Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?

The extent of the response of plant growth to atmospheric CO₂ enrichment depends on the availability of resources other than CO₂. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO₂ response of plants. The effect of elevated CO₂ (60 Pa) partial pressure (pCO₂) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO₂, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO₂, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO₂ than under ambient pCO₂ in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO₂ was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO₂, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N₂-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO₂. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N₂-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO₂. Received: 11 November 1996 / Accepted: 20 May 1997     

Temperature Sensitivity of Photosynthesis in Lolium perenne Swards: A Comparison of Two Methods for Deriving Photosynthetic Parameters from in vivo Measurements

The seasonal variation in photosynthetic rate of grass swards is partly the result of changes in the environment and partly the result of changes in the photosynthetic capacity of the sward itself. We evaluated two types of photosynthesis equations regarding their capacity to analyse seasonal and short-term temperature effects on photosynthesis of ryegrass (Lolium perenne L.). Intact cores of a field-grown ryegrass sward were taken to the laboratory 10 d after cutting for measurement of photosynthesis under controlled conditions. This was done during two four-week periods, in summer and autumn. Net photosynthetic rate (P _N) of the sward was lower in autumn than in summer. Both a simple negatively exponential photosynthesis irradiance-response curve and the Farquhar equations for photosynthesis were applied to the in vivo canopy measurements. Application of the irradiance-response curve showed that irradiance-saturated gross photosynthetic rate increased linearly with increasing temperature and was higher in summer than in autumn. The initial radiation use efficiency did not differ between the seasons but decreased with the temperature rise. This explains the observation that total canopy photosynthetic rate decreased after short-term temperature increases in both seasons. The parameters in Farquhar equations that represent the temperature sensitivity of the maximum electron transport rate and of the Michaelis-Menten constants for CO₂ and O₂ fixation could not be quantified satisfactorily. Parameterisation of the Farquhar equations was hampered by a lack of robust information on many biochemical parameters, and the use of simple empirical response-functions may be preferable in the case of in vivo canopy measurements on grass swards.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

Effects of temperature,oxygen concentration,leaf age and seasonal variations on the CO2 compensation point of Lolium perenne L.

Various factors affect the CO₂ compensation point of detached leaves of Lolium perenne L. These include oxygen concentration, temperature, leaf age, and season (spring and summer). Analysis of the results using the model of G.D. Farquhar, S. von Caemmerer and J.A. Berry (1980) Planta 149, 78–90, indicates that some of the CO₂ evolved by leaves in the light is derived from sources other than photorespiration. It is suggested that the operation of the tricarboxylic acid in the light can account for most of this CO₂.Azcón-Bieto—experimental work was done in Barcelona, the further analyses white at the A.N.U.     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO₂ concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The ¹³C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO₂. In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO₂ but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO₂ whereas only the C:N ratio of L. perenne roots increased under elevated CO₂. Root biomass of L. perenne doubled under elevated CO₂ and again under high N fertilization. Total soil C was unaffected by CO₂ treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F‐value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO₂ had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO₂ concentration (overall average of 8.5 g C kg^?1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO₂ and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO₂.     

Sexual reproduction of Lolium perenne L. and Trifolium repens L. under free air CO2 enrichment (FACE) at two levels of nitrogen application

Field experiments in managed grassland have shown that the response of vegetative growth to elevated CO₂ is nitrogen‐dependent in grasses, but independent in N₂‐fixing legumes. In the present study, we tested whether this is also true for reproduction. We evaluated reproductive growth, flowering phenology, seed development, reproductive success and seed germination in the grass Lolium perenne L. and the legume Trifolium repens L., growing in monocultures in a free air carbon dioxide enrichment (FACE) system at ambient (35 Pa) and elevated (60 Pa) partial pressure of CO₂ and two levels of nitrogen fertilization (14 and 56 g N m^?2 a^?1). In both species, elevated CO₂ had no significant effect on sexual reproduction. In L. perenne, reproduction was mainly nitrogen‐dependent. The weak interactions between CO₂ and mineral N supply (13% more flowers and 8% more grains per spike at high N, 7% less flowers and 8% less grains at low N) were not significant. Under elevated CO₂, grain maturation was slightly enhanced and grain weight tended to decrease. No influence could be ascertained in the date of anthesis, the temporal pattern of grain growth, the rate of grain abortion and germination. Trifolium repens, grown under CO₂ enrichment at both levels of N fertilization, flowered 10 d earlier, tended to form more inflorescences per ground area and more flowers (8–12%) and seeds (>18%) per inflorescence than at ambient CO₂. The temporal pattern of seed growth was about the same in all treatments; embryo development, however, was accelerated in fumigated plants. The number of aborted seeds per pod, seed size, thousand‐seed weight and germinability did not show any influence of CO₂. Fumigated plants at high N were attacked slightly more frequently by seed‐eating weevils, which lowered the seed output per pod. In summary, the reproductive response of L. perenne and T. repens to CO₂ enrichment on the flower and inflorescence level was far weaker than expected from the results on vegetative growth.     

The effects of elevated CO2 concentrations on the root growth of Lolium perenne and Trifolium repens grown in a FACE* system

Lolium perenne and Trifolium repens were grown in a Free Air CO₂ Enrichment (FACE) system at elevated (600 μimol mol-¹) and ambient (340 μmol mol-¹) carbon dioxide concentrations during a whole growing season. Using a root ingrowth bag technique the extent to which CO₂ enrichment influenced the growth of L, perenne and T. repens roots under two contrasting nutrient regimes was examined. Root ingrowth bags were inserted for a fixed time into the soil in order to trap roots. It was also possible to follow the mortality of roots in bags inserted for different time intervals. Root ingrowth of both L. perenne and T. repens increased under elevated CO₂ conditions. In L. perenne, root ingrowth decreased with increasing nutrient fertilizer level, but for T. repens the root ingrowth was not affected by the nutrient application rate. Besides biomass measurements, root length estimates were made for T, repens. These showed an increase under elevated CO₂ concentrations. Root decomposition appeared to decrease under elevated CO₂ concentrations. A possible explanation for this effect is the observed changes in tissue composition, such as the increase in the carbon: nitrogen ratio in roots of L. perenne at elevated CO₂ concentrations.     

Photosynthesis and Photorespiration in Lolium multiflorum and Lolium perenne

The response of net photosynthesis to changing light-flux densityby leaves of Lolium multiflorum (S. 22) and L. perenne (S. 321)is more adequately described by current models when a term allowingfor photorespiration is included. The magnitude of this termwas determined from the changes in the slope of the light-responsecurves for net photosynthesis. A pseudo first-order rate-constantfor photorespiration, and a pseudo second-order rate-constantfor photosynthesis calculated by this technique for L. multiflorumwere found to be similar to corresponding parameters calculatedfrom light-compensation-point measurements using a simple modeldescribed by Brown (1969). The relative magnitudes of respirationand photosynthesis at light saturation for both Lolium specieswere similar to reported values for other temperate species(Lake, 1967). Two selection lines of L. perenne (S. 321) with contrastingdry-matter yields were found to have the same parameters forrespiration and photosynthesis.     

Assimilation of 14CO2 by the Inflorescence of Poa annua L. and Lolium perenne L.

The relative assimilatory activity of the inflorescence, itsindividual components, and the leaves of flowering tillers ofPoa annua L. and Lolium perenneL. was determined over the periodfrom inflorescence emergence to seed shedding. The pattern of¹⁴CO₂ fixation was similar for both species and the inflorescencewas by far the most important assimilatory organ of the reproductivetiller, particularly over the latter period of seed developmentas leaf senescence progressed. With the exception of the seedsall parts of the inflorescence showed significant assimilatoryactivity and the lemmas and paleas accounted for 40–50per cent of the total ¹⁴C fixed by the inflorescence in bothspecies. The importance of the grass inflorescence as a photosyntheticstructure is discussed in relation to similar studies on cereals. Poa annua, Lolium perenne, carbon dioxide assimilation, inflorescence     

Elevated CO2 and the mineral content of herbaceous and woody plants

The CO₂ enrichment effects (300–650 µmol mol^-1) on mineral concentration (N, P, K, Ca, Mg, Mn, Fe, Zn), absolute total mineral contents per individual and of whole stands of four herbaceous (Trifolium repens L.,Trifolium pratense L.,Lolium perenne L.,Festuca pratensis HUDS.) and two woody species (Acer pseudo-platanus L.,Fagus sylvatica L.) were investigated.In general, the mineral concentration of the plant tissues decreased (all six species: N>Ca>K>Mg) with the exception of P. Mn and Fe were only determined for the tree species. Both decreased in concentration (Mn>Fe). Zn was only analysed forTrifolium pratense andFestuca pratensis and decreased significantly in the grass.Despite of decreases in concentrations of as much as 20% in some cases there were increases in absolute amounts per individual and, therefore, in the whole vegetation up to 25% because of the enhanced dry matter accumulation at elevated CO₂ supply.Dedicated to Prof. Dr. R. Bornkamm, TU-Berlin, on behalf of his 60th birthday     

不同水分处理下喀斯特土层厚度异质性对两种草本叶片解剖结构和光合特性的影响

为了探究不同水分处理下草本植物对喀斯特土层厚度变化的叶片形态建成和光合生理响应,以黑麦草(Lolium perenne L.)和苇状羊茅(Festuca arundinacea Schreb.)为研究对象,通过盆栽水分受控试验,研究了3种水分处理[正常供水(W_(ck)),减水1组(D1)和减水2组(D2)]下3种土层厚度[浅土组(S_S)、对照组(S_(CK))和深土组(S_D)]对两种草本叶片解剖结构和光合特性的影响。结果表明:(1)正常供水下(W_(ck)),黑麦草和苇状羊茅在浅土组(S_S)的气孔密度和气孔限制值(Ls)均显著高于对照组(S_(CK)),净光合速率(Pn)、胞间CO_2浓度(Ci)和蒸腾速率(Tr)降低;在深土组(S_D),两种植物的气孔密度都有所下降,黑麦草的叶脉密度、Pn和Tr均低于对照组,而苇状羊茅的叶脉密度和Pn表现出增加;(2)D1水分条件下,黑麦草在浅土组的气孔密度较对照组增加,叶脉密度、Pn和Tr均降低,而苇状羊茅的气孔密度有所降低,叶脉密度、Pn和Tr未受到显著影响;在深土组中,黑麦草的气孔密度不变,叶脉密度增加,而Pn和Tr均降低;苇状羊茅的气孔密度降低,但叶脉密度、Pn和Tr均升高;(3)D2水分条件下,两种植物在浅土组的叶脉密度较对照组均增加,气孔密度、Pn和Tr均受到抑制;在深土组,黑麦草的远轴面气孔密度较对照组下降,两种植物的其他指标未受到明显影响。可见,在不同水分条件下,植物的叶片解剖结构和光合特性对不同土层厚度的响应不一,且不同物种间也有差异。总体上随着水分减少,土层厚度降低对植物的光合抑制作用增强,而厚度增加对深根植物的光合促进作用和对浅根植物的光合抑制作用先增强后减弱。植物气孔和叶脉性状特征随水分条件的变化在一定程度上与叶面积和叶片宽度的变化有关。     

The development of sequence-tagged sites (STSs) in Lolium perenne L.: the application of primer sets derived from other genera

Genetic analysis, particularly the development of genetic linkage maps in forage grass species, lags well behind other members of the Poaceae. Comparative mapping within this family has revealed extensive conservation in gene and marker synteny among chromosomes of diverse genera. Recently, the ability to transfer mapped STS markers between barley and wheat has been demonstrated. The transfer of mapped STS markers between cereals and forage grasses could provide PCR-based markers for comparative mapping in these species providing they amplify homologous sequences. In this study, primers derived from three barley genes of defined function and a gene from Phalaris coerulescens were used to amplify homologous fragments in Lolium perenne. Primers derived from two barley and two oat cDNA clones were also tested along with eight barley and two Triticum tauchii STS markers. Twenty one primer pairs derived from 18 loci were tested. Eleven primer pairs (52%) amplified homologous sequences in L. perenne from ten (55%) of the loci targetted. Thirteen new STS markers were generated in L. perenne, of which ten have been mapped in barley or rye and amplify homologous sequences in L. perenne. Received: 20 October 2000 / Accepted: 13 January 2001     

Effects of long-term elevated atmospheric carbon dioxide onLolium perenne andTrifolium repens,using a simple photosynthesis model

Changes in gross canopy photosynthetic rate (PGc), produced by long-term exposure to an elevated atmospheric CO₂ level (626±50 µmol mol^-1), were modelled forLolium perenne L. cv. Vigor andTrifolium repens L. cv. Blanca, using a simple photosynthesis model, based on biochemical and physiological information (leaf gross CO₂ uptake in saturating light, P_max, and leaf quantum efficiency, ) and structural vegetation parameters (leaf area index, LAI, canopy extinction coefficient, k, leaf transmission, M). Correction of PGc for leaf respiration allowed comparison with previously measured canopy net CO₂ exchange rates, with the average divergence from model prediction amounting to about 6%. Sensitivity analysis showed that for a three-week old canopy, the PGc increase in high CO₂ could be attributed largely to changes in P_max and , while differences in canopy architecture were no longer important for the PGc-stimulation (which they were in the early growth stages). As a consequence of this increasing LAI with canopy age, the gain of daytime CO₂ uptake is progressively eroded by the increasing burden of canopy respiration in high-CO₂ grownLolium perenne. Modelling canopy photosynthesis in different regrowth stages after cutting (one week, two weeks,...), revealed that the difference in a 24-h CO₂ balance between the ambient and the high CO₂ treatment is reduced with regrowth time and completely disappears after 6 weeks.Abbreviations C350 ambient CO₂ treatment - C625 high CO₂ treatment - k canopy extinction coefficient - LAI leaf area index - LAI_max fitted LAI-maximum - M leaf transmission - NCER net CO₂ exchange rate - PGc gross canopy photosynthetic rate - Q photosynthetic photon flux density - Q₀ photosynthetic photon flux density at the top of the canopy - RDc canopy dark respiration rate - RDl leaf dark respiration rate - t regrowth time after cutting - T air temperature - leaf quantum efficiency - _LAI rate of initial LAI-increase with time     

Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory

We compared the CO₂- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 μl l^–1 CO₂. Naturally-established saplings were fumigated with a free-air CO₂ enrichment system. Light-saturated photosynthetic rates were 159–190% greater for Ce. canadensis saplings grown and measured under elevated CO₂. This species had the greatest CO₂ stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO₂ enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO₂ stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO₂. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO₂. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO₂ make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO₂ did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO₂. Received: 16 February 1999 / Accepted: 30 August 1999     

Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

Elevated atmospheric CO₂ may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO₂ decomposes differently from plant material produced under ambient CO₂. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO₂ made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO₂ at two levels of N fertilizer (140 vs. 560 kg ha^?1 yr^?1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO₂ under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO₂ and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO₂ did not affect decomposition rates of the plant material. If under prolonged elevated CO₂ changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO₂. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the ¹³C‐CO₂ signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO₂, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.     

Vegetation establishment on chalk marl spoil: the role of nurse grass species and fertiliser application

Abstract. The Channel Tunnel workings on the UK side have yielded nearly 4 million m³ of chalk-marl spoil which now forms a 36 ha landscaped reclamation platform. To establish vegetation of amenity and conservation interest on the spoil, seed mixtures of native wild flowers and grasses were sown with Lolium perenne (perennial rye grass) as a nurse species. Potentially, L. perenne is a suitable nurse species for grassland creation on infertile substrates as it provides rapid initial cover and stability, but it is non-persistent and declines in vigour with time, allowing wild flower species sown alongside to expand their cover and spread in the longer term. On very low fertility substrates like chalk marl, an initial application of fertilizer is needed to encourage plant growth. Results are reported of a fertilizer experiment on Channel Tunnel spoil to determine appropriate levels of fertilizer for establishment of species-rich grassland vegetation. An area hydroseeded with L. perenne and wild flowers in autumn 1992 was subjected to factorial treatment of four levels each of N and P in spring 1993. The results the following summer showed significant positive effects of nitrogen and phosphorus on L. perenne biomass and a negative impact of nitrogen on densities of wild flower species, especially legumes, establishing in the L. perenne sward. In general, low fertilizer applications encouraged low productivity and maximal species richness in the vegetation. Conversely high applications encouraged high productivity and competitive exclusion of sown wild flower species. Fertilizer applications must therefore balance encouragement of the stabilising nurse grass sward, while preventing competitive exclusion of wild flowers by the nurse grass.     

Root hydrocarbons as potential markers for determining species composition

Grasslands can be a complex mixture of plant species. A method is described to allow the identification of both roots and shoots of five different grass species, thus permitting greater knowledge about whole plant allocation and competition in mixed pastures. The five species were Lolium perenne, Festuca ovina, Festuca rubra, Poa trivialis and Agrostis capillaris. N‐alkanes with odd‐numbers of carbon atoms in the chains predominate in plants and in the five grass species studied, concentrations of alkanes of chain length of C₂₉, C₃₁ and C₃₃ were highest. Average concentrations of C₂₇‐C₃₃ alkanes in shoots and roots were 187 and 11 mg kg ^{? 1}, respectively. This wide range of values required considerable modifications to the method of analysis, including expressing concentrations on an organic matter basis and scaling‐down the procedure. The n‐alkane concentrations in roots are different from those in shoots and therefore values from shoots cannot be used to predict the composition in roots. Using a canonical variate analysis, all five grass species could be separated using concentrations of C₂₆, C₃₁ and C₃₃ values in the roots. The greatest difference occurred between A. capillaris and the others, whereas discrimination was least between the two Festuca species. Defoliation had contrasting effects on the concentration of a few n‐alkanes, but not in the n‐alkanes used to discriminate between grass species. Alkane analysis shows great potential as a method to quantify the species composition of the root biomass beneath mixed pasture species.     

A Comparison of the Growth of the C4 Grass Spartina anglica with the C3 Grass Lolium perenne at Different Temperatures

Dunn, R., Thomas, S. M., Keys, A. J. and Long, S. P. 1987. Acomparison of the growth of the C₄ grass Spartina anglica withthe C₃ grass Lolium perenne at different temperatures.—J.exp. Bot. 38: 433–441. S. anglica is one of the few C₄ species which occurs naturallyin cool temperate zones. It is known to attain photosyntheticrates which equal or exceed those of C₃ grasses over the temperaturerange typical of the spring and summer in cool temperate climates.This study examines whether S. anglica can also attain comparablegrowth rates at these temperatures. Seedlings of S. anglicaand L. perenne cv. S23 were grown in controlled environmentsat 10,15,20 and 25 °C. Quantitative growth analysis wasconducted by taking frequent harvests to determine the progressionsof leaf area and plant weight of individual plants with time.Quadratic regressions were found to describe these progressionswell. Instantaneous derived growth parameters were calculatedfrom the fitted regressions. Both absolute and relative growthrates of S. anglica were significantly lower than for L. perenne,this being largely attributable to a lower ratio of leaf areaproduction per unit of plant dry weight. Although the amountof dry matter invested into leaves was similar, the leaf areaper unit of leaf dry weight was lower in S. anglica. In comparisonto L. perenne, the rate of dry matter accumulation per unitof leaf area (ULR) was higher in S. anglica at 25 °C andinitially equal at 10 °C. Prolonged exposure to 10 °Csteadily reduced ULR in S. anglica which approached zero at80 d. Although growth in S. anglica is reduced more by low temperaturethan it is in L. perenne, by comparison to other C₄ speciesthe assimilatory capacity of S. anglica is more tolerant oflow temperature exposure. Key words: C4 photosynthesis, temperature, quantitative growth analysis     

Fungal endophytes of native grasses decrease insect herbivore preference and performance

Endophytic fungal symbionts of grasses are well known for their protective benefit of herbivory reduction. However, the majority of studies on endophyte–grass symbioses have been conducted on economically important, agricultural species—particularly tall fescue (Lolium arundinaceum) and perennial ryegrass (Lolium perenne)—raising the hypothesis that strong benefits are the product of artificial selection. We examined whether fungal endophytes found in natural populations of native grass species deterred insect herbivores. By testing several native grass–endophyte symbiota, we examined phylogenetic signals in the effects of endophytes on insects and compared the relative importance of herbivore and symbiotum identity in the outcome of the interactions. Preference was assessed using three herbivore species [Spodoptera frugiperda (Lepidoptera), Schistocerca americana (Orthoptera), Rhopalosiphum padi (Hemiptera)] and ten native symbiota, which spanned seven grass genera. We also assessed herbivore performance in a no choice experiment for five native symbiota against S. frugiperda. We compared greenhouse and laboratory trials with natural levels of herbivory measured in experimental field populations. In all cases, we included the agronomic grass species, L. arundinaceum, to compare with results from the native grasses. Both in the field and in experimental trials, herbivores showed a significant preference for endophyte-free plant material for the majority of native grasses, with up to three times lower herbivory for endophyte-symbiotic plants; however, the degree of response depended on the identity of the herbivore species. Endophyte presence also significantly reduced performance of S. frugiperda for the majority of grass species. In contrast, the endophyte in L. arundinaceum had few significant anti-herbivore effects, except for a reduction in herbivory at one of two field sites. Our results demonstrate that the mechanisms by which native symbionts deter herbivores are at least as potent as those in model agricultural systems, despite the absence of artificial selection.