Cell Walls of Seed Hairs fromLygeum spartum: Ultrastructure, Composition and Mechanical Properties

The silky hairs covering the whole surface ofLygeum spartumdispersalunits are unicellular, unlignified epidermal structures whoselength varies from 5 to 17 mm. The cell walls present a stratifiedtexture after extraction of matrix material. Their compositiondepends on the geographical origin of the grasses. Polysaccharidesextracted with boiling water represent approx. 18% of the cellwall dry matter in samples harvested in Algerian arid high-plateausand only 3% in samples harvested in semi-arid coastal zones.In the latter case, hemicellulose and cellulose contents amountedto approx. 50 and 40%, respectively. Slightly lower values werefound in high-plateau samples. Cinnamic acids were detectedonly in coastal region specimens which exhibited a higher mechanicalresistance and a lower extensibility. These results supportthe existence of two distinct populations colonizing arid orsemi-arid areas respectively.Copyright 1998 Annals of BotanyCompany Cell wall; extensibility; polysaccharides;Lygeum spartum; seed hairs.     

Leaf traits of two Mediterranean perennial tussock grass species in relation to soil nitrogen and phosphorus availability

Studying relationships of plant traits to ecosystem properties is an emerging approach aiming to understand plant's potential effect on ecosystem functioning. In the current study, we explored links between morphological and nutritional leaf traits of two Mediterranean perennial grass species Stipa tenacissima and Lygeum spartum, widely used to prevent desertification process by stabilizing sand dunes. We evaluated also relationships in terms of nitrogen (N) and phosphorus (P) availability between leaves of the investigated species and the corresponding soil. Our results showed that leaf P was very low in comparison with leaf N for the two investigated species. In fact, chlorophyll content, photosynthesis capacity and water conservation during photosynthesis are mainly linked to leaf nitrogen content. Our findings support previous studies showing that at the species levels, morphological and nutritional leaf traits were not related. On the other hand, significant relationships were obtained between soil N and leaf N for S. tenacissima (P = 0.011) and L. spartum (P = 0.033). However, leaf P was not significantly related to soil P availability for both species. We suggest that any decrease in soil N with the predicted increasing aridity may result in reduction in leaf N and thus in worst dysfunction of some biological processes levels.     

Seasonal growth and translocation of some major and trace elements in two Mediterranean grasses (Stipa tenacissima Loefl. ex L. and Lygeum spartum Loefl. ex L.)

The rangelands of Stipa tenacissima and Lygeum spartum (Poaceae) constitute one of the main typical ecosystems in the Iberian Peninsula and North Africa. This study examines the seasonal changes in aboveground biomass accumulation and translocation of some major (Ca and K) and trace elements (Br, Cr, Cu, Fe, Mn, Sr and Zn) from topsoil to shoots of these perennial grasses. Species, season and their interaction significantly affected the dry biomass (DW) and chemical composition of both species and their surrounding soil. The maximum DW was found in spring due to high physiological activity and was correlated positively with rainfall. A significant relationship between seasons and chemical elements was found. For both species the maximum concentrations of Ca, Cu and Zn were found in spring season. However L. spartum had the highest concentrations of K, Cr, Br, and Sr in autumn season, indicating exceptional ability of these species to accumulate large contents of these elements during the active growth periods. By way of contrast, in the topsoil the highest concentrations of almost all chemical elements were found in summer and autumn. Principal component analyses (PCA) showed that growth of L. spartum was highly associated with K, Ca, Zn, Br and Sr, whereas topsoil was correlated with Cu, Cr, Fe and Mn concentrations. Translocation factor (TF_x) of chemical elements was not identical across the two species, demonstrating inter–specific variability to uptake chemical elements. The maximum values of TF_x were recorded for K, Ca and Sr especially for L. spartum. To cope with arid conditions, S. tenacissima and L. spartum sprout quickly by increasing their rate of growth and nutrient uptake as soon as soil water is available after the rain.     

Do Tree Canopies Enhance Perennial Grass Restoration in California Oak Savannas?

Scattered trees in grass‐dominated ecosystems often act as islands of fertility with important influences on community structure. Despite the potential for these islands to be useful in restoring degraded rangelands, they can also serve as sites for the establishment of fast growing non‐native species. In California oak savannas, native perennial grasses are rare beneath isolated oaks and non‐native annual grasses dominate. To understand the mechanisms generating this pattern, and the potential for restoration of native grasses under oaks, we asked: what are the effects of the tree understory environment, the abundance of a dominant non‐native annual grass (Bromus diandrus), and soils beneath the trees on survival, growth, and reproduction of native perennial grass seedlings? We found oak canopies had a strong positive effect on survival of Stipa pulchra and Poa secunda. Growth and reproduction was enhanced by the canopy for Poa but negatively impacted for Stipa. We also found that Bromus suppressed growth and reproduction in Stipa and Poa, although less so for Stipa. These results suggest the oak understory may enhance survival of restored native perennial grass seedlings. The presence of exotic grasses can also suppress growth of native grasses, although only weakly for Stipa. The current limitation of native grasses to outside the canopy edge is potentially the result of interference from annual grasses under oaks, especially for short‐statured grasses like Poa. Therefore, control of non‐native annual grasses under tree canopies will enhance the establishment of S. pulchra and P. secunda when planted in California oak savannas.     

Tradeoffs Between Irradiance Capture and Avoidance in Semi-arid Environments Assessed with a Crown Architecture Model

Plants in arid environments cope with stress from excessiveirradiance by physiological photoprotection of the photosyntheticapparatus and by structurally reducing the leaf area exposedto the sun (structural photoprotection). We assessed the ecologicalrelevance of structural photoprotection in two plants of contrastingarchitecture co-occurring in a semi-arid environment, usingthe three-dimensional canopy model YPLANT. We compared the roleof crown geometry in avoiding excessive radiation, analysedthe costs of structural photoprotection in terms of reductionof potential carbon gain, and compared these costs with thosedue to seasonal constraints of photosynthesis and tissue ageing.The results of the model simulations indicated that canopy architectureofStipa tenacissima(a tussock grass) andRetama sphaerocarpa(aleafless leguminous shrub) minimized the risk of overheatingand photo-oxidative destruction of the photosynthetic apparatuswith steeply oriented foliage and moderate self-shading. Butthis structural photoprotection imposed an increased cost interms of potential carbon gain. Diurnal and seasonal patternsof light interception by the crown of these plants translatedinto a simulated potential carbon gain only half that of anequivalent, horizontal photosynthetic surface. This reductionin potential carbon gain, due to irradiance avoidance, was similarto that imposed by water shortage.S. tenacissima,which ceasesphotosynthetic activity during periods of drought, exhibitedmore structural avoidance of irradiance thanR. sphaerocarpa,whichremains active throughout the year. This illustrates the influenceof the capacity of plants to utilize light for carbon fixationon the trade-offs between irradiance capture and avoidance.Structural avoidance of excessive radiation efficiently preventsthe risk of damage by intense irradiance, has no special maintenancecosts, and is biomechanically cheaper than enhanced light harvestingby a horizontal canopy, which points to structural photoprotectionas a very effective strategy to cope with high irradiance stressin poor and adverse habitats.Copyright 1999 Annals of BotanyCompany Canopy model, functional architecture, irradiance avoidance, light harvesting, photoinhibition, photoprotection, photosynthesis model, plant architecture,Retama sphaerocarpa,semi-arid environment,Stipa tenacissima,structural photoprotection.     

Diversity of Cellulose Microfibril Arrangement in the Cell Walls of Lygeum spartum Leaves

Extraction of matrix material, cytochemical methods and stereoanalysiswere used to study cell-wall architecture in leaves of Lygeumspartum, a grass species growing in sub-and regions of the Mediterraneanzone Most tissues were lignified with the exception of mesophyllcells Better results were obtained with NaOH extraction thanwith methylamine extraction The different types of fibres (sub-epidermal,axial and bundle sheath fibres), parenchyma and epidermal cellswere characterized by specific cell-wall organization The differentwall textures were variations of the basic helicoidal model Lygeum spartum L, lignified leaf fibres, cell wall, helicoidal texture, polysaccharide cytochemistry     

Limitation to Carbon Assimilation of Two Perennial Species in Semi-Arid South-East Spain

Diurnal and seasonal changes of net photosynthetic rate (P_n) and the efficiency of photosystem 2 (F_v/F_m) were measured on two perennial species growing on a soil catena in semi-arid south-east Spain. Stipa tenacissima, a tussock grass, grows on shallow soil at the top of the catena and Retama sphaerocarpa, a leguminous shrub, grows in the valley bottom. A linear relationship was found between light saturated photosynthetic rates (P_max) and diffusive leaf conductance (g_l) in both Retama and Stipa indicating that the intercellular CO₂ concentration (c_i) was maintained constant in both species diurnally. Relatively high values of calculated c_i in Retama cladodes suggested that was not the primary limitation to carbon assimilation. F_v/F_m for the two species when well watered was around 0.8. Although Retama cladodes maintained this value throughout the year, F_v/F_m decreased to a minimum of 0.43 in Stipa leaves, at the end of the dry season. Our data suggest that plants in the Rambla Honda can substantially reduce transpiration without reducing photosynthetic rates to the same extent by closing their stomata, because P_n is reduced primarily by high respiration, decreased mesophyll conductance and by photoinhibition or permanent damage of photosystem 2.     

Effects of biological soil crusts on a Mediterranean perennial grass (Stipa tenacissima L.)

In the last decades, there has been a growing interest on evaluating the effects of biological soil crusts (hereafter, biocrusts) on vascular plants. The purpose of this paper was to investigate the influence of biocrusts on Stipa tenacissima, key species in arid and semi-arid environments of the Mediterranean Basin. In fact, we explored the impact of biocrusts removal underneath S. tenacissima on soil chemical properties and leaf mineral uptake. Indeed, we studied the effect of mixed biocrusts on the seed germination of S. tenacissima under laboratory conditions. The research showed a greater availability of soil resources correlated with enhanced leaf nutrient levels of bioessential elements in the presence of intact biocrusts compared to fragmented biocrusts underneath S. tenacissima tussocks. We suggest that this is related to the loss of soil moisture as a result of biocrusts removal. In addition, we found that mixed biocrusts favored seed germination and viability of S. tenacissima. By improving soil chemical properties and leaf nutrient uptake, biocrusts can provide enhanced growth conditions for Stipa tenacissima. These findings with the positive effect on seed germination suggest that biocrusts can be used in the restoration of S. tenacissima.     

Ecophysiological significance of chlorophyll loss and reduced photochemical efficiency under extreme aridity in Stipa tenacissima L.

Stipa tenacissima L., a perennial tussock grass widely found in semi-arid environments of the Iberian Peninsula and North Africa, is subjected to multiple stresses during the extreme summer conditions of south-east Spain. We characterised the photoprotective mechanisms of S. tenacissima during the transition from spring to summer and autumn. S. tenacissima experienced a marked water deficit (_{{ pd}} < -8.4 MPa) and the complete suppression of CO₂ assimilation in August, associated with a 72% reduction of maximal photochemical efficiency of PSII (F_{{ v}}/F_{{ m}}). These reduced F _{{ v}}/F_{{ m}} values were related to the pre-dawn maintenance of high levels of epoxidized forms of xanthophyll-cycle pigments (DPS_{{ pd}}, ca. 42% higher than spring values), and with a 60% reduction in the concentration of total chlorophyll (Chl a+b). These changes were associated with a low capacity of dissipation of the excitation energy non-radiatively (measured as NPQ). Leaves showed a complete recovery of F _{{ v}}/F_{{ m}} and xanthophyll and chlorophyll concentrations after the autumn rainfall, which reached levels similar to that of spring. This poikilohydric-type response of S. tenacissima to stress allows for a greater tolerance of water shortage, high temperature and high light intensity, which are typical in these semi-arid environments and accounts for its distinctive opportunistic growth.     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

Seed dormancy and longevity in <Emphasis Type="Italic">Stipa tenacissima</Emphasis> L. (Poaceae)

In the present paper we studied the life history traits related to seed germination of Stipa tenacissima, a key species in semiarid environments of western Mediterranean areas. S. tenacissima is a perennial tussock grass, which has traditionally been considered to expand mainly by vegetative propagation with little or no sexual reproduction. We analysed seed longevity as well as the type of seed dormancy and the role of the seed covers from seeds collected from different populations in SE Spain. We also studied the variation in seed germinability among populations, individuals, and years and the ability of seeds of S. tenacissima to form soil seed banks. There was significant variation in seed germination among individuals, populations and years. Lemma and palea were the main factor controlling these differences since their removal promoted higher and faster germination and eliminated the differences in germination parameters among populations. However, the control of dormancy by lemma and palea was independent of their weight, suggesting that their chemical nature plays a more important role than does size in controlling seed germination. Mechanical scarification treatments (via abrasion with sand) did not affect seed germination. The decay in seed germinability two years after seed collection and the low density of viable seeds in soils one year after seed dispersal indicated that S. tenacissima forms transient soil seed banks.     

δ13C values of grass species collected in the northern Sahara desert

Summary ¹³C values were measured for 45 Poaceae species collected in the northern Sahara desert, at the foot of the Saharan Atlas. The results indicate a clear relationship between carbon isotope discrimination and phytogeographical distribution of the grasses. Mediterranean species predominantly had ¹³C values indicating the C₃ pathway of photosynthesis. By contrast, nearly all species belonging to the Saharo-Arabian and /or Sudanian group showed a C₄ like carbon isotope composition. Leaf material of two species, Lygeum spartum and Stipa tenacissima, had ¹³C values in the region of-20, i.e. intermediate between the mean ¹³C values of C₃ and C₄ plants. However, additional speciments of both these grasses obtained from a different source (herbarium of the Hebrew University, Jerusalem) yielded a C₃ like carbon isotope composition.     

Tillering in Tussock Grasses in Relation to Defoliation and Apical Bud Removal

Experiments with five caespitose grass species from temperateand tropical environments showed that the number of lateralshoots (tillers) which emerged following defoliation was notincreased by leaving a greater residual leaf area. Increasedavailability of photosynthate (and perhaps other resources)was effective, however, in increasing the rate of growth anddegree of flowering of new lateral shoots in one tropical species,Panicum maximum. In two temperate Agropyron tussock grasses, decapitation (apicalbud removal) did not stimulate lateral shoot growth. This indicatedthat apical dominance was not a factor preventing growth oflateral buds just prior to inflorescence emergence on the parenttillers. However, defoliation, where both terminal buds andfoliage were removed from the parent tillers stimulated lateralbud growth. Hormones other than those produced by the apicalbud or light quality or intensity may control lateral bud growthin these species. In contrast to the temperate species, lateralbud growth was stimulated by both decapitation and defoliationin the three tropical species. This response is consistent withthe model of correlative inhibition by apical dominance. Agropyron desertorum, Agropyron spicatum, Heteropogon contortus, Panicum maximum, Themeda triandra, crested wheatgrass, bluebunch wheatgrass, black speargrass, green panic grass kangaroo grass, apical dominance, tillering, regrowth, grazing, tussock grasses     

Savannah woodland vegetation in the South‐East District of South Australia: the influence of evaporative aerodynamics on the foliage structure of the understorey invaded by introduced annuals

Abstract Evaporative aerodynamics determine the foliage projective cover of the understorey of perennial tussock grasses and associated perennial herbs in the savannah woodland dominated by Eucalyptus camalduknsis on gleyed podsolic soils in the Mediterranean climate of the South‐East District of South Australia. By the mid 1940s, winter‐spring evapotranspiration from the ‘thin’ leaves (with low leaf specific weight) of introduced annual plants was depleting surface soil water and thus reducing the annual growth of the summer‐growing savannah understorey; perennial herbs between the tussock grasses were the first to succumb to this competition. During spring, the percentage of the ground covered by the savannah understorey was increased by 10% in the subhumid zone to 30% in the humid zone as the pre‐European perennial herbs between the tussock grasses were replaced by introduced annuals. Application of phosphatic fertilizer to the understorey increased the growth of introduced annuals, which formed a dense stratum during their winter‐spring growing season, increasing evapotranspiration and leading eventually to the extinction of the native perennial grasses. When the savannah understorey, invaded by introduced annuals in the mid‐1940s, was converted to improved pasture, the percentage of ground covered by the seasonal foliage was increased by 20–30%; 100% coverage of overlapping foliage resulted in the humid zone.     

Water Relations Only Partly Explain the Distributions of Three Perennial Plant Species in a Semi-arid Environment

The water relations and stomatal conductances of three perennial plant species, Stipa tenacissima L., Anthyllis cytisoides L., and Retama sphaerocarpa (L.) Boiss., dominant on the upper slopes, mid-slopes and floor of a valley, respectively, in semi-arid south-east Spain, were investigated to test the hypothesis that differences in plant-soil water relations could account for the different distributions of each species in the catena. Diurnal measurements of water potential (Ψ_w), relative water content (RWC) and stomatal conductance (g_s) of leaves were made over one year. Leaf temperature, air humidity, wind-speed and incident quantum flux density were measured concurrently. Soil water content was determined gravimetrically at 0 – 5 cm and 15 – 20 cm depths. Measurements of Ψ_w, RWC and g_s were analysed according to meteorological conditions, based on the maxima for daily air temperature and atmospheric saturation water vapour deficit and on soil moisture content. The hypothesis that plant-soil water relations can explain the distribution of the three species along the catena from valley side to valley floor was rejected for Anthyllis and Stipa but confirmed for Retama. This revised version was published online in July 2006 with corrections to the Cover Date.     

Water Use Characteristics of Dactylis glomerata L., Lolium perenne L. and L. multiflorum Lam. Plants

Cocksfoot (Dactylis glomerata L.). perennial ryegrass (Loliumperenne L.) and Italian ryegrass (L. multiflorum Lam.) plantswere grown on deep (75–95 cm) columns of soil in glasshousesand growth rooms with and without irrigation. The species inwhich growth declined least rapidly after water had been withheldwere those which transpired most slowly. During early establishmentin the glasshouse cocksfoot transpired least because of slowroot growth. In the growth room, when root systems were deeperand denser, perennial ryegrass transpired least because of lowleaf water conductance. Results are discussed in relation to(a) drought resistance in the three species; (b) breeding forincreased drought resistance through modifying root distributionand leaf water conductance; and (c) the use of isolated soilcolumns in water relations studies. Dactylis glomerata L., Lolium perenne L., Lolium multiflorum Lam., cocksfoot, perennial ryegrass, Italian ryegrass, transpiration, roots, leaf water conductance     

Interactive Effects of Three Ecosystem Engineers on Infiltration in a Semi-Arid Mediterranean Grassland

The redistribution of water in semi-arid environments is critical for the maintenance and survival of vegetation patches. We used a systems approach to examine the interactive effects of three engineers—Stipa tenacissima, biological soil crusts, and the European rabbit (Oryctolagus cuniculus)—on infiltration processes in a model gypseous semi-arid Mediterranean grassland. We measured the early (sorptivity) and later (steady-state infiltration) stages of infiltration at two supply potentials using disk permeameters, which allowed us to determine the relative effects of different engineers and soil micropores on water flow through large macropores. We detected few effects under tension when flow was restricted to matrix pores, but under ponding, sorptivity and steady-state infiltration adjacent to Stipa tussocks were 2–3 times higher than in intact or rabbit-disturbed biological soil crusts. Structural Equation Modeling (SEM) showed that both Stipa and biological soil crust cover exerted substantial and equal positive effects on infiltration under ponding, whereas indirectly, rabbit disturbance negatively affected infiltration by reducing crust cover. Under tension, when macropores were prevented from conducting water, Stipa had a direct negative effect and biological soil crust cover was relatively unimportant. More complex SEM models demonstrated that (1) Stipa primarily influenced biological soil crusts by reducing their richness, (2) rabbits exerted a small negative effect on crust richness, and (3) lichens were negatively, and mosses positively, correlated with a derived “infiltration” axis. Our results highlight the importance of biological soil crusts as key players in the maintenance of infiltration processes in Stipa grasslands, and demonstrate the modulating role played by rabbits through their surface disturbances.     

Differential effects of the crustose Diploschistes diacapsis and the squamulose Fulgensia bracteata on the establishment of a Mediterranean grass species

The interest of the scientific community in biological soil crusts has grown exponentially over the last decades. One of the scientific research interests is the study of the effect of these crusts on plant establishment. Findings in this topic have been controversial, and some differences were attributed to crust types. Biological soil crusts dominated by lichens are common components of Stipa tenacissima steppes in arid and semi‐arid environments of the southern Mediterranean. In the current study, we conducted growth chamber experiments to investigate the differential effects of two lichen species with continuous crustose thalli (Diploschistes diacapsis) and with squamulose semicontinuous thalli (Fulgensia bracteata) on seed germination, root penetration, shoot emergence and seed viability of the tussock grass species S. tenacissima. Our results showed that under laboratory conditions, two distinct lichen species had significantly different effects on the establishment of S. tenacissima. Our findings clearly demonstrated that D. diacapsis significantly decreased germination, root penetration and shoot emergence of S. tenacissima compared to F. bracteata. This can be related to differences in morphological and physiological characteristics between crustose and squamulose lichens. Overall, we suggest that D. diacapsis and crustose lichens generally can act as natural barrier to the establishment of S. tenacissima.     

Temporal dynamics of herbivory and water availability interactively modulate the outcome of a grass–shrub interaction in a semi‐arid ecosystem

The study of plant–plant interactions along grazing and abiotic stress gradients is a major research topic in plant ecology, but the joint effects of both stressors on the outcome of plant–plant interactions remains poorly understood. We used two different factorial experiments conducted in a semi‐arid Mediterranean steppe to assess: 1) the role of the perennial grass Stipa tenacissima, a low‐palatability species, providing protection from rabbit herbivory to the shrub Retama sphaerocarpa (experiment 1), and 2) the effects of environmental amelioration provided by Stipa on the recovery of Retama after rabbit damage under two contrasted levels of water availability (experiment 2). In the experiment 1, water stress worked as an indirect modulator of herbivore protection by Stipa. This species protected Retama seedlings from rabbit herbivory during the wetter conditions of spring and winter, but this effect dissapeared when rabbit pressure on Retama increased during summer drought due to the decrease in alternative food resources. In the experiment 2, Stipa exerted a negative effect on the survival of Retama seedlings during the three years of the experiment, regardless of inter‐annual differences in rainfall or the watering level applied. This negative effect was mainly due to excessive shading. However, Stipa increased Retama recovery after initial rabbit impact, overriding in part this negative shade effect. Conversely, Stipa impact on the Fv/Fm of Retama seedlings depended on the intra‐annual water dynamics and its experimental manipulation, overall contradicting predictions from the stress–gradient hypothesis. The complex interactions found between herbivory, microclimatic amelioration from Stipa, and water availability as drivers of Retama performance illustrate the importance of considering the temporal dynamics of both biotic and abiotic stressors to fully understand the outcome of plant–plant interactions.     

Leaf Extension Rate in Temperate Pasture Grasses in Relation to Assimilate Pool in the Extension Zone

In two growth cabinet experiments the leaf extension rate (LER)was studied under a 14 h photoperiod followed by prolonged darkness,in tillers of the perennial temperate pasture grasses Phalaristuberosa L. cv. Sirosa and Dactylis glomerata L. cv. Currie.Levels of soluble non-structural carbohydrates and total