A comprehensive study of plant density consequences on nitrogen uptake dynamics of maize plants from vegetative to reproductive stages

Nitrogen (N) use efficiency (NUE), defined as grain produced per unit of fertilizer N applied, is difficult to predict for specific maize (Zea mays L.) genotypes and environments because of possible significant interactions between different management practices (e.g., plant density and N fertilization rate or timing). The main research objective of this study was to utilize a quantitative framework to better understand the physiological mechanisms that govern N dynamics in maize plants at varying plant densities and N rates. Paired near-isogenic hybrids [i.e., with/without transgenic corn rootworm (Diabrotica sp.) resistance] were grown at two locations to investigate the individual and interacting effects of plant density (low—54,000; medium—79,000; and high—104,000 pl ha⁻¹) and sidedress N fertilization rate (low—0; medium—165; and high—330 kg N ha⁻¹) on maize NUE and associated physiological responses. Total aboveground biomass (per unit area basis) was fractionated and both dry matter and N uptake were measured at four developmental stages (V14, R1, R3 and R6). Both plant density and N rate affected growth parameters and grain yield in this study, but hybrid effects were negligible. As expected, total aboveground biomass and N content were highly correlated at the V14 stage. However, biomass gain was not the only factor driving vegetative N uptake, for although N-fertilized maize exhibited higher shoot N concentrations than N-unfertilized maize, the former and latter had similar total aboveground biomass at V14. At the R1 stage, both plant density and N rate strongly impacted the ratio of total aboveground N content to green leaf area index (LAI), with the ratio declining with increases in plant density and decreases in N rate. Higher plant densities substantially increased pre-silking N uptake, but had relatively minor impact on post-silking N uptake for hybrids at both locations. Treatment differences for grain yield were more strongly associated with differences in R6 total biomass than in harvest index (HI) (for which values never exceeded 0.54). Total aboveground biomass accumulated between R1 and R6 rose with increasing plant density and N rate, a phenomenon that was positively associated with greater crop growth rate (CGR) and nitrogen uptake rate (NUR) during the critical period bracketing silking. Average NUE was similar at both locations. Higher plant densities increased NUE for both medium and high N rates, but only when plant density positively influenced both the N recovery efficiency (NRE) and N internal efficiency (NIE) of maize plants. Thus plant density-driven increases in N uptake by shoot and/or ear components were not enough, by themselves, to increase NUE.     

Effects of plant density and nitrogen and potassium fertilization on cotton yield and uptake of major nutrients in two fields with varying fertility

As the most important cultural practices for cotton production, the single effects of plant density and [nitrogen (N) and potassium (K)] fertilization on yield and yield components are well documented but their combined effects on Bt cotton are poorly understood. Using a split–split plot design with four replications, we conducted a two-year field experiment in two fields, one with lower fertility and the other with higher fertility, in the Yellow River Valley of China. The aim was to evaluate both the individual and combined effects of plant density and nitrogen and potassium fertilization on yield, yield components and uptake of major nutrients. The main plots were assigned to plant density (4.5 and 7.5 plants/m²), while nitrogen (0 and 240 kg N/ha) and potassium fertilization (0 and 150 kg K/ha) were assigned to the sub- and sub–subplots. Lint yield was improved with high plant density (7.5 plants/m²) in the lower fertility field, particularly without N and K application, but not in the higher fertility field. Nitrogen or K application also increased lint yield, and a combination of high plant density, N and K application further improved lint yield in the lower fertility field, while only K application increased lint yield in the higher fertility field. Lint percentage was not affected by any of the variables studied. Thus, the yield increase due to plant density, fertilization or their combinations was attributed to increases in boll number or boll weight. The ratio of seed cotton to stalk (RSS) was linearly correlated with harvest index, and thus can be a simple indicator of dry matter allocation to reproductive structures. Increased yield due to plant density and fertilization was mainly attributed to the enhanced biological yield in the lower fertility field, while the yield increase due to K fertilization was mainly due to increased RSS in the higher fertility field. The plants used approximately equal N and P to produce 100 kg lint in both fields, but the uptake of K to produce 100 kg lint in the higher fertility field was about 21% more than that in the lower fertility field. Ratios of N:P:K were 1:0.159:0.604 in the lower fertility field and 1:0.159:0.734 in higher fertility field. This study suggests that K fertilization was extremely important for maintaining high yield, although luxury consumption occurred in the higher fertility field; N was applied more than required in the highly fertile field, and increased plant density would be beneficial to cotton yield in the lower fertility field.     

The type of relationship between plant density and reproductive yield of cotton in a subtropical environment

The type of relationship between yield of seed-cotton and plant populations ranging from 1 to 21 plants per m², was investigated in several cultivars of theGossypium hirsutum L. andG. barbadense L. species, under irrigated and dryland conditions over several seasons, localities and dates of sowingThe simple and quadratic equations and the simplified exponential reciprocal expression, as proposed by Holliday and by Bleasdale, respectively, similarly and very adequately described the investigated relationship at the plant level over the whole range of populations tested. The -constant of Bleasdale's equation exhibited a high degree of cultivar specificity. In the dryland experiments its intercultivar values were linearly positively correlated with estimated maximum yields of seed-cotton per unit area.At the unit area level the estimated relationship was of two types: asymptotic and parabolic. It actually was dynamic in type and assumed transient forms throughout the boll-opening period, generally being parabolic at first and asymptotic at later stages. The ultimate response type exhibited a great intercultivar variability and cultivar constancy, being asymptotic in Acala 4–42 and Acala 1517 C and parabolic in Coker 100 W and in two Pima-type cultivars. Environmental growth factors enhancing prolific vegetative growth during the reproductive phase deviated from a specific asymptotic response pattern to a parabolic type.The estimated optimum plant density of the parabolic response types ranged between 4.0 and 5.7 plants per m². This range was extended by an average of 0.7 plants per m² when 5% of the maximum yield, estimated for asymptotic types, was discounted. The top estimated maximum yields were 1940 and 4600 kg seed-cotton per ha in the dryland and irrigated environments, respectively.

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Zusammenfassung Es wurde die Beziehung zwischen dem Ertrag bei Baumwolle und der Bestandsdichte — zwischen 1 bis 21 Pflanzen pro m² — an verschiedenen Sorten vonGossypium hirsu tum L. undG. barbadense L. in Trockengebieten mit und ohne Bewässerung über mehrere Vegetationszeiten, an verschiedenen Standorten und bei unterschiedlichen Aussaatterminen untersucht.Durch die einfache und quadratische sowie die vereinfachte exponentiel-reziproke Gleichung, wie von Holliday bzw. von Bleasdale vorgeschlagen, werden auf sehr einfache und ausreichende Weise die untersuchten Beziehungen im Bereich der Einzelpflanze innerhalb der geprüften Population beschrieben. Die -Konstante der Bleasdalschen Gleichung weist eine hohe Sortenspezifität auf. Bei den Dürre-Versuchen bestand bei den Werten zwischen den Sorten eine lineare positive Korrelation mit den errechneten Höchsterträgen der Baumwollsamen pro Flächeneinheit.Bezogen auf die Flächeneinheit waren die Beziehungen von zweierlei Typus: asymptotisch und parabolisch. Tatsächlich waren sie dynamischer Natur und lassen während der Kapselöffnungsperiode Übergangs-Formen vermuten; im allgemeinen waren sie zunächst parabolisch und in späteren Stadien asymptotisch. Im Endergebnis stellte sich zwischen den Sorten eine große Variabilität und innerhalb der Sorten eine Stabilität heraus. Diese war asymptotisch bei Acala 4–42 und Acala 1517 C und parabolisch bei Coker 100 W und bei zwei Pima-Typ-Sorten. Umweltbedingungen jedoch, die das vegetative Wachstum währed der reproduktiven Phase begünstigen, wandeln eine typisch asymptotische Beziehung in eine parabolische um.Der errechnete optimale Pflanzenbestand der parabolischen Beziehungstypen schwankte zwischen 4,0 und 5,7 Pflanzen pro m². Dieser Bereich erweitert sich um durchschnittlich 0.7 Pflanzen pro m², wenn 5% des errechneten Höchstertrages für die asymptotischen Beziehungstypen abgezogen werden. Der höchste errechnete Maximal ertrag von Baumwollsamenfasern war 1940 kg/ha für die Dürre- und 4600 kg/ha für die bewässerten Versuche.

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Résumé Le type de relation entre le rendement en coton-graine et la densité de population fut étudié pour des populations variant entre 1 et 21 plantes per m², sur plusieurs variétés de coton deGossypium hirsutum L. etG. barbadense L., en culture irriguée et en culture sèche, pendant plusieurs saisons en différentes localités et dates de semis.L'équation exponentielle simplifiée ainsi que l'équation reciproque simple et quadratique proposées par Bleasdale et par Holliday, respectivement, décrivent convenablement les relations étudiées pour l'ensemble des densités testées. La constante de l'équation de Bleasdale a montré une forte spécificité varietale. Dans les essais en culture sèche, il y avait une corrélation lineaire positive entre les valeurs intervariétales et les rendement maximum respectifs, éstimés en coton-graine par unité de surface.Au niveau de l'unité de surface, les relations estimées étaient de deux types: asymptotique et parabolique. En fait elles étaient de type dynamique et prenaient des formes transitoires pendant la période d'ouverture des capsules. En général, elles étaient de type parabolique au début et asymptotique plus tard. Le type final de réponse a montré une grande variabilité intervariétale et une stabilité variétale; il était asymptotique pour les variétés Acala 4–42 et Acala 1517C et parabolique pour Coker 100 W et deux variétés de type Pima. Les facteurs de croissance qui ont induit une végétation prolifique pendant le stade de reproduction, ont fait dévier un réponse de type asymptotique en type parabolique.La densité optimale de plantes éstimée pour les résponses de type parabolique variait entre 4.0 – 5.7 plantes par m². Ces valeurs étaient accrues de 0.7 plantes par m² lorsqu'on tenait compte des 5% du rendement maximum éstimé pour les types asymptotiques. Les estimations les plus élevées des rendements maximum en cotongraine se chiffraient a 1940 kg et 4600 kg par ha, pour la culture sèche et irriguée, respectivement.

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Contribution from the Agricultural Research Organization, The Volcany Center, Bet Dagan, Israel. 1972 Series, No. 2242-E.     

Changes in plant population density, composition and sward structure of a hill pasture during a pastoral fallow

Appropriate pre-sowing methods for the introduction of improved forage legume and grass germplasm are an important issue for hill pasture improvement in New Zealand. A pastoral fallow, which involves not defoliating pasture for a period generally from late spring/early summer to autumn, could create a potentially favourable environment for introducing improved germplasm. A field study was conducted on two aspects (shady and sunny) of moist, low-fertility hill country with or without added fertilizer (phosphorus and sulphur) in the southern North Island of New Zealand, to investigate the changes in plant population density and sward structure during a full or partial pastoral fallow, compared with a rotationally grazed pasture. A 7-month (October to May) pastoral fallow dramatically decreased the densities of grass tillers by 72% (P < 0·01), white clover (Trifolium repens L.) growing points by 87% (P < 0·01) and other species by 87% (P < 0·05). The decline in tiller density by pastoral fallow was enhanced on the shady aspect. Fertilizer application increased white clover growing-point density on the shady aspect (P < 0·05) and grass tiller density on the sunny aspect (P < 0·05). Decreased plant density during pastoral fallowing was attributed to aboveground biomass accumulation, which altered sward structure, leading to interplant competition and mortality by self-thinning and completion of the life cycle of some matured plants. The plant size-density relationship during pastoral fallowing in this mixed-species sward followed the serf-thinning rule, particularly when the calculation was based on all plant species rather than grass alone. There was no significant (P > 0·05) difference in final plant population density between the 7-month pastoral fallow and a shorter term (October to December) pastoral fallow. It is concluded that pastoral fallowing effectively reduced the plant population density and altered sward structure of a hill pasture. Such changes create a more favourable environment for the introduction of improved forage species.     

Effects of streamline complexity on the relationships between urban land use and ecological communities in streams

Urban land use can adversely affect the water quality of adjacent streams through interactions at the edges of the two ecosystems. From a landscape ecological perspective, edges control the flow rate of materials between two adjacent systems. Based on the rationale that the streamline complexity formed between land uses and water bodies may function in this manner, we investigated the potential role of streamline complexity in the relationship between stream ecological communities and urban land use in Korea. Various indices of biological indicators including the diatom assemblage index for organic pollution, the trophic diatom index, the Korean saprobic index, and the index of biotic integrity were applied at 80 sampling sites in the Nakdong River system. We computed the fractal dimensions of the streamlines within 1-km buffers and the proportion of urban land use in the watershed within 5-km buffers around the sampling sites. A moderation model was adopted to investigate the role of streamline complexity in the relationship between urban land use and the biological indices of ecological communities. A comparison between a simple geometry regression model and complex geometry models indicated that streamline complexity may moderate the adverse impacts of urban land use on ecological communities in streams. The moderating effect of streamline complexity was particularly significant for assemblages of macroinvertebrates and fishes. Therefore, to enhance the ecological integrity of streams, we strongly recommend considering streamline complexity when restoring channelized streams in developed areas.     

氮肥运筹对鄂南直播稻生长、产量及氮素吸收的影响

采用田间试验,研究了氮肥运筹对直播稻生长、产量及氮素吸收的影响,以期为氮肥的合理施用提供依据。结果表明,施用氮肥增产效果明显,与不施氮处理(T1)相比,平均增产1 010 kg/hm2,增产率17.0%,其中氮肥分3次施用的处理增产效果最佳,增产1 468 kg/hm2,增产率24.7%;氮肥的施用能够显著增加水稻吸氮量,各施氮处理在苗期、分蘖末期、齐穗期和收获期吸氮量与T1相比分别增加34%、125%、80%和39%;氮肥的合理运筹显著提高了氮肥吸收利用率,与氮肥全部作基肥处理相比,氮肥50%作基肥、30%作苗肥、20%作分蘖肥(T4)和氮肥35%作基肥、30%作苗肥、15%作分蘖肥、20%作穗肥(T5)分别提高了22个和28个百分点。综合分析认为,本试验条件下,T4处理的氮肥运筹效果最好,可以在鄂南直播稻生产上推广应用。     

The voluntary intake and in vivo digestibility of herbage harvested from indigenous hill plant communities

Voluntary intake and in vivo digestibility of herbage harvested from five indigenous communities– Agrostis-Festuca, Nardus and Molinia grasslands and Eriophorum vaginatum- and Trichophorum caespitosum -dominated varieties of blanket bog–and also from sown Lolium perenne and Trifolium repens swards were measured over two years. First growths of each sward were harvested at three stages of maturity and stored at — 20°C before feeding. Voluntary intake was measured while feeding ad libitum to 15% excess of the intake on the previous day and digestibility when feeding at 80% of voluntary intake.
There was a common positive relationship (r²= 0·86***) between intake (adjusted for year of feeding, range 72·8–16·7 g organic matter (kg LW)^0·75 d^-1) and organic matter digestibility (range 0·796–0·371), covering Lolium perenne and most of the indigenous communities. However, there were significant differences between the regressions of intake on the concentration of neutral detergent fibre for ryegrass and the indigenous communities, and also between the regressions for individual communities.
The results demonstrated the general value of organic matter digestibility as a predictor of intake potential, and the high digestibility and intake potential of immature vegetation from the grass and grass heath communities.     

Optimum plant density of Digitaria eriantha for herbage accumulation and hydrological performance in a summer dominant rainfall zone

In the frost‐prone, summer dominant rainfall zone of northern NSW and southern Queensland, sown tropical grass pastures commonly establish with plant densities >26 plants per m², yet the optimum density for maximum herbage accumulation and hydrological performance is not known. A replicated, field study was established in northern NSW in November 2011, using five densities of Digitaria eriantha (digit grass): 0, 1, 4, 9 and 16 plants per m² and a range of agronomic and hydrologic measures was assessed. The results showed that the largest differences between plant densities in herbage accumulation, root depth, soil water extraction and rainfall refill efficiency occurred during the first 2 years; after this time, digit grass pastures with densities of ≥4 plants per m² responded similarly. Plant frequency increased with increasing plant density but the treatments did not converge. During the first two growing seasons, there was no effect of plant density on water use efficiency; however, by the third growing season, pastures with 4 plants per m² were the most water efficient. Overall, the results indicated that 1 plant per m² was too low to efficiently use resources, 16 plants per m² utilized the resources quickly then tended to stall, while 4–9 plants per m² was a good compromise between agronomic and hydrological response, achieving both herbage accumulation and sustainability goals. A practical management advantage of pastures with 4–9 plants per m² is they allow soil water resources to be available for a longer period of time and potentially provide the opportunity to establish legumes in the ensuing 24‐month period.     

Nitrogen uptake by grasses: changes induced by competing neighbour plants differing in frequency of defoliation

Nitrogen uptake can be modified by both frequency of defoliation and competing neighbour plants. The objective of this study was to investigate nitrogen uptake by Lolium perenne and Festuca rubra subject to weekly or monthly defoliation with five neighbours: either none, L perenne clipped weekly, L. perenne clipped monthly, F. rubra clipped weekly or F. rubra clipped monthly. Plants were grown in sand culture with nutrients supplied as solutions. Nitrogen uptake over a I-week period commencing at the time of a defoliation event was estimated using ^l5N as a tracer. L. perenne had greater dry weight and nitrogen uptake than F. rubra. Neighbours reduced nitrogen uptake per plant. When neighbours were present, clipping them more frequently resulted in greater uptake by sample plants. Over 57% of total plant uptake was partitioned towards the two youngest leaves. In mixtures of L. perenne with F. rubra , the percentages of the total pot dry weight and N content recovered in sample L. perenne plants increased when they were clipped less frequently, compared with when the accompanying F. rubra plants were clipped less frequently.     

Response of Brazilian maize hybrids from different eras to changes in plant density

Maize (Zea mays L.) hybrids differ in their response to plant population density. Enhancements in tolerance to crowding have been obtained around the world as the result of selecting the best yielding hybrids at dense stands over a wide testing area. However, the understanding of morpho-physiological determinants of maize endurance to the population density stress still needs improvement. This study aimed to evaluate the response of maize hybrids from different eras grown extensively in Brazil to the increase in plant sowing density and to identify agronomic traits that can account for the contrasting tolerance to high interplant competition. The trial was conducted in Lages, southern Brazil, during the growing seasons of 1999/2000 and 2000/2001. A split plot design was used. The double-cross hybrids Agroceres 12, Agroceres 303, and the single-cross hybrid Cargill 929, commercially released during the 1970s, 1980s and 1990s, respectively, were evaluated in the main plots. Four plant population densities were tested in the split plots: 25,000, 50,000, 75,000 and 100,000 plants ha⁻¹. Maize was sown in no-tillage system, having black oat as the preceding winter crop. Canopy architecture parameters, tassel dry matter accumulation, flower synchrony, grain yield and its components were evaluated. The older hybrids Agroceres 12 and Agroceres 303 out-yielded modern-hybrid C 929 at the lowest plant population density. The increase in plant population increased barrenness, lengthened the anthesis–silking interval and decreased kernel set per ear more drastically for the older than for the modern-hybrid. Consequently, the higher the number of plants per unit area, the greater the edge between grain yield and kernel number per area of C 929 in relation to its older counterparts. Current hybrid’s better performance at supraoptimum plant population densities was favored by three sets of traits. First, by lower dry matter partition to the tassel, stimulating a more balanced allometric relationship between male and female inflorescence. Second, by a more compact canopy architecture, with shorter plants, fewer and more upright leaves, enhancing solar radiation interception. Third, by low ear insertion to plant height ratio, promoting greater resistance to stalk lodging. These traits contributed to higher grain yield and positive response to higher densities in the modern-hybrid.     

Effect of plant density and removal of ears on the quality and quantity of forage maize in a temperate climate

The present study was aimed at testing the hypothesis that a dense maize stand (320–720·20³ plants ha^-1) will produce more dry matter of acceptable quality than a stand sown at the density generally advocated (10⁵ plants ha^-1). It was also aimed at proving that grain is not essential in order to obtain a high yield of good-quality forage maize.
It was found that dry matter yield increased with density, especially at the early stages of growth. When ear-formation was depressed by increasing plant density, the resulting reduction of ear yield and its quality due to the absence of ear was partly compensated for by the increased yield and quality of the stem.     

苏东台棉花栽培密度与植棉效益的思考

近年来,随着杂交棉花的普及和用肥水平的提高,加之高效立体种植的推广,棉花生产的栽培密度呈逐年下降趋势.但是一些地方的棉花栽培密度盲目降低,有些甚至不到15000株/hm2,一些种子经营单位为了提高棉花的单株成桃,也在片面宣传低密度种植.密度己成为影响棉花生产的一项重要因素,如何科学确定棉花栽培密度,是当前棉花栽培工作中的又一课题.     

Nitrogen concentration in the upper leaves of the canopy is a reliable indicator of plant N nutrition in both pure and mixed grassland swards

Effective indicators of plant nitrogen (N) nutrition are needed to improve N management in grasslands. This is particularly the case for mixtures that rely on N fixation by legumes as a major N input, because no reference tool such as the nitrogen nutrition index (NNI) exists under these conditions. The aims of this study were to test the reliability of a plant-based index, the N concentration of upper leaves in the canopy (Nup), as a possible alternative for NNI in both pure and mixed grasslands. Data were gathered from four experiments covering a range of pure and mixed grasslands under different N fertilization levels. A cross-validation of Nup predictions against NNI in pure stands, and against two NNI-derived indices in mixtures, was performed. The Nup values appeared to be linearly related to NNI in pure stands of both grasses and legumes. The relationship was identical for the two groups of species and explained up to 86% of NNI variability. In mixtures, Nup also displayed a linear relationship with the two other tested indices, explaining 65% and 78% of variability. The conclusions of the three indices diverged with respect to strongly unbalanced mixtures, where the assumptions regarding the computation of NNI-derived indices were not met. Excluding these situations, the overall relationship between Nup and NNI proved to be identical for mixtures and pure stands. The results suggest that Nup is a valid criterion for plant N nutrition which applies to a broad range of grassland species and to mixture conditions.     

Correlation and variability analysis of yield and quality related traits in different peanut varieties across various ecological zones of China

Peanut cultivation in China spans various ecological zones, each with unique environmental conditions. Identifying suitable peanut varieties for these regions has been challenging due to significant phenotypic variations observed across environments. This study, based on a comprehensive analysis of 256 peanut varieties, selected nine representative varieties(Huayu23, Yuanza9102, Silihong, Wanhua2, Zhonghua6, Zhonghua16, Zhonghua21,Zhonghua215, Zhonghua24) for cultivation in five distinct ecologi...     

Sowing date,transplanting, plant density and nitrogen fertilization affect indigo production from Isatis species in a Mediterranean region of Spain

The increasing interest in natural products from a renewable source has encouraged growers to reintroduce indigo-producing crops into the European agriculture. We studied agronomic conditions (sowing date, plant density, nitrogen fertilization, irrigation rate, seedling transplanting) influencing production of the blue pigment indigo, from Isatis tinctoria and I. indigotica crops in a Mediterranean region of Spain (Valencia). I. tinctoria was more suitable for cultivation in our climate conditions than I. indigotica. Indigo yield from Spanish I. tinctoria trials was greater than in Northern and Central Europe. Furthermore, indigo production was maintained when water and nitrogen supplies were significantly restricted, showing that I. tinctoria is not a high-demanding crop.     

Limited impacts of extensive human land use on dominance,specialization, and biotic homogenization in boreal plant communities

Background

Niche theory predicts that human disturbance should influence the assembly of communities, favouring functionally homogeneous communities dominated by few but widespread generalists. The decline and loss of specialists leaves communities with species that are functionally more similar. Evenness of species occupancy declines, such that species become either widespread of rare. These patterns have often been observed, but it is unclear if they are a general result of human disturbance or specific to communities that are rich in species, in complex, spatially heterogeneous environments where the problem has often been investigated. We therefore tested whether human disturbance impacts dominance/evenness of species occupancy in communities, specialism/generalism of species, and functional biotic homogenization in the spatially relatively homogeneous, species poor boreal forest region of Alberta, Canada. We investigated 371 boreal vascular plant communities varying 0 – 100% in proportion of human land use.

Results

Rank species occupancy curves revealed high species dominance regardless of disturbance: within any disturbance class a few species occupied nearly every site and most species were found in a low proportion of sites. However, species were more widespread and displayed more even occupancy in intermediately disturbed communities than among communities of either low or high disturbance. We defined specialists and generalists based on turnover in co-occupants and thereby assessed impacts of human disturbance on specialization of species and community homogenization. Generalists were not disproportionately found at higher disturbance sites, and did not occupy more sites. Communities with greater human disturbance were not more functionally homogeneous; they did not harbor communities with more generalists.

Conclusions

We unexpectedly did not observe strong linkages between species specialism/generalism and disturbance, nor between community homogenization and disturbance. These results contrast previous findings in more species rich, complex or spatially heterogeneous systems and ecological models. We suggest that broad occupancy-based intercommunity patterns are insensitive to human land use extent in boreal vascular plants, perhaps because of ubiquity of generalists, low species richness, and history of natural disturbance. The poor sensitivity of these metrics to disturbance presents challenges for monitoring and managing impacts to biodiversity in this region.     

Effect of plant density on the inflorescence production of stems and the distribution of flower production in potato

Summary A knowledge of the pattern of flowering and seed production is required for the development of large-scale field production of True Potato Seed (TPS). At the highland experimental station of the International Potato Center in Peru, data on flowering and seed production were collected from three cultivars planted at three densities. Main stems in which flowering was delayed ceased shoot growth at an earlier stage and produced fewer inflorescences. Inflorescences produced later had fewer flowers, a lower berry set and yielded less seed. Inflorescences flowering at the same time performed similarly, irrespective of their position on the plant. Increasing plant density resulted in cessation of shoot growth at an earlier stage and concentrated inflorescence and flower production at primary positions of early-flowering shoots. With cvs Renacimiento and Yungay a higher plant density increased the percentage of flowers produced in the first three weeks of the flowering period, but with cv. Atzimba the effect of plant density on the distribution of flower production was off-set by a slower stem development.     

Comparison of hydroponic culture and culture in a peat/sand mixture and the influence of nutrient solution and plant density on seed potato yields

Summary Two culture systems for propagating first generation potatoes were compared; the traditional system used a peat/sand mixture with mineral fertilizer, and hydroponic culture used perlite and nutrient solution. Total production and the number of tubers obtained using the hydroponic system were significantly higher than using the traditional culture system. Tuber yields from in vitro plants and minitubers depended upon time of year. During the autumn/winter cycle yield from minitubers was double that from in vitro plants, whereas the reverse was true during the spring/summer cycle. Four hydroponic test cultures were carried out to study the influence of seed density. The number of tubers obtained increased significantly with seed density but there was no decrease in the number of large-diameter tubers.     

Effects of ecological,landscape and management factors on plant species composition,biodiversity and forage value in Alpine meadows

Due to decades of loss of grassland diversity across Europe, there is a need to identify factors affecting species composition and diversity in managed meadows. The aim of the current study was to assess how ecological, landscape and management factors may affect plant species composition, biodiversity and forage value in Alpine hay meadows. Species composition, Shannon index and forage value were obtained from phytosociological relevés. Twenty explanatory variables were selected from a set of ecological, landscape and management factors. Their effects on plant species composition, Shannon index and forage value were analysed by applying the variation partitioning approach. Plant species composition was related to sixteen factors, explaining 35·6% of the variability. Shannon index and forage value were related to eleven factors, explaining 47·8 and 40·8% of their total variation respectively. Ecological factors were the main set explaining species composition and diversity, whereas none of the three individual groups of factors (ecological, landscape, management) significantly explained variability within forage value. Overall, the effects of the three groups of factors accounted for 70% of the total variability in plant species composition, but less than half that of Shannon index and of forage value.