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.     

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.     

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.     

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.     

杂交稻新组合宜优673在不同生态条件下的表现

宜优673系福建省农科院水稻所用宜香A与福恢673配组而成的杂交水稻新组合,2006年通过福建省品种审定.为考察宜优673在不同生态条件种植的适应性,2005-2006年在福建省不同生态区域安排种植试验,现将试验结果分析如下.     

The prediction of the in vivodigestibility of the diet of sheep and cattle grazing indigenous hill plant communities by in vitrodigestion, faecal nitrogen concentration or indigestible' acid-detergent fibre

Herbage and faeces samples were retained from an intake and in vivo digestibility trial using material harvested at three different stages from five indigenous hill plant communities (those dominated by Agrostis-Festuca., Nardus stricta., Molinia caerulea., Eriophorum vaginatum and Tri-chophorum caespitosum) and from sown swards of ryegrass or white clover. Samples of the herbages and of their separate components, together with extrusa samples of the same herbages recovered from pen-fed sheep and cattle oesophageal fistulates, were digested in vitro using rumen liquor. Measurements were made also of faecal nitrogen concentration (FN) and of indigestible acid-detergent fibre (IADF) using the samples from the original trial.
The range of in vitro disappearance values of the herbages (IVOMD; 0809-0278) was slightly wider than those of in vivo digestibility (OMD; 0-796-0-37I). Differences between OMD and IVOMD were greatest at low quality and relationships were best described by two separate linear regressions of OMD on IVOMD for (a) ryegrass, white clover, Agrostis-Festuca and Nardus (RSD; 0 0185) and, (b) Molinia, Tricho-phorum and Eriophorum (RSD; 00246). In vitro disappearance values of extrusa were higher than those of herbages offered by 00503 and 00156 units for sheep and cattle respectively, partly because of greater levels of ensalivation, especially in the sheep. The relationships of OMD to IADF and to FN were poor, though inclusion of faeces output and fitting parallel lines for communities in the regression of OMD on FN reduced the RSD to 0020. We conclude that in vitro digestion of samples of extrusa, using appropriate standards, is the best method of diet digestibility prediction for sheep and cattle grazing these communities.     

Characteristics of Nitrogen Uptake,Use and Transfer in a Wheat-Maize-Soybean Relay Intercropping System

Abstract

Intercropping and relay intercropping systems, which significantly improve land use efficiency, are used worldwide to increase crops yield. The wheat-maize-soybean relay intercropping system has been widely employed by famers in Southwestern China for years, but the detailed mechanisms through which the nitrogen fertilizer use efficiency reach the high level in this system remain unclear. In the present study, two separate pot experiments were performed by ¹⁵ N isotope dilution (ID) labeling and direct ¹⁵N foliar feeding (FF) assays, and a solid barrier was employed to prevent the roots intergrowth and N movement among crops in the first experiment, using no barrier as the control. The results showed that, under the no-barrier condition, the grain yields, ¹⁵N uptake and ¹⁵N recovery efficiency of wheat and maize were significantly increased, but those measures in soybean were decreased compared to the solid barrier condition. Furthermore, bi-directional N transfer was detected during the co-growing stage of crops, the amount (Ntransfer) and percentage (%NT) of ¹⁵N transferred varied significantly with the fertilizer-N rate, and the maximum reached at 150 – 300 kg N ha^–1 level. The Ntransfer from maize to wheat was 16.1% – 163.0% higher than that from wheat to maize; the Ntransfer from soybean to maize was 1.7 – 6.0 times higher than those from maize to soybean, while the %NT from soybean to maize were 6.7 – 22.2 times higher than those from maize to soybean. Conclusively, this study revealed that the interaction of the roots among crops significantly increased the uptake efficiency and recovery efficiency, and further, the positive N competition and bi-directional N transfer of each crops were the main contributors to improve the N use efficiency in the wheat-maize-soybean relay intercropping system.     

Different Nitrogen Sources Affect Biomass Partitioning and Quality of Potato Production in a Hydroponic System

Crop production systems should reduce nitrogen application costs and assure that the appropriate form of nitrogen is used. Thus, three potato cultivars in a hydroponic system were supplied with two different nitrogen sources to determine the effect on biomass accumulation and partitioning, and total plant nitrogen content. Cultivars Agata, Atlantic and Bintje received, on alternate days, nutritive solutions differing only in either calcium nitrate or urea. Urea stimulated biomass accumulation and total nitrogen in shoots. Moreover, urea reduced the ratios tuber:shoot and tuber:root, and increased the ratio shoot:root, indicating competition for biomass partitioning between shoots and tubers. Urea stimulated greater tuber volume in the Atlantic cultivar, and increased tuber protein content, which is undesirable for industry. These results suggested that salts containing nitrate could be more appropriate for hydroponic potato cultivation, since urea compromised tuber quality and biomass partitioning in all cultivars studied.     

Scavenging Capacity of Marine Carotenoids against Reactive Oxygen and Nitrogen Species in a Membrane-Mimicking System

Carotenoid intake has been associated with the decrease of the incidence of some chronic diseases by minimizing the in vivo oxidative damages induced by reactive oxygen (ROS) and nitrogen species (RNS). The carotenoids are well-known singlet oxygen quenchers; however, their capacity to scavenge other reactive species, such as peroxyl radical (ROO^•), hydroxyl radical (HO^•), hypochlorous acid (HOCl) and anion peroxynitrite (ONOO⁻), still needs to be more extensively studied, especially using membrane-mimicking systems, such as liposomes. Moreover, the identification of carotenoids possessing high antioxidant capacity can lead to new alternatives of drugs or nutritional supplements for prophylaxis or therapy of pathological conditions related to oxidative damages, such as cardiovascular diseases. The capacity to scavenge ROO^•, HO^•, HOCl and ONOO⁻ of seven carotenoids found in marine organisms was determined in liposomes based on the fluorescence loss of a fluorescent lipid (C₁₁-BODIPY^581/591) due to its oxidation by these reactive species. The carotenoid-bearing hydroxyl groups were generally more potent ROS scavengers than the carotenes, whilst β-carotene was the most efficient ONOO⁻ scavenger. The role of astaxanthin as an antioxidant should be highlighted, since it was a more potent scavenger of ROO^•, HOCl and ONOO⁻ than α-tocopherol.     

Impact of deferred grazing and fertilizer on plant population density,ground cover and soil moisture of native pastures in steep hill country of southern Australia

The impact of deferred grazing (no defoliation of pastures for a period generally from spring to autumn) and fertilizer application on plant population density, ground cover and soil moisture in a hill pasture (annual grass dominated, with Australian native grasses being the major perennial species) were studied in a large‐scale field experiment from 2002 to 2006 in southern Australia. Three deferred grazing strategies were used: short‐term deferred grazing (no defoliation between October and January each year), long‐term deferred grazing (no defoliation from October to the autumn break, that is the first significant rainfall event of the winter growing season) and optimized deferred grazing (withholding time from grazing depends on morphological development of the plants). These treatments were applied with two fertilizer levels (nil fertilizer and 50 kg P ha^?1 plus lime) and two additional treatments [continuous grazing (control) and no grazing for year 1]. Deferred grazing increased (P < 0·05) perennial grass tiller density compared with the control. On average, the tiller density of the three deferred grazing treatments was 27–88% higher than the control. There was a negative (P < 0·01) relationship between perennial and annual grass tiller density. Fertilizer application increased (P < 0·05) legume plant density. The densities of annual grasses, legumes, onion grass (Romulea rosea) and broadleaf weeds varied between years, but perennial grass density and moss cover did not. The ground cover of the deferred grazing treatments in autumn was on average 27% higher than the control. Soil moisture differed between treatments at 15–30 cm depth, but not at 0–15 depth over autumn and winter. The results imply that deferred grazing can be an effective tool for rejuvenating degraded native pastures through increases in native grass tiller density and population and through improving farm productivity and sustainability.     

Simulation of nitrogen uptake, fixation and leaching in a grass/white clover mixture

To represent nitrogen cycling in a low input grass/legume pasture system, a previously developed, weather-driven grass/white clover growth model has been adapted to become the crop growth component of the soil nitrogen dynamics model SOILN. This provides a means of simulating nitrogen uptake by a grass/white clover crop, an important component of the overall nitrogen balance in low-input grassland systems.
Crop growth is represented by a photosynthesis equation adapted to take account of competition between the two crops for resources of light, water and nitrogen in the soil. Water shortage is represented by linked simulations with the soil water and heat model SOIL, and nitrogen shortage by links with the SOILN model. Nitrogen fixation has been introduced according to an equation for potential fixation reduced by environmental factors, particularly temperature. Transfer of nitrogen-rich clover plant material to the soil nitrogen pools of SOILN (from where it becomes available as a nutrient for grass) is also represented. The model is tested by comparing simulated cut crop yields and nitrogen content of cut material with measured data from perennial ryegrass/white clover at a test site. Soil nitrogen processes in the model are tested by comparing simulated and measured nitrate in drainflows. Apart from some discrepancies between simulated and measured results attributable to the inherent instability of a mixed crop system, agreement is reasonable by the standards of biological system models, indicating that the combined model gives a realistic representation of carbon and nitrogen processes in grassland with a grass, legume mixed crop.     

Effects of plant density on the yield and yield components of true potato seed (TPS) hybrids in early and main crop potato production systems

This study was conducted to evaluate true potato seed (TPS) technology for use in ware or seed potato production in two contrasting environments in Turkey during 2002 and 2003. The field experiments were carried out in the Hatay and Nevsehir provinces in Turkey, which represent a Mediterranean early crop potato production area and a temperate main crop potato production area, respectively. The plug seedlings of six TPS hybrids were transplanted to the fields at four densities (15, 20, 25 or 30 plants m⁻²). The seed tubers of the medium early cultivar Marfona were also planted in the experimental plots to compare the performance of the TPS hybrids with traditional seed tubers. Transplanting of the seedlings was significantly delayed in Hatay due to unsuitable weather conditions in both years. The seedlings needed an adaptation period of 2–4 weeks after transplanting depending on the location and the growing conditions. The adaptation period was longer in Hatay due to high air temperatures after transplanting. Although the yield performance of the TPS hybrids differed depending on the location and year, the TPS hybrids produced noticeably higher total tuber yields in Nevsehir location (ranging from 43.1 to 62.5 t ha⁻¹ in 2002 and from 39.5 to 50.6 t ha⁻¹ in 2003) than in Hatay (ranging from 15.3 to 19.6 t ha⁻¹ in 2002 and from 15.1 to 19.1 t ha⁻¹ in 2003). The percentage of marketable tubers (>28 mm) was also considerably higher in Nevsehir. The optimal plant density varied between 25 and 30 plants m⁻² with regard to the total yield, while the optimal density with regard to the marketable yield was 20 or 25 plants m⁻² depending on hybrids in Nevsehir. However, none of the tested plant densities caused competition between plants in Hatay, where the environmental conditions during the growing period considerably restricted the growth of individual TPS seedlings.It was concluded that transplanting of TPS seedlings can be considered a feasible alternative for ware or seed potato production in temperate environments like Nevsehir that have growing periods of at least 4 months. However, there are several obstacles, such as difficulties with the timing of transplanting, long adaptation period that threaten the practicability of TPS technology in Mediterranean-type environments. Further agronomical studies focused on reducing inter- and intra-plant competition are needed for both environments in order to improve the acceptability of TPS technology to farmers.