Geographical distribution and chorology of grasses in the Arabian Peninsula

Arid regions of Saudi Arabia occupy most of the area of the Arabian Peninsula. These areas are at the meeting position of plants from Mediterranean, Irano-Turanian, Saharo-Arabian, and Sudanian phytogegraphical regions. Geomorphology of the area reveals a wide diversity of landforms including coastal lines, desert plains, and high mountains. Grasses are well represented in the flora of Saudi Arabia and form an appropriate group for studying the relation of grass distribution, chorology, and photosynthetic pathways. In this paper, geographical distribution of C₃ and C₄ grasses was studied in an area extending between latitude 17°N and latitude 31°N. Two regions were recognized in the study area, namely; a (relatively) cold region north of latitude 24°N with ample winter rainfall, and a hot region south of latitude 24°N with scarce summer rainfall. Work involved field observations and collection of grass species in the study area. Work also depended on published carbon discrimination values of grasses and biochemical analysis of C₄ species subtypes. Climatic conditions in the study area vary considerably, and the distribution of grass species was found to follow patterns that reveal adaptive advantages of different photosynthetic pathways. Grass species in the cold northern region with ample winter rainfall are generally C₃ grasses belonging mainly to Mediterranean/Irano-Turanean chorotypes. C₃ grass species found in the southern hot region were recorded at high altitudes of southern mountains characterized by low temperatures. Grass species recorded at low altitudes in the south hot region with scarce summer rainfall were mainly C₄ grasses belonging to Tropical and Saharo-Arabian-Sudanean chorotypes. Pronounced spatial variations of temperature profoundly control the geographical distribution of C₃ and C₄ grasses. Low temperatures in the northern cold region and at high altitudes of the southern hot region limit the occurrence of C₄ grasses and shift the ecological balance in favor of C₃ grasses. Results are discussed in terms of heat sensitivity of the CO₂ carboxylating enzyme of C₃ grasses and high temperature optima for CO₂ assimilation of C₄ grasses. Results are also discussed in comparison with geographical distribution of grasses in other parts of the world.     

Geographical and environmental distribution of C3 and C4 grasses in the Sinai,Negev, and Judean deserts

Summary The relation between photosynthetic pathway and habitat of the grass species recorded in the desert regions of Sinai, Negev, and Judea was investigated. The climatic conditions and micro-environments in the study area vary considerably, and the distribution of the various species is found to conform to specific patterns which reveal the adaptive advantages of the different photosynthetic pathways. There is also a distinct correlation between the phytogeographic origin of the grass species and the photosynthetic pathways that they utilize.The survey shows that the majority of the grass species in the region are of the C₃ type and all except one of these species belong to the Holarctic domain. This is in accordance with the fact that the region forms part of the Mediterranean winter rainfall regime and that C₃ species have an adaptive advantage where minimum temperatures are low during the winter growing season.The occurence of C₄ species increases with decreasing rainfall and they dominate in those districts where temperatures are high throughout the year. These C₄ grasses are of both Holarctic and Palaeotropic origin according to the classification adopted here, but they are essentially all elements of the Saharo-Arabian, Irano-Turanian, Sudanian, or Tropical phytogeographic regions and are not typical of the Mediterranean or Euro-Siberian floras. The plants with multi-regional distributions that occur in Mediterranean communities may well be intrusive.Analysis of the three subtypes of the C₄ species suggests that the malate-forming NADP-me grasses grow where water stress is not a dominating factor, while the aspartateforming NAD-me grasses are more successful under xeric conditions. The PEP-ck species are not abundant and form an intermediate group between the NADP-me and NAD-me subtypes.     

Photosynthetic pathways and ecological distribution ofEuphorbia species in Egypt

Summary The photosynthetic pathways of 42 species of the genusEuphorbia growing wild, naturalized or cultivated in Egypt were investigated. The criteria used included the δ¹³C- and δD-values and Kranz anatomy of the leaves. There is a relationship between the photosynthetic pathway and the ecological conditions in the habitat of a particular species. All 4 CAM species are succulent shrubs, wild or cultivated. The 11 species with C₄ pathways are mainly summer annuals of tropical origin and flourish under the hot summer conditions. The 27 C₃ species are either winter annuals, perennials, perennials flourishing in winter or shrubs; the majority are Mediterranean, European or Saharo-Arabian. Summer annuals with C₃ pathways grow under the shade of the summer crops. Generally, C₃ plants grow under conditions of relatively better water resources and lower temperature than the C₄ plants. The majority of the CAM and C₄ species occur in the southern part of the country, where high temperature is a common feature of the climate.     

Photosynthetic pathways and the ecological distribution of the chenopodiaceae in Isreal

Summary Fifty-four species of the Chenopodiaceae in Israel were examined for their anatomical features, ¹³C values, habitat and phytogeographical distribution. 17 species have ¹³C values between -20 and -30and non-Kranz anatomy (NK) and are therefore considered as C₃ plants. 37 species have ¹³C values between -10 and -18 and Kranz or C₄-Suaeda type anatomy and are therefore considered as C₄ plants. Some C₄ plants have leaf structure which seems to be intermediate between the Kranz and the C₄-Suaeda type of leaf anatomy.The segregation of the species into photosynthetic groups shows tribal and phytogeographical grouping. Most of the C₃ Chenopods are either mesoruderal plants or coastal halophytes, with a distribution area which covers the Euro-Siberian as well as the Mediterranean phytogeographical regions. The C₄ Chenopods are mainly desert or steppe xerohalophytes with a distribution area which includes the Saharo-Arabian and/or Irano-Turanian phytogeographical regions.     

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.     

Photosynthetic pathways inChenopodiaceae from Africa, Asia and Europe with their ecological, phytogeographical and taxonomical importance

Photosynthetic pathways are reported for 305 species ofChenopodiaceae from Africa, Asia and Europe. Ecological characteristics, phytogeography and life forms of all species are given, and their correlation with relevant CO₂-fixation types are discussed. 205 species (67.2%) exhibit the C₄-pathway and 100 species (32.8%) the C₃-pathway of CO₂-fixation. Most of the C₄ species are of Irano-Turanian origin. The diversity of C₄ species of the Irano-Turanian phytochoria, with very harsh winters, are interpreted by the active period of Chenopods in summer. There is a close relationship between some special morphotypes and respective photosynthetic type. Halophytes and xerophytes with articulated stems and stem succulents ofAnabasis-type are exclusively C₄. Leaf succulent halophytes and xerohalophytes are predominantly C₄. Hygrohalophytes with leaf or stem succulence are often C₃. Probably many C₄ Salsoleae have been evolved in the Afroasiatic arid zone after the climatic changes of the Miocene. Among them there is a high proportion of annuals that have a younger origin. There is some evidence that the present Chenopods of the subfamily Salsoloideae may have their ancient stock in NW Africa. We found close taxonomic relationships between photosynthetic pathways and infrageneric classification in genera with both types of C₃ and C₄, likeAtriplex andSuaeda. Dedicated to emer. Univ.-Prof. DrFriedrich Ehrendorfer, Wien, on the occasion of his 70th birthday     

Investigations on the spontaneous emitted and hydrodistilled volatiles of Platycladus orientalis (L.) Franco grown in four different biogeographic zones of Jordan

The essential oil of the fresh leaves of Platycladus orientalis (L.), grown in four different biogeographic zones of Jordan,- (the Mediterranean, Irano-Turanian, Saharo-Arabian, and Sudanian penetration) -, were obtained by hydrodistillation and analysed by gas chromatography (GC) and gas chromatography-mass-spectrometry (GC/MS). The actual composition of the spontaneous emitted volatiles was obtained using the solid-phase-micro-extraction (SPME) method and investigated using the same chromatographic and spectroscopic methods. Hydrocarbon monoterpenes dominated the hydrodistilled oils and emissions of all regions. Bicyclic monoterpenes (sabinene, α-pinene, and α-thujene) and monocyclic α-terpinene were detected as the major constituents of the oils and emissions. Additionally, hierarchical cluster analysis (HCA) revealed that the clustering is based on the region of collection rather than the applied methodology. Differences were observed in the quantity of the obtained oils (P-values <0.01); the highest amount of volatile oil was obtained from samples grown in the Irano-Turanian biogeographic zone.     

Photosynthesis pathways,ecological characteristics,and the geographical distribution of the Cyperaceae in Japan

Summary The nature of the photosynthetic pathways of Cyperaceae found in Japan were investigated on the basis of Kranz anatomy, the CO₂ compensation concentration and previously reported data. Among 301 species (96% of all cyperaceous species recorded in the region), 58 species were classified as being C₄ plants. These C₄ species were scattered among the tribes Fimbristylideae, Lipocarpheae, Cypereae and Rhynchosporeae in the subfamily Cyperoideae. The genera Cyperus, Eleocharis and Rhynchospora included, in Japan, both C₃ and C₄ species within a single genus. Using these data, an analysis was made of the ecological characteristics and geographical distribution of the C₃ and C₄ species in Japan. Although cyperaceous species grow in markedly different environments, the majority were found in wet and aquatic areas (61%) or shaded areas, such as forest floors (20%). Most of the C₃ species were also hygrophytes (58%) and forest-living species (25%), and C₃ species growing in mesic and dry areas were relatively rare. The C₄ species inhabited wet and aquatic (75%), mesic (13%) and dry areas (6%) and showed marked ecological characteristics with respect to soil-moisture conditions, unlike other C₄ plants, although they were absent from shaded habitats. In order to determine the climatic factors that influence the relative floristic abundance of C₃ and C₄ members of the Cyperaceae in Japan, the ratios of number of C₄ species to the total number of members of Cyperaceae (C₄ percentage) in 16 representative locales were examined in terms of various climatic variables. There were strong positive correlations between the C₄ percentage and temperature. Among the C₃ groups of three subfamilies, there were different distributional trends for various temperature regimes. The C₃ subfamily Caricoideae increased its relative contribution to the cyperaceous flora with a decrease in mean annual temperature, while the C₃ subfamily Sclerioideae exhibited the opposite pattern. The C₃ group of the subfamily Cyperoideae did not show any marked change in pattern along temperature gradients, unlike the two other C₃ subfamilies, and seemed to be heterogeneous in terms of its response to temperature. The relationships between the C₄ biochemical subtypes and ecological characteristics are also discussed.     

Photosynthetic Pathways,Life Forms,and Reproductive Types for Forage Species Along the Desertification Gradient on Hunshandake Desert,North China

Floristic compositions, life forms, reproductive types for forage species, and their responses to desertification in Hunshandake desert were studied. 164 species, in 30 families and 94 genera, were identified with C₃ (137 species), C₄ (25 species), and CAM (2 species) photosynthesis. Of the 25 C₄ species, 76 % were grasses and Chenopodiaceae species (hereafter chenopods). This suggests that the C₄ species mainly occurred in a few families in the desert region. The reduction of C₃ species and the increase of C₄ species with desertification indicated that C₄ species might have higher tolerance to environmental stresses (e.g. dry and poor soil). Relatively more hemicrytophyte and therophyte forms in the desert are related to the local temperate climate and vegetation dynamics. Relatively greater proportions of C₄/C₃ and clonal species/sexual species at mobile dune showed that the C₄ species and clonal species could make greater contribution to sand land restoration in the Hunshandake desert.     

Phytogeographical significance of Wadi Wateer (Sinai), Egypt, with special reference to Cocculus hirsutus (L.) W.Theob.

The botanical survey of Wadi Wateer, located in the southeastern Sinai, revealed the presence of a plant that provides an important new record to the Flora of Egypt: Cocculus hirsutus (L.) Theob. This plant has not been recorded previously as existing in its wild form in the Flora of Egypt. The plant cover in Wadi Wateer is dominated by plant taxa belonging to the Saharo-Sindian phytogeographical element. However, many plants in the wadi have been attributed to the Sudanian chorotype, such as Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan, Chrozophora brocchiana Vis., Lycium shawii Roem & Schult., Moricandia sinaica (Boiss.) Boiss., Cocculus hirsutus (L.) Theob. and Cocculus pendulus (J.R. & G. Forst.) Diels. The presence of Sudanian chorotype plant taxa [including the newly recorded Cocculus hirsutus (L.) Theob.] provides evidence that Wadi Wateer and Aqaba Gulf functioned as migratory tracks for these African plants, assisting their penetration into the Saharo-Sindian, Mediterranean and Irano-Turanian phytogeographical regions in the Sinai and Asia. This study suggests that Wadi Wateer should be declared a protected area for its habitat and climatic diversity as well as for its phytogeographical significance.     

Co-function of C3-and C4-photosynthetic pathways in C3, C4 and C3-C4 intermediate Flaveria species

The potential for C₄ photosynthesis was investigated in five C₃-C₄ intermediate species, one C₃ species, and one C₄ species in the genus Flaveria, using ¹⁴CO₂ pulse-¹²CO₂ chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating ¹⁴CO₂ into the C₄ acids malate and aspartate, following an 8-s pulse. The proportion of ¹⁴C label in these C₄ products ranged from 50–55% to 20–26% in the C₃-C₄ intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, ¹⁴CO₂ into aspartate as into malate. Generally, about 5–15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C₄ photosynthesis among the intermediate species based on the apparent rate of conversion of ¹⁴C label from the C₄ cycle to the C₃ cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C₄-cycle products and an increase in percentage label in C₃-cycle products during chase periods with ¹²CO₂, although the rate of change was slower than in the C₄ species, F. palmeri. In these C₃-C₄ intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C₃-C₄ intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C₄-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C₄ and C₃ cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C₃ species, 7–18% of the initial label was in malate+aspartate. However, only 40–50% of this label was in the C-4 position, indicating C₄-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O₂, the absorbed quantum yields for CO₂ uptake (in mol CO₂·[mol quanta]^-1) averaged 0.053 in F. cronquistii (C₃), 0.051 in F. trinervia (Spreng.) Mohr (C₄), 0.052 in F. ramosissima (C₃-C₄), 0.051 in F. anomala (C₃-C₄), 0.050 in F. linearis (C₃-C₄), 0.046 in F. floridana (C₃-C₄), and 0.044 in F. pubescens (C₃-C₄). In 2% O₂ an enhancement of the quantum yield was observed in all of the C₃-C₄ intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O₂ were intermediate in value to the C₃ and C₄ species, indicating a co-function of the C₃ and C₄ cycles in CO₂ assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O₂ presumably reflect an ineffcient transfer of carbon from the C₄ to the C₃ cycle. The response of the quantum yield to four increasing O₂ concentrations (2–35%) showed lower levels of O₂ inhibition in the C₃-C₄ intermediate F. ramosissima, relative to the C₃ species. This indicates that the co-function of the C₃ and C₄ cycles in this intermediate species leads to an increased CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O₂.Abbreviations PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate     

δ<Superscript>13</Superscript>C values,photosynthetic pathways,and plant functional types for some plants from saline meadows,Northeastern China

Based on stable carbon isotope ratio (δ¹³C) measurements, photosynthetic pathway types were determined for 61 species in 54 genera and 24 families of flowering plants from the saline meadows of Northeastern China. Of these total vascular plants, 18 species in 17 genera from 6 families were found to have C₄ photosynthesis; 43 species in 38 genera from 20 families had C₃ photosynthesis. Six dicotyledonous species exhibited C₄ pathway, 12 monocotyledonous species were found with C₄ photosynthesis. The dicotyledonous C₄ species had relative greater mean δ¹³C value and less total carbon content than both monocotyledonous C₄ and C₃ species. Most dicotyledonous C₄ species were annual forbs and halophytes. Some C₄ species had been previously documented, but their δ¹³C values varied remarkably from those of the present study. Even though there are some fluctuations for the δ¹³C values of some C₄ species, δ¹³C value was still more reliable for C₃ and C₄ identification than the use of the enzyme ratio method and of low CO₂ compensation concentration.     

Altitudinal Changes in Photosynthetic Pathways of Floristic Elements in Southern Sinai, Egypt

High altitude profoundly influenced plant diversity and distribution on mountains of southern Sinai (Egypt). Plants exhibiting the C₃-mode of photosynthesis were widely distributed along the altitudinal transect. Plants exhibiting the C₄-mode were restricted below an altitude of about 1400 m above sea level. The transition from C₃-dominated areas to C₄-dominated areas occurred between 1200 and 1400 m a.s.l. This revised version was published online in August 2006 with corrections to the Cover Date.     

Occurrence of C3 and C4 photosynthesis in cotyledons and leaves of Salsola species (Chenopodiaceae)

Most species of the genus Salsola (Chenopodiaceae) that have been examined exhibit C₄ photosynthesis in leaves. Four Salsola species from Central Asia were investigated in this study to determine the structural and functional relationships in photosynthesis of cotyledons compared to leaves, using anatomical (Kranz versus non-Kranz anatomy, chloroplast ultrastructure) and biochemical (activities of photosynthetic enzymes of the C₃ and C₄ pathways, ¹⁴C labeling of primary photosynthesis products and ¹³C/¹²C carbon isotope fractionation) criteria. The species included S. paulsenii from section Salsola, S. richteri from section Coccosalsola, S. laricina from section Caroxylon, and S. gemmascens from section Malpigipila. The results show that all four species have a C₄ type of photosynthesis in leaves with a Salsoloid type Kranz anatomy, whereas both C₃ and C₄ types of photosynthesis were found in cotyledons. S. paulsenii and S. richteri have NADP- (NADP-ME) C₄ type biochemistry with Salsoloid Kranz anatomy in both leaves and cotyledons. In S. laricina, both cotyledons and leaves have NAD-malic enzyme (NAD-ME) C₄ type photosynthesis; however, while the leaves have Salsoloid type Kranz anatomy, cotyledons have Atriplicoid type Kranz anatomy. In S. gemmascens, cotyledons exhibit C₃ type photosynthesis, while leaves perform NAD-ME type photosynthesis. Since the four species studied belong to different Salsola sections, this suggests that differences in photosynthetic types of leaves and cotyledons may be used as a basis or studies of the origin and evolution of C₄ photosynthesis in the family Chenopodiaceae.This revised version was published online in October 2005 with corrections to the Cover Date.     

Mechanism of the gas phase reactions of the C5H5Ni and C5H5Co ions with substituted pyridines. A combined experimental and theoretical study

Different products have been observed in the reactions of C₅H₅Co⁺ and C₅H₅Ni⁺ ions with halogen-substituted pyridines (XPy) that have been studied by ion trap mass spectrometry (ITMS) techniques. In particular, an addition product C₅H₅M(XPy)⁺ and a product ion C₅H₄M(Py)⁺ corresponding to a loss of a HX molecule (X = F, Cl, Br) have been detected. The relative yield of these products is determined by the nature of the metal and by the nature and position of the halogen on the pyridine ring. A computational study at the DFT level on model-systems formed by 2-fluoro and 2-bromopyridine reacting either with the C₅H₅Ni⁺ or the C₅H₅Co⁺ ion has been carried out. This study shows the existence of a general mechanistic pattern. The rate-determining step of this mechanism is the migration of the halogen from the pyridine ring to the metal. A final hydrogen abstraction step carried out by the halogen leads to the expulsion of a HX molecule. The existence of avoided crossings between surfaces of different multiplicities (ground and first excited state) allows the system to follow lower energy reaction pathways. The barrier determined for the reactions involving 2-bromopyridine is significantly lower than that found for 2-fluoropyridine. This is mainly due to the poor migrating/leaving character and low polarizability of fluorine compared to that of bromine.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Photosynthetic Pathways and Life Form Types for Native Plant Species from Hulunbeier Rangelands,Inner Mongolia,North China

Photosynthetic pathway Types (C₃, C₄, and CAM) and life forms of native species from Hulunbeier rangelands, north China were studied. Of the total 258 species, 216 species in 132 genera and 42 families had C₃ photosynthetic pathway, including dominant herbs, e.g. Stipa baicalensis Roshev. and Leymus chinensis (Trin.) Tzvel., Filifolium sibiricum Kitam. and Arudinella hirta (Thunb.) Koidz. 38 species in 28 genera and 10 families were found with C₄ photosynthesis, and 4 species in 2 genera and 1 family had CAM photosynthetic pathway. The occurrence of C₄ species was common in Gramineae and Chenopodiaceae, and the two families were leading ones within C₄ plants. More than 52 % of the total 258 species were in H form, 21 % in Th form, 19 % in G form; the other life form Types, e.g. Ch, M, N, and HH, formed less than 3 %. 68 % of C₄ species were in Th form and 24 % in H form, indicating that these Types were the dominant life forms for C₄ species in the rangeland region. The occurrence of C₄ species was closely related with plant habitats, disturbed lands had the highest C₄ abundance (55 % of the total C₄ species), followed by grasslands and sandy soil, and forests had the lowest C₄ abundance (8 %). Hence the occurrence of C₄ species could be efficient indicator for rangeland dynamics in Hulunbeier rangelands.     

Redefining the dose of the entomopathogenic fungus Metarhizium brunneum (Ascomycota,Hypocreales) to increase Fe bioavailability and promote plant growth in calcareous and sandy soils

Background and aims

Plants differ in their ability to use different nitrogen (N) chemical forms, these differences can be related to their ecology and drive community structure. The capacity to uptake intact organic N has been observed in plants of several ecosystems. However, soil organic N uptake by Mediterranean plants is unknown despite organic N being abundant in Mediterranean ecosystems. We compare the uptake of different N forms in two widespread coexisting Mediterranean forest trees with contrasting ecophysiological characteristics: Quercus ilex and Pinus halepensis.

Methods

To estimate root uptake rate of each N form we used equimolar solutions (1 mM N) of ¹⁵NO₃ ^?, ¹⁵NH₄ ⁺ and ¹⁵N-¹³C glycine.

Results

NH₄ ⁺ and glycine were taken up at a similar rate, but faster than NO₃ ^? in both species. Intact dual labeled glycine was found in both species, demonstrating that both species can absorb intact organic N.

Conclusions

Despite their ecological differences, both species had similar preference for N forms suggesting no fundamental niche complementarity for N uptake. The higher preference for NH₄ ⁺ and glycine over NO₃ ^? possibly reflects adaptation to the differing proportions of N forms in Mediterranean soils.

     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

Carbonic Anhydrase Activity and Localization in Some Plant Species

The aim of this work concerned the study of the differences in the carbonic anhydrase activity and localization between plant species, the photosynthesis of which is carried out according to the C₃ and C₄ pathways respectively. The measurement of enzymatic activity was made with a titrimetric evaluation of the rate of the reaction CO₂+ H₂O ? H⁺+ HCO^?₃. The C₃ plant species showed higher activities than the C₄ species. The localization of carbonic anhydrase was carried out with a histochemical method. The carbonic anhydrase appeared in the chloroplasts both in the mesophyll and the bundle sheath without any difference between C₃ and C₄ plants.