,10 cm条件下无花序形成,50 cm水深下的花粉量、P/O比和花序数显著高于其他处理组,且深水处理的结实数和结实率均显著高于30 cm组。这表明钝脊眼子菜可通过调整形态可塑性和生物量分配,并采取不同的繁殖策略,以达到对水深的最佳适应,其中最适水深生长范围在50 cm左右。     

根茎型植物扁秆荆三棱对光照强度和养分水平的生长响应及资源分配策略

自然界中光照和养分因子常存在时空变化,对植物造成选择压力。克隆植物可通过克隆生长和生物量分配的可塑性来适应环境变化。尽管一些研究关注了克隆植物对光照和养分因子的生长响应,但尚未深入全面了解克隆植物对光照和养分资源投资的分配策略。以根茎型草本克隆植物扁秆荆三棱(Bolboschoenus planiculmis)为研究对象,在温室实验中,将其独立分株种植于由2种光照强度(光照和遮阴)和4种养分水平(对照、低养分、中养分和高养分)交叉组成的8种处理组合中,研究了光照和养分对其生长繁殖及资源贮存策略的影响。结果表明,扁秆荆三棱的生长、无性繁殖及资源贮存性状均受到光照强度的显著影响,在遮阴条件下各生长繁殖性状指标被抑制。且构件的数目、长度等特征对养分差异的可塑性响应先于其生物量积累特征。在光照条件下,高养分处理的总生物量、叶片数、总根茎分株数、长根茎分株数、总根茎长、芽长度、芽数量等指标大于其他养分处理,而在遮阴条件下,其在不同养分处理间无显著差异,表明光照条件可影响养分对扁秆荆三棱可塑性的作用,且高营养水平不能补偿由于光照不足而导致的生长能力下降。光照强度显著影响了总根茎、总球茎及大、中、小球茎的生物量分配,遮阴条件下,总生物量减少了对地下部分根茎和球茎的分配,并将有限的生物量优先分配给小球茎。总根茎的生物量分配未对养分发生可塑性反应,而随着养分增加,总球茎分配下降,说明在养分受限的环境中球茎的贮存功能可缓冲资源缺乏对植物生长的影响。在相同条件下,根茎生物量对长根茎的分配显著大于短根茎,以保持较高的繁殖能力;而总球茎对有分株球茎的生物量分配小于无分株球茎,表明扁秆荆三棱总球茎对贮存功能的分配优先于繁殖功能。研究为进一步理解根茎型克隆植物对光强及基质养分环境变化的生态适应提供了依据。     

Allometry of Salsola collina in response to soil nutrients,water supply and population density

The allometry of greenhouse‐grown Salsola collina Pall. in response to variation in soil nutrient content, water supply and population density has been compared. The results showed that the biomass allocation was size‐dependent. Root, stem, leaf and reproductive allocation showed a ‘true’ plasticity in response to soil nutrient variation. At low soil nutrient content, plants tended to allocate more biomass to the development of reproductive organs than to stem and leaf, but root allocation was consistent due to a tradeoff between the effects of plant size and soil nutrient content. The plasticity of stem allocation and reproductive effort was ‘true’, while the plasticity of root allocation was ‘apparent’, but there was no plasticity for leaf allocation in response to soil water variation. At lower soil water content, plants tended to allocate more biomass to the stem than to development of reproductive organs. With the exception of ‘apparent’ plasticity of root allocation, no plasticity was detected in biomass allocation when population density was varied.     

Changes in reproductive investment with altitude in an alpine plant

Aims In perennial species, the allocation of resources to reproduction results in a reduction of allocation to vegetative growth and, therefore, impacts future reproductive success. As a consequence, variation in this trade-off is among the most important driving forces in the life-history evolution of perennial plants and can lead to locally adapted genotypes. In addition to genetic variation, phenotypic plasticity might also contribute to local adaptation of plants to local conditions by mediating changes in reproductive allocation. Knowledge on the importance of genetic and environmental effects on the trade-off between reproduction and vegetative growth is therefore essential to understand how plants may respond to environmental changes.Methods We conducted a transplant experiment along an altitudinal gradient from 425 to 1?921 m in the front range of the Western Alps of Switzerland to assess the influence of both altitudinal origin of populations and altitude of growing site on growth, reproductive investment and local adaptation in Poa alpina .Important findings In our study, the investment in reproduction increased with plant size. Plant growth and the relative importance of reproductive investment decreased in populations originating from higher altitudes compared to populations originating from lower altitudes. The changes in reproductive investment were mainly explained by differences in plant size. In contrast to genetic effects, phenotypic plasticity of all traits measured was low and not related to altitude. As a result, the population from the lowest altitude of origin performed best at all sites. Our results indicate that in P. alpina genetic differences in growth and reproductive investment are related to local conditions affecting growth, i.e. interspecific competition and soil moisture content.     

The effects of temporal variation in soil carbon inputs on resource allocation in an annual plant

Aims Resource allocation in plants can be strongly affected by competition. Besides plant–plant interactions, terrestrial plants compete with the soil bacterial community over nutrients. Since the bacterial communities cannot synthesize their own energy sources, they are dependent on external carbon sources. Unlike the effect of overall amounts of carbon (added to the soil) on plant performance, the effect of fine scale temporal variation in soil carbon inputs on the bacterial biomass and its cascading effects on plant growth are largely unknown. We hypothesize that continuous carbon supply (small temporal variance) will result in a relatively constant bacterial biomass that will effectively compete with plants for nutrients. On the other hand, carbon pulses (large temporal variance) are expected to cause oscillations in bacterial biomass, enabling plants temporal escape from competition and possibly enabling increased growth. We thus predicted that continuous carbon supply would increase root allocation at the expense of decreased reproductive output. We also expected this effect to be noticeable only when sufficient nutrients were present in the soil.Methods Wheat plants were grown for 64 days in pots containing either sterilized or inoculated soils, with or without slow-release fertilizer, subjected to one of the following six carbon treatments: daily (1.5mg glucose), every other day (3mg glucose), 4 days (6mg glucose), 8 days (12mg glucose), 16 days (24mg glucose) and no carbon control.Important findings Remarkably, carbon pulses (every 2–16 days) led to increased reproductive allocation at the expense of decreased root allocation in plants growing in inoculated soils. Consistent with our prediction, these effects were noticeable only when sufficient nutrients were present in the soil. Furthermore, soil inoculation in plants subjected to low nutrient availability resulted in decreased total plant biomass. We interpret this to mean that when the amount of available nutrients is low, these nutrients are mainly used by the bacterial community. Our results show that temporal variation in soil carbon inputs may play an important role in aboveground–belowground interactions, affecting plant resource allocation.     

根系隔离条件下的谷子亲缘识别

在根系隔离情况下, 通过研究邻株身份(亲缘株、非亲缘株、陌生株)及其与种植密度(高、低)和土壤养分水平(高、低)交互作用对谷子(Setaria italica)形态学特征和生物量分配的影响, 探索谷子地上部分是否能够识别亲缘邻株, 以及谷子的这种亲缘识别能力对环境因子如何响应。结果显示: 1)亲缘组谷子叶生物量分配显著降低, 茎秆显著增粗, 暗示着亲缘组谷子植株间减少竞争, 并增强对当地多风气候的适应。而非亲缘组谷子叶生物量分配显著增加, 表明非亲缘组谷子植株间竞争较强。2)与非亲缘组相比, 陌生组谷子种子生物量分配显著增加, 株高显著减少, 表明陌生组谷子植株通过不对称竞争(与邻株糜(Panicum miliaceum)植株相比, 株高显著增加), 进一步限制邻株(糜)生长, 从而增强竞争能力, 同时, 将更多的生物量投资于繁殖, 增加适合度。3)在高密度种植条件下, 谷子茎生物量和叶生物量分配在各邻株身份处理间无显著差异, 而在低密度种植条件下, 与非亲缘组相比, 亲缘组谷子茎生物量显著增加, 叶生物量分配显著减小; 随着种植间距的增大(种植密度减小), 亲缘组谷子叶生物量分配显著减少, 而非亲缘组和陌生组叶生物量分配在高、低种植密度条件下无显著差异。4)在低土壤养分条件下, 亲缘组和非亲缘组谷子叶生物量分配无显著差异, 前者穗长显著小于后者, 而在高土壤养分条件下, 亲缘组谷子叶生物量分配显著小于非亲缘组, 前者穗长显著大于后者。结果表明, 在根系隔离的情况下,谷子能够识别亲缘邻株, 且谷子地上部分竞争信号在亲缘识别过程中扮演重要角色。较低种植密度和较高土壤养分水平有利于谷子亲缘识别能力的表达。     

SEXUAL DIMORPHISM IN BIOMASS ALLOCATION AND CLONAL GROWTH OF XANTHOXYLUM AMERICANUM

Reproduction can have a high resource cost. It has been suggested that greater investments in sexual reproduction by female dioecious plants leads to a lower rate of vegetative growth in females than in males. In this study, we investigated sexual dimorphism in biomass allocation and genet growth of the dioecious clonal shrub, northern prickly ash (Xanthoxylum americanum). The allocation of biomass over the course of one growing season to reproductive tissue, leaves, and growth of aboveground first-year wood, was compared in 18 clones growing in fields and six clones in woods in southeastern Wisconsin during 1985 and 1986. In addition, the number of shoots per clone, and weight of nonfirst-year wood (accumulated biomass) above- and below-ground were estimated. In open field sites, male clones allocated more biomass to new wood and less to reproduction than females, although males allocated more to flowers alone. Accordingly, male clones had significantly more shoots and more accumulated biomass both above- and below-ground than female clones. In the woods, where fruit set was near zero, there were few significant differences between male and female clones in either biomass allocation or accumulated biomass. These results support the hypothesis that the high resource investment in fruit production by females reduces their vegetative growth relative to males.     

Kin recognition in Setaria italica under the condition of root segregation

Aims Kin recognition may play an important part in the performance and productivity of crop plants. However, so far, little is known about whether crop plants can recognize their kin neighbors. The aim of this study was to explore kin recognition in Setaria italica, and its responses to changes in environmental and biological conditions.Methods A field experiment was conducted in the suburb of Shanghai. Setaria italica grew with different neighbors (kin, non-kin and strangers), under the condition of root segregation and different plant densities (high and low) and soil nutrient levels (high and low), respectively. We investigated how neighbor identity and its interactions with plant density and soil nutrient level affected the morphology and biomass allocation of S. italica.Important findings Under the condition of root segregation, 1) Leaf biomass allocation and stem diameter of plants in the kin groups significantly decreased and increased, respectively, suggesting that plants of S. italica in the kin groups reduced inter-individual competition, and adapted to the local windy climate. 2) Compared with the non-kin groups, plants in the stranger groups significantly increased the biomass allocation to seeds, while plant height decreased significantly, suggesting that the plants of S. italica in the stranger groups may reduce the growth of their neighbors due to asymmetric competition (S. italica significantly increased height compared with the neighboring plants, Panicum miliaceum). Therefore, the S. italic plants in the stranger groups allocated more biomass to reproduction and increased fitness than those in non-kin groups. 3) Under the condition of high plant density, no significant differences were found in stem biomass and leaf biomass allocation of plants among different neighbor identity treatments. While under the condition of low plant density, compared with the non-kin groups, biomass allocation to stem and leaf in the kin groups significantly increased and decreased, respectively. As the plant density decreased, plants in the kin groups decreased leaf biomass allocation significantly, while plants in the non-kin and stranger groups did not show such a response. 4) Under the condition of low soil nutrient level, no significant difference was found in leaf biomass allocation between the kin and non-kin groups, while the ear length of plants in the kin groups decreased significantly. Under the condition of high soil nutrient level, the biomass allocated to leaves in the kin groups decreased significantly, while ear length increased significantly compared with the non-kin groups. Therefore, under the condition of root segregation, plants of S. italica showed the ability to recognize their kin neighbors, and the aboveground competitive cues may play important roles in the course of kin recognition in S. italica. Lower plant density and higher soil nutrient level may facilitate the ability of kin recognition in S. italica.     

柔毛淫羊藿生物量与生殖配置研究

探索了柔毛淫羊藿在有性繁殖时期不同生境和不同发育阶段的生殖配置,以探讨柔毛淫羊藿的生物量及生殖配置的内在规律.结果如下:随着生长季节的变化,柔毛淫羊藿总生物量呈现出低-高-低的趋势,柔毛淫羊藿生物量积累最快的时间为柔毛淫羊藿花期和果期.其中地上部分生物量在不断增加,地下部分生物量变化较小,从而根冠比在有性繁殖时期逐渐下...     

Different Responses of Sexual and Asexual Reproduction of Arundinella hirta to Flooding

The sexual and asexual reproductive features of the graminoid species Arundinella hirta growing at riversides of the Jialing River were analyzed.It was found that the total seed mass,seed number per plant,and sexual reproductive allocation of A.hirta decreased with decreasing bank elevation,and plants growing at the lowest elevations of banks subjected to intense flooding did not show sexual reproduction.The total plant biomass and the number of ramifications per plant increased with flooding intensity,which implies that,contrary to sexual reproduction,asexual reproduction of A.hirta was enhanced by flooding.     

Resource manipulation reveals flexible allocation rules to growth and reproduction in a Mediterranean evergreen oak

Aims In plants, resource allocation to growth and reproduction may depart from trade-off expectations if (i) investment in growth and reproduction relies on different resource pools, (ii) allocation to reproduction is dependent upon reaching some growth threshold or (iii) reproduction is developmentally linked to growth, both functions relying on the same resource pool. We examined the effects of enhanced resource level on patterns of resource allocation to growth and reproduction in holm oak (Quercus ilex sbsp. ballota), a Mediterranean evergreen tree.Methods In the experimental year (2003), we manipulated the amount of soil nutrients in autumn (to increase nutrient uptake during shoot elongation in the following spring) and soil water in summer (to increase water uptake during acorn growth). Indicators of growth and male and female reproduction were estimated in the pre-experimental (2002), experimental (2003) and post-experimental (2004) years.Important findings Fertilized trees produced significantly longer shoots, but the number of female flowers per shoot was not affected by treatments. The production of male catkins was also enhanced by fertilization. Irrigation did not affect the production of female flowers or abortion rates. Growth and female reproduction showed no consistent relationship in untreated trees, but resource addition elicited a growth-female reproduction trade-off in the experimental year. The sign of this significant relationship changed in the post-experimental year, indicating the existence of lagged effects of resource manipulation on acorn production. Overall, patterns of allocation to growth and reproduction varied as a function of sex, resource availability and year, a result consistent with extreme allocational plasticity in holm oak.     

Effects of water depth and substrate type on growth dynamics and biomass allocation of Oryza rufipogon

Oryzarufipogon is known as the ancestor of Asian cultivated rice (Oryzasativa) and the most important germplasm for rice improvement. In this study, we investigated its growth and biomass allocation in response to water depth and substrate type. The results demonstrated that individual plant had an average weight of 18.98 g, 9.36 g, and 10.59 g and produced 77.7 seeds, 35.5 seeds, and 41.6 seeds in deep-, fluctuated-, and shallow water, respectively. Root-to-shoot ratios were 0.37 in deep water, 0.61 in fluctuated water level, and 0.56 in shallow water, reflecting larger investment in roots under water-level fluctuation and shallow water conditions. However, biomass allocation for sexual reproduction was no significant different among the three water levels. In substrates experiment, total biomass and seed production were significantly different between sediment types, and the averages were 10.59 g, 3.56 g, and 0.76 g in biomass and 41.6 seeds, 13.8 seeds, and 0.5 seeds per plant in lake sediment, river sand and clay, respectively. The number of tillers was significantly fewer in sandy and clay substrates than in lake sediments. Individual plant in clay substrates was significantly shorter than those in sandy and in lake sediment substrates. Our study suggests that the plasticity in morphology and biomass allocation of O.rufipogon appears to be an adaptation to the variability in water level and substrate conditions.     

Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae)>

We examined sexual dimorphism in reproductive allocation and its effects on growth and subsequent reproduction in a natural population of the dioecious woody shrub Lindera benzoin (L.) Blume. In addition to comparing natural patterns of growth and reproduction in a marked population of 251 females and 87 males, we experimentally examined the effect of reduced reproduction on future growth and reproduction in female plants and examined sexual dimorphism in carbon/nutrient balance. Our results suggest that females of L. benzoin bear greater reproductive costs in terms of both biomass and minerals. These costs were measurable in terms of current biomass and allocation of nitrogen to reproduction, as well as subsequent growth, reproduction, and tissue carbon/nutrient balance. Based upon the results of a fruit-thinning experiment and path analysis, fruit production in 1991 appeared to have direct negative effects on fruit production in 1992, an effect that was not necessarily mediated through effects on plant growth. We discuss our results in the context of other intrinsic and extrinsic factors that can influence growth, reproductive costs, and mortality in this species.     

Effects of water depth and substrate type on growth dynamics and biomass allocation of Oryza rufipogon

Oryzarufipogon is known as the ancestor of Asian cultivated rice (Oryzasativa) and the most important germplasm for rice improvement. In this study, we investigated its growth and biomass allocation in response to water depth and substrate type. The results demonstrated that individual plant had an average weight of 18.98 g, 9.36 g, and 10.59 g and produced 77.7 seeds, 35.5 seeds, and 41.6 seeds in deep-, fluctuated-, and shallow water, respectively. Root-to-shoot ratios were 0.37 in deep water, 0.61 in fluctuated water level, and 0.56 in shallow water, reflecting larger investment in roots under water-level fluctuation and shallow water conditions. However, biomass allocation for sexual reproduction was no significant different among the three water levels. In substrates experiment, total biomass and seed production were significantly different between sediment types, and the averages were 10.59 g, 3.56 g, and 0.76 g in biomass and 41.6 seeds, 13.8 seeds, and 0.5 seeds per plant in lake sediment, river sand and clay, respectively. The number of tillers was significantly fewer in sandy and clay substrates than in lake sediments. Individual plant in clay substrates was significantly shorter than those in sandy and in lake sediment substrates. Our study suggests that the plasticity in morphology and biomass allocation of O.rufipogon appears to be an adaptation to the variability in water level and substrate conditions.     

不同高程短尖苔草对水位变化的生长及繁殖响应

在淡水湿地生态系统中,水位通常是制约植被生长和繁殖动态的关键因素,进而对物种组成、群落演替和植被分布格局产生决定性影响。无性繁殖是洞庭湖湿地克隆植物适应环境胁迫的重要策略之一,以洞庭湖湿地典型克隆植物-短尖苔草(Carex brevicuspis C.B.Clarke)为对象,研究了不同分布高程(23.7 m和25.8 m)的植物对水位变化(0 cm,-15 cm,-30 cm)的生长和繁殖特征响应。结果表明:水位变化对不同分布高程分布短尖苔草的生长和克隆繁殖特征均产生显著影响(P0.05)。对高程区的短尖苔草而言,克隆繁殖特征如分株数、分株总生物量、芽数和芽生物量随水位的降低而增加,而对生长特征(株高及总生物量)无显著影响(P0.05),表明适当干旱有利于高程区苔草的克隆繁殖。对于低程区分布短尖苔草而言,水位变化对其生长特征有显著影响(P0.05),如株高和总生物量随着水位的降低而增加;分株数和总芽生物量等克隆繁殖特征则随水位的降低而减少,而水位对低程区短尖苔草的分株总生物量和总芽数影响不显著(P0.05)。因此,短尖苔草的克隆繁殖特征除受到水位的影响外,还受其分布高程的影响。可见,同一种短尖苔草因长期适应于不同生境而对相同的环境胁迫表现出了不同的生长繁殖策略,     

Ecological analysis of the vegetation in a summer-drained fishpond

Distributions of individual species and plant communities growing on the bottom of a medium-sized, 0.35 km², Starý Vdovec fishpond near T?eboň, South Bohemia, were investigated during the summer-drainage of the pond. The community characteristics (phytocenological relevés, cover assessed by point-quadrat method, biomass and production, amounts of nutrients in biomass, species diversity, interspecies associations) as well as environmental characteristics (depth of pond sediment, position of water table, mineral nutrient concentrations and contents in the soil, soil pH) were evaluated. The sediment depth and position of water level were distinguished as the most important environmental factors; other characteristics of both the vegetation and environment have been related to their gradients. The results were discussed and related, above all, to the ecological properties of the constituent species as well as to pond management.     

Environmental constraints on the growth,photosynthesis and reproductive development of Dryas octopetala at a high Arctic polar semi-desert,Svalbard

Opportunities exist in high Arctic polar semidesert communities for colonisation of unvegetated ground by long-lived clonal plants such as Dryas octopetala. This can be achieved by lateral spread of vegetative ramets, or by sexual reproduction and seedling recruitment. The objectives of this study were (1) to determine whether these two means of proliferation show differential sensitivity to contrasting components of the abiotic environment (temperature, soil nutrient and water availability) and (2) to evaluate the potential for D. octopetala to respond to climate change by an increase in cover and biomass in polar semi-desert communities. Factorial environmental manipulations of growing season temperature, soil nutrient and water status were conducted over 3 years at a polar semi-desert community in Svalbard, Norway (78°56.12N, 11°50.4E) and both clonal and sexual reproductive performance, together with instantaneous net photosynthesis (P_n), were recorded during the third season (1993). D. octopetala capitalised rapidly on an amelioration in the availability of inorganic nutrients (N, P and K) by an expansion in leaf area and biomass supported by increased P_n per unit leaf weight, and by apparent luxury uptake of nutrients (particularly P). Several facets of sexual reproductive development and seed viability were markedly improved by elevated temperatures or soil nutrient availability. Thus although D. octopetala is a long-lived clonal plant, with many traits characteristic of stress resistance syndrome, it showed considerable phenotypic plasticity in response to environmental manipulations. The results support the hypothesis that clonal growth confers survival potential during unfavourable years, together with the ability to capitalise on nutrient flushes and recycle nutrients internally. Continued investment in sexual reproduction ensures that seed setting is successful during favourable years, even if these occur infrequently. Cimate warming in the high Arctic could thus result in changes in the cover, biomass and the relative significance of clonal versus sexual proliferation of D. octopetala (and thus the genetic diversity of the population) but the long-term responses will probably be constrained by lack of available nutrients.     

Density-dependent reproductive and vegetative allocation in the aquatic plant Pistia stratiotes (Araceae)

Pistia stratiotes is an aquatic macrophyte that grows in temporary-ponds in the southern Pantanal, Brazil. It reproduces both sexually and asexually and is usually observed forming dense mats on the water surface, a condition favored by the plant's vegetative reproduction coupled with an ability for rapid growth. In this study we examined the effect of densely crowded conditions on the production of reproductive and vegetative structures. In addition, we verified whether there is a trade-off between clonal growth and investment in sexual reproductive structures, and whether there is an allocation pattern with plant size. Individual plant biomass and the number of the rosettes producing sexual reproductive structures and vegetative growth structures both increased with density. Increase in plant size resulted in increased proportional allocation to sexual reproductive structures and vegetative growth structures. Allocation of biomass to reproduction did not occur at the expense of clonal growth. Thus, the density response appears as a increase of rosettes producing sexual reproductive structures and vegetative growth structures. Therefore, long leaves and stolons may be adaptive under densely crowded conditions where competition for light is intense. An important aspect in the study of trade-offs is the size-dependency of the allocation patterns .Usually, larger plants produce more biomass. Therefore, larger plants can allocate more biomass to both vegetative and sexual reproduction than smaller plants and thus show a positive correlation between both traits rather than the expected negative one.