Molecular characterisation and origin of the Coffea arabica L. genome

Restriction fragment length polymorphism (RFLP) markers were used in combination with genomic in situ hybridisation (GISH) to investigate the origin of the allotetraploid species Coffea arabica (2n = 44). By comparing the RFLP patterns of potential diploid progenitor species with those of C. arabica, the sources of the two sets of chromosomes, or genomes, combined in C. arabica were identified. The genome organisation of C. arabica was confirmed by GISH using simultaneously labelled total genomic DNA from the two putative genome donor species as probes. These results clearly suggest that C. arabica is an amphidiploid formed by hybridisation between C. eugenioides and C. canephora, or ecotypes related to these diploid species. Our results also indicate low divergence between the two constituent genomes of C. arabica and those of its progenitor species, suggesting that the speciation of C. arabica took place relatively recently. Precise localisation in Central Africa of the site of the speciation of C. arabica, based on the present distribution of the coffee species, appears difficult, since the constitution and extent of tropical forest has varied considerably during the late Quaternary period. Received: 6 June 1998 / Accepted: 10 November 1998     

Genetic and physical mapping of the S<Subscript>H</Subscript>3 region that confers resistance to leaf rust in coffee tree (<Emphasis Type="Italic">Coffea arabica</Emphasis> L.)

Resistance to coffee leaf rust is conferred by S_H3, a major dominant gene that has been introgressed from a wild coffee species Coffea liberica (genome L) into the allotetraploid cultivated species, Coffea arabica (genome C^aE^a). As the first step toward the map-based cloning of the S_H3 gene, using a bacterial artificial chromosome (BAC) library, we describe the construction of a physical map in C. arabica spanning the resistance locus. This physical map consists in two homeologous BAC-contigs of 1,170 and 1,208 kb corresponding to the subgenomes C^a and E^a, respectively. Genetic analysis was performed using a single nucleotide polymorphism detection assay based on Sanger sequencing of amplicons. The C. liberica-derived chromosome segment that carries the S_H3 resistance gene appeared to be introgressed on the sub-genome C^a. The position of the S_H3 locus was delimited within an interval of 550 kb on the physical map. In addition, our results indicated a sixfold reduction in recombination frequency in the introgressed S_H3 region compared to the orthologous region in Coffea canephora.     

In situ hybridization identifies the diploid progenitor species of Coffea arabica (Rubiaceae)

 The most important commercial coffee species, Coffea arabica, which is cultivated in about 70% of the plantations world-wide, is the only tetraploid (2n=4x=44) species known in the genus. Genomic in situ hybridization (GISH) and fluorescent in situ hybridization (FISH) were used to study the genome organization and evolution of this species. Labelled total genomic DNA from diploid species (C. eugenioides, C. congensis, C. canephora, C. liberica) closely related to C. arabica was separately used as a probe in combination with or without blocking DNA to the chromosome spreads of C. arabica. GISH discriminated between chromosomes of C. arabica only in the presence of an excess of unlabelled block DNA from the species not used as a probe. Among the range of different species combinations used, DNA from C. eugenioides strongly and preferentially labelled 22 chromosomes of the tetraploid C. arabica, while the remaining 22 chromosomes were labelled with C. congensis DNA. The similarity of observations between C. arabica and the two diploid species using two ribosomal genes with FISH with respect to metaphase chromosomes provided additional support to the GISH results. These results confirm the allopolyploid nature of C. arabica and show that C. congensis and C. eugenioides are the diploid progenitors of C. arabica. Received: 2 February 1998 / Accepted: 12 May 1998     

Prediction of the heterosis of <Emphasis Type="Italic">Laminaria</Emphasis> hybrids with the genetic distance between their parental gametophyte clones

Eighteen microsatellite markers were used to determine the genetic distances between the parental gametophyte clones of 14 Laminaria hybrids, which were then used to establish a linear relationship with the heterosis (hybrid vigor) of economic traits including yield, mean blade try weight, mean blade fresh weight, blade length, blade width and mean blade thickness using regression analysis. Significant regression was found between the genetic distance (x) and the heterosis (y) of yield (y = 115.10x − 77.97, r = 0.8151, p = 0.00038), mean blade dry weight (y = 115.23x  −77.97, r = 0.8154, p = 0.00038), mean blade fresh weight (y = 100.08x − 57.85, r = 0.7306, p = 0.0030) and blade length (y = 204.11x − 46.77, r = 0.6963, p = 0.00566). The prediction of the heterosis of Laminaria hybrids with the genetic distance between parental gametophyte clones will facilitate the selection of elite Laminaria hybrids by avoiding the time-consuming and labor-intensive trait evaluation of a large number of hybridization combinations.     

Karyology and cytotaxonomy of the genus Passiflora L. (Passifloraceae)

 The chromosomes of 31 species of Passiflora, distributed throughout the subgenera Astrophea, Calopathanthus, Distephana, Dysosmia, Passiflora, Plectostemma and Tacsonia were analysed. Three different karyotypes were observed: 2n = 12, 24, 36; 2n = 18, 72 and 2n = 20. The karyotype of these species was almost always constituted of metacentric and submetacentric chromosomes with variable karyotype symmetry. In the group with x = 6, represented by the subgenus Plectostemma, six diploid species with 2n = 12, one tetraploid with 2n = 24 (P. suberosa) and an intraspecific polyploid with 2n = 12, 36 (P. misera) were analysed. P. pentagona (subgenus Astrophea) may also be included in this karyological group since it presents 2n = 24 and may be of polyploid origin, with x = 6. The interphase nuclei in this group were areticulate, except those of P. morifolia and P. pentagona with semi-reticulate characteristics. Two small terminal heterochromatic blocks, positive for chromomycin A₃, were identified in the largest chromosome pair of P. capsularis and P. rubra, species very closely related, while P. tricuspis displayed four chromosomes with proximal blocks. In the group with x = 9, represented mainly by subgenus Passiflora, 20 species with 2n = 18 and one with 2n = 72 were studied. They presented chromosomes larger than those species with x = 6 and interphase nuclei of semi-reticulate type, except for P. mixta with areticulate nuclei. Four terminal CMA⁺ blocks were observed in P. edulis, six blocks in P. caerulea and P. racemosa, while five blocks were observed in the single P. amethystina plant analysed. P. foetida (subgenus Dysosmia), the only species with 2n = 20, exhibited six chromosomes with CMA⁺ blocks and interphase nuclei of the areticulate type. The meiotic analysis of representatives of the three groups (P. foetida, P. suberosa, P. cincinnata and P. racemosa) always presented regular pairing and regular chromosome segregation, except in P. jilekii where a tetravalent was observed. The analysis of the chromosome variation within the genus and the family suggests that the base number of Passiflora may be x₁ = 6 or x₁ = 12, whereas x₂ = 9 is only an important secondary base number. Received April 11, 2000 Accepted October 5, 2000     

Meiosis in hybrids between tree coffees (C. liberica,C. dewevrei) and Nandi coffee (C. eugenioides)

Meiotic behaviour of hybrids between Coffea dewevrei var. excelsa and C. eugenioides, and between C. liberica and C. eugenioides, and of open pollinated progeny of these hybrids reveals a close genome relationship in spite of striking morphological differences and distinct geographic separation. In the cross C. dewevrei excelsa X C. eugenioides several maternal types occurred, attributed to parthenogenesis. The hybrids are self-sterile and set few seeds on open pollination, again partly involving parthenogenesis, and participation of some unreduced gametes in hybridization.Subdivision of the section Eucoffea into different subsections (C. dewevrei and C. liberica belonging to subsection Pachycoffea, C. eugenioides to Erythrocoffea) has no cytological basis.     

The allotetraploid <Emphasis Type="Italic">Arabidopsis thaliana–Arabidopsis lyrata</Emphasis> subsp. <Emphasis Type="Italic">petraea</Emphasis> as an alternative model system for the study of polyploidy in plants

Polyploidy is known to be common in plants and recent work has focused on the rapid changes in genome structure and expression that occur upon polyploidization. In Arabidopsis, much of this work has been done on a synthetic allotetraploid obtained by crossing a tetraploid Arabidopsis thaliana (2n = 4x = 20) with A. arenosa (2n = 4x = 32). To explore an alternative route to polyploidy in this model species, we have developed a synthetic allopolyploid by crossing two diploid species: A. thaliana (2n = 2x = 10) and Arabidopsis lyrata subsp. petraea (2n = 2x = 16). F₁ hybrids were easy to obtain and phenotypically more similar to A. lyrata. Spontaneous chromosome doubling events occurred in about 25% of the F₁s, thus restoring fertility. The resulting allotetraploids (2n = 26) exhibited many genomic changes typically reported upon polyploidization. Nucleolar dominance was observed as only the A. lyrata rDNA loci were expressed in the F₁ and allotetraploids. Changes in the degree of methylation were observed at almost 25% of the loci examined by MSAP analysis. Finally, structural genomic alterations did occur as a large deletion covering a significant portion of the upper arm of chromosome II was detected but no evidence of increased mobility of transposons was obtained. Such allotetraploids derived from two parents with sequenced (or soon to be sequenced) genomes offer much promise in elucidating the various changes that occur in newly synthesized polyploids.     

Relationship between parental chromosomic contribution and nuclear DNA content in the coffee interspecific hybrid C. pseudozanguebariae×C. liberica var ‘dewevrei’

 F₁ hybrids were obtained between two coffee species with the same chromosome number (2n=22) but with different nuclear DNA contents [C. pseudozanguebariae (PSE) 2C=1.13 pg and C. liberica var ‘dewevrei’ (DEW) 2C=1.42 pg]. G2 hybrids were obtained by open-pollination of the F₁ hybrids. Genomic in situ hybridisation (GISH) and flow cytometry were used on six F₁ hybrids and seven G2 hybrids to determine their parental chromosomic contribution and their nuclear DNA content (qDNA), respectively. GISH efficiently identified chromosomes from both species. F₁ hybrids had a qDNA intermediate between that of the parental species and contained the expected 11 chromosomes from each species. There was a linear relationship between the number of PSE chromosomes and the nuclear DNA content, which indicates that flow cytometry can be used to give a rough estimate of the parental chromosomic contribution in G2 hybrids. Received: 1 August 1997/Accepted: 25 August 1997     

Determination of shelf life of <Emphasis Type="Italic">Spirulina platensis</Emphasis> (MI<Subscript>2</Subscript>) grown in the Philippines

Gamma linolenic acid (GLA) degradation in Spirulina followed first-order reaction kinetics. At an accelerated temperature range of 45 to 55°C, the degradation rate constants (k _r) of GLA obtained were 4.0 × 10⁻² to 8.8 × 10⁻² day⁻¹. The energy of activation (E _a) was 16.53 kcal mol⁻¹, and the Q₁₀ was 2.22. Based on 20% GLA degradation, the shelf life of sun-dried Spirulina at 30°C is 263 days or 8.6 months using the Arrhenius plot, and 258 days or 8.5 months using the Q ₁₀ approach. Presented at the 6th Meeting of the Asia Pacific Society of Applied Phycology, Manila, Philippines.     

Analysis of alien introgression in coffee tree (<Emphasis Type="Italic">Coffea</Emphasis><Emphasis Type="Italic">arabica</Emphasis> L.)

The transfer of desired traits from related wild diploid Coffea species into the cultivated allotetraploid C. arabica is essential in coffee breeding to develop pest/disease-resistant cultivars. The present work is an attempt to gain insights into alien introgression in C. arabica. An F2 population derived from a cross between T5296 and Et6 was analysed with simple sequence repeat (SSR; microsatellite) and amplified fragment length polymorphism (AFLP) molecular markers. The T5296 is a derivative of an interspecific hybrid introgressed by the diploid C. canephora species and Et6 is a wild Ethiopian accession of C. arabica. The origin of the revealed polymorphism was determined by comparisons using representative accessions from C. arabica and its two diploid parental species, C. eugenioides and C. canephora. The number and mode of inheritance of canephora-introgressed segments were investigated, as well as their sub-genome localisation and rate of recombination. The results suggested that the transfer of desirable genes into C. arabica from C. canephora is not limited by the ploidy level differences or the suppression of recombination between the different genomes.     

A new karyotype for <Emphasis Type="Italic">Cavia magna</Emphasis> (Rodentia: Caviidae) from an estuarine island and <Emphasis Type="Italic">C. aperea</Emphasis> from adjacent mainland

We describe a new karyotype for Cavia magna Ximenez, 1980 from an estuarine island and the karyotype of Cavia aperea Erxleben, 1777 from an adjacent mainland. The species have differences in diploid number (2n), autosomal fundamental number, quantity, and distribution of heterochromatin as dissimilar distributions of the nucleolus-organizing regions (Ag-NORs). The C. aperea karyotype has a diploid number of 64 as previously reported for C. aperea and most other Cavia species. In contrast, this new C. magna karyotype exhibits a variant diploid number of 2n = 62, considering that previous work reported a karyotype of 2n = 64 for C. magna. The discovery of a distinct diploid number within C. magna represents the first record of intra-specific chromosomal variation in a species of Caviidae. The diploid number of 2n = 62, heterochromatin quantity, Ag-NOR distributions, and inversed X chromosome from this population of C. magna are as seen in the geographically proximate (Cavia intermedia Cherem Olimpio and Ximenez; intermediate Cavy). These data provide further evidence supporting C. magna as the sister species of C. intermedia.     

Contributions of C3 and C4 plants to higher trophic levels in an Amazonian savanna

We studied the energy flow from C₃ and C₄ plants to higher trophic levels in a central Amazonian savanna by comparing the carbon stable-isotope ratios of potential food plants to the isotope ratios of species of different consumer groups. All C₄ plants encountered in our study area were grasses and all C₃ plants were bushes, shrubs or vines. Differences in δ¹³C ratios among bushes (xˉ = −30.8, SD = 1.2), vines (xˉ = −30.7, SD = 0.46) and trees (xˉ = −29.7, SD = 1.5) were small. However the mean δ¹³C ratio of dicotyledonous plants (xˉ = −30.4, SD = 1.3) was much more negative than that of the most common grasses (xˉ = −13.4, SD = 0.27). The insect primary consumers had δ¹³C ratios which ranged from a mean of −29.5 (SD = 0.47) for the grasshopper Tropidacris collaris to a mean of −14.7 (SD = 0.56) for a termite (Nasutitermes sp.), a range similar to that of the vegetation. However, the common insectivorous and omnivorous vertebrates had intermediate values for δ¹³C, indicating that carbon from different autotrophic sources mixes rapidly as it moves up the food chain. Despite this mixing, the frogs and lizards generally had higher values of δ¹³C (xˉ = −21.7, SD = 1.6; xˉ = −21.9, SD = 1.8, respectively) than the birds (xˉ = −24.8, SD = 1.8) and the only species of mammal resident in the savanna (xˉ = −25.4), indicating that they are generally more dependent on, or more able to utilise, food chains based on C₄ grasses. Received: 7 May 1998 / Accepted: 30 November 1998     

The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (permeability coefficient, P_s), and reflection coefficients (σ_s) were measured on internodes of Chara corallina, Klein ex Willd., em R.D.W.. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 °C which provided evidence about pathways of water and solutes as they move across the plasma membrane (water channel and bilayer arrays). Test solutes used to check for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accuracy, Q₁₀ values for Lp and for the diffusive water permeability (P_d) were identical (Q₁₀ for Lp = 1.29 ± 0.17 (± SD; n = 15 cells) and Q₁₀ for P_d = 1.25 ± 0.16 (n = 5 cells)). The Q₁₀ values were equivalent to activation energies of E_a = 16.8 ± 6.4 and 16.6 ± 10.0 kJ · mol⁻¹, respectively, which is similar to that of self-diffusion or of viscous flow of water. The Q₁₀ values and activation energies for P_s of the alcohols were significantly larger (ethanol: Q₁₀ = 1.68 ± 0.16, E_a = 37.1 ± 5.9 kJ · mol⁻¹; n-propanol: Q₁₀ =  1.75 ± 0.40, E_a = 43.1 ± 15.3 kJ · mol⁻¹; iso-propanol: Q₁₀ = 2.12 ± 0.42, E_a =  52.2 ± 14.6 kJ · mol⁻¹; tert-butanol: Q₁₀ = 2.13 ± 0.56, E_a = 51.6 ± 17.1 kJ · mol⁻¹; ±SD; n = 5 to 6 cells). Effects of temperature on reflection coefficients were most pronounced. With increasing temperature, σ_s values of the alcohols decreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary and isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in σ_s values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of σ_s with temperature was theoretically predicted from the temperature dependence of P_s and Lp. Differences between predicted and measured values of σ_s allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of test solutes. It is concluded that water channels are much less selective than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of σ_s. The σ_s of HDO was extremely low. Its response to temperature was opposite to that for the σ_s of the alcohols. This suggested a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a differential sensitivity of different mechanisms to temperature. Received: 10 October 1996 / Accepted: 2 December 1996     

Production and characterization of intergeneric somatic hybrids between <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">Orychophragmus violaceus</Emphasis> and their backcrossing progenies

Alien chromosome addition lines have been widely used for identifying gene linkage groups, assigning species-specific characters to a particular chromosome and comparing gene synteny between related species. In plant breeding, their utilization lies in introgressing characters of agronomic value. The present investigation reports the production of intergeneric somatic hybrids Brassica napus (2n = 38) + Orychophragmus violaceus (2n = 24) through asymmetric fusions of mesophyll protoplasts and subsequent development of B. napus-O. violaceous chromosome addition lines. Somatic hybrids showed variations in morphology and fertility and were mixoploids (2n = 51–67) with a range of 19–28 O. violaceus chromosomes identified by genomic in situ hybridization (GISH). After pollinated with B. napus parent and following embryo rescue, 20 BC₁ plants were obtained from one hybrid. These exhibited typical serrated leaves of O. violaceus or B. napus-type leaves. All BC₁ plants were partially male fertile but female sterile because of abnormal ovules. These were mixoploids (2n = 41–54) with 9–16 chromosomes from O. violaceus. BC₂ plants showed segregations for female fertility, leaf shape and still some chromosome variation (2n = 39–43) with 2–5 O. violaceus chromosomes, but mainly containing the whole complement from B. napus. Among the selfed progenies of BC₂ plants, monosomic addition lines (2n = 39, AACC + 1O) with or without the serrated leaves of O. violaceus or female sterility were established. The complete set of additions is expected from this investigation. In addition, O. violaceus plants at diploid and tetraploid levels with some variations in morphology and chromosome numbers were regenerated from the pretreated protoplasts by iodoacetate and UV-irradiation. Z. Zhao and T. Hu make equal contributions to this work.     

Observations on the division characterization of diploid nuclear in <Emphasis Type="Italic">Porphyra</Emphasis> (Bangiales,Rhodophyta)

Nuclear divisions of carpospores, conchocelis and conchospores of Porphyra yezoensis, P. haitanensis, P. katadai var. hemiphylla and P. oligospermatangia from China were investigated. The observations showed diploid chromosome numbers of 2n = 6 for P. yezoensis and P. oligospermatangia, and 2n = 10 for P. haitanensis and P. katadai var. hemiphylla. For all four species, somatic pairing of chromosome sets was observed in late prophase. Sister chromosomes separated at anaphase as mitosis took place in carpospores, conchocelis filamentous cells, conchosporangial branch cells and sporangial cells (conchospore formation). Chromosome configurations of tetrad and ring-shaped in conchospore germination were observed, demonstrating the occurrence of meiosis. The characteristics of diploid nuclear division in 2n = 6 species are the same as those of 2n = 10 species. The influence of somatic pairing on nuclear division of diploid cells in Porphyra was discussed.     

Ochratoxin A production in aniseed-based media by selected fungal strains and in anise fruits (<Emphasis Type="Italic">Pimpinella anisum</Emphasis> L.)

The growth conditions and ochratoxin A (OTA) production of Aspergillus strains were studied in aniseed (Pimpinella anisum L.)-based media. The results showed that methanol/NaHCO₃ (50:50, v/v) mixture for extraction and competitive direct ELISA analytical method are capable of detecting low OTA concentrations in this raw material, which were confirmed by HPLC with fluorescence detection (R ² = 0.994). In aniseed meal extract agar artificially contaminated with selected fungi, the higher OTA values obtained were 283.8 ± 28.1 μg L^-1 for A. carbonarius and between 1.7 ± 0.1 μg L^-1 and 16.5 ± 12.8 μg L^-1 for A. steynii strains. While the optimal conditions of growth for A. carbonarius and A. steynii are 28°C and 0.98 a _w, the optimal production of OTA was observed at 0.99 a _w for both A. carbonarius and A. steynii but at 22°C and 28°C, respectively. Except in one sample, all the aniseed samples analysed were negative for OTA natural contamination. This study demonstrates that aniseed can be a matrix capable to contamination with OTA, at least produced by A. carbonarius and A. steynii strains, regardless of the antimicrobial properties of aniseed essential oil.     

Distribution of <Emphasis Type="Italic">Divo</Emphasis> in <Emphasis Type="Italic">Coffea</Emphasis> genomes,a poorly described family of angiosperm LTR-Retrotransposons

Coffea arabica (the Arabica coffee) is an allotetraploid species originating from a recent hybridization between two diploid species: C. canephora and C. eugenioides. Transposable elements can drive structural and functional variation during the process of hybridization and allopolyploid formation in plants. To learn more about the evolution of the C. arabica genome, we characterized and studied a new Copia LTR-Retrotransposon (LTR-RT) family in diploid and allotetraploid Coffea genomes called Divo. It is a complete and relatively compact LTR-RT element (~5 kb), carrying typical Gag and Pol Copia type domains. Reverse Trancriptase (RT) domain-based phylogeny demonstrated that Divo is a new and well-supported family in the Bianca lineage, but strictly restricted to dicotyledonous species. In C. canephora, Divo is expressed and showed a genomic distribution along gene rich and gene poor regions. The copy number, the molecular estimation of insertion time and the analysis at orthologous locations of insertions in diploid and allotetraploid coffee genomes suggest that Divo underwent a different and recent transposition activity in C. arabica and C. canephora when compared to C. eugenioides. The analysis of this novel LTR-RT family represents an important step toward uncovering the genome structure and evolution of C. arabica allotetraploid genome.     

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish <Emphasis Type="Italic">Pterygoplichthys disjunctivus</Emphasis> (Siluriformes,Loricariidae)

A feeding trial was performed in the laboratory with the catfish species Pterygoplichthys disjunctivus to determine stable carbon (¹³C) and nitrogen (¹⁵ N) turnover rates and discrimination factors in non-lethally sampled tissues (red blood cells, plasma solutes, and fin). A second feeding trial was conducted to determine what P. disjunctivus could assimilate from low-quality wood-detritus—refractory polysaccharides (e.g., cellulose), or soluble wood-degradation products inherent in wood-detritus. This was performed by feeding the fish an artificial wood-detritus diet with fibrous (δ¹³C = −26.36‰; δ¹⁵  N = 2.13‰) and soluble portions (δ¹³C = −11.82‰; δ¹⁵  N = 3.39‰) that had different isotopic signatures and monitoring the dynamics of isotopic incorporation in the different tissues over time. Plasma solutes turned over more quickly than red blood cells for ¹³C and ¹⁵ N. However, in contrast to previous studies of juvenile fishes, C and N incorporation was primarily driven by catabolic tissue turnover as opposed to growth rate. Tissue-diet discrimination factors for ¹⁵ N varied from 4.08 to 5.17‰, whereas they were <2‰ for ¹³C (and less than 0.3‰ for plasma and red blood cells). The results of trial two suggested that P. disjunctivus could not assimilate refractory polysaccharides. Moreover, the δ¹³C and δ¹⁵ N signatures of wild-caught P. disjunctivus from Florida confirmed their detrital trophic standing in Floridian aquatic ecosystems.     

Comparison of Nitrogen Fixation for North- and South-facing <Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> Stands in Central Korea

The nitrogenase activity, root nodule biomass, and rates of nitrogen (N) fixation were measured in 25-year-old pure north- and south-facing Robinia pseudoacacia stands in an urban forest of Seoul (Kkachisan Mountain) in central Korea. The nitrogenase activity was estimated using an acetylene reduction (AR) assay, which showed an increasing trend during the early growing season, with sustained high rates from June through to September with a decrease thereafter. July had the highest nitrogenase activity rate (micromoles C₂H₄ per gram dry nodule per hour), averaging 95.8 and 115.1 for the north- and south-facing stands, respectively. The maximum root nodule biomass (kilograms per hectare) was 45.7 and 9.1 for the north- and south-facing stands in July, respectively. The AR rate appeared to be strongly correlated to the soil temperature (r ² = 0.68, P < 0.001) and soil pH (r ² = 0.59, P < 0.001) while root nodule biomass was correlated to the soil temperature (r ² = 0.36, P < 0.01) and water content (r ² = 0.35, P < 0.05). The soil temperature showed clear differences between seasons, while there was a significant difference in soil pH, organic matter, total N concentrations, and available phosphorus between the north- and south-facing stands. The N₂ fixation rates during the growing season varied from 0.1 to 37.5 kg N ha⁻¹ month⁻¹ depending on the sampling location and time. The annual N₂ fixation rate (kg N per hectare per year) was 112.3 and 23.2 for the north- and south-facing stands, respectively. The differences in N₂ fixation rate between the two stands were due mainly to the differences in total nodule biomass.     

Compensatory effects of elevated CO<Subscript>2</Subscript> concentration on the inhibitory effects of high temperature and irradiance on photosynthetic gas exchange in carrots

We determined the interactive effects of irradiance, elevated CO₂ concentration (EC), and temperature in carrot (Daucus carota var. sativus). Plants of the cv. Red Core Chantenay (RCC) were grown in a controlled environmental plant growth room and exposed to 3 levels of photosynthetically active radiation (PAR) (400, 800, 1 200 μmol m⁻² s⁻¹), 3 leaf chamber temperatures (15, 20, 30 °C), and 2 external CO₂ concentrations (C _a), AC and EC (350 and 750 μmol mol⁻¹, respectively). Rates of net photosynthesis (P _N) and transpiration (E) and stomatal conductance (g _s ) were measured, along with water use efficiency (WUE) and ratio of internal and external CO₂ concentrations (C _i/C _a). P _N revealed an interactive effect between PAR and C _a. As PAR increased so did P _N under both C _a regimes. The g _s showed no interactive effects between the three parameters but had singular effects of temperature and PAR. E was strongly influenced by the combination of PAR and temperature. WUE was interactively affected by all three parameters. Maximum WUE occurred at 15 °C and 1 200 μmol m⁻² s^{− 1} PAR under EC. The C _i /C _a was influenced independently by temperature and C _a. Hence photosynthetic responses are interactively affected by changes in irradiance, external CO₂ concentration, and temperature. EC significantly compensates the inhibitory effects of high temperature and irradiance on P _N and WUE.