Carbon and nitrogen dynamics of decomposing leaf litter in a tropical hill evergreen forest

This study was carried out in a tropical hill evergreen forest in northern Thailand. Two tree species Castanopsis accuminatissima and Schima wallichii were the dominant species in the study plot. Decomposition processes of these two species were studied using a litter bag method to evaluate the roles of these species in the nutrient cycling in this hill evergreen forest. The decomposition rates (k) were 0.99–1.05 and 0.55–0.61 in C. accuminatissima and S. wallichii, respectively. In this monsoon forest, the relationship between the onset of rain and the accumulation and decomposition of litter is an important mechanism for linking climate and availability of plant nutrient. Weight loss for both species showed a similar pattern shaped by the seasonal availability of water. However, the nitrogen dynamics were apparently different in the two species. During the rainy season, C. accuminatissima released nitrogen during the decomposition processes, while S. wallichii nitrogen was immobilized in the leaf litter over the rainy season. C. accuminatissima released nitrogen, subsequently available for uptake by trees, while S. wallichii accumulated nitrogen over 15 months and so their roles are important for the retention of nutrients in the hill evergreen forest ecosystem. Thus the two species have different functions in the nutrient dynamics in the forest floor.     

Observations on two narrowly endemic plants, Moehringia minutiflora bornm. and silene viscoriopsis bornm., from prilep,Yugoslavia

Silene viscariopsis and Moehringia minutiflora are Yugoslav endemic plants of very restricted distribution. The habitat of both species at their locus classicus, Markova Kula near Prilep, is described. The two species are dissimilar ecologically, S. viscariopsis being a plant of unshaded grassland on north-facing slopes whereas M. minutiflora is restricted to heavily shaded areas under granite rock formations. M. minutiflora was found in almost all suitable localities investigated at Markova Kula whereas S. viscariopsis is confined to two small colonies in spite of the presence of apparently similar grassland elsewhere on the hill. The possible effects of past changes in the vegetation of Markova Kula on both species are discussed and their present status is assessed. The habitat of S. viscariopsis makes it much more vulnerable than M. minutiflora to changes in land use.     

Analysis of clonal germplasm from five Saccharum species: S. barberi , S. robustum, S. officinarum, S. sinense and S. spontaneum . A study of inter- and intra species relationships using microsatellite markers

In order to study relationships within and among species of the highly complex polyploid genus Saccharum, 30 clones from 5 species (S. barberi, S. officinarum, S. robustum, S. sinense, and S. spontaneum) and 6 cultivated clones were analyzed using 15 sugarcane simple sequence repeat (SSR) markers and two multivariate statistical methods. The origins of two species, S. barberi and S. sinense, are unclear, and it has been hypothesized that they arose from crosses of S. officinarum × S. spontaneum. S. robustum is suggested to be ancestral to S. officinarum, which, when crossed with S. spontaneum, principally comprise the genomes of commercial cultivars. Although our analyses supported S. robustum as being an ancestor of S. officinarum, and show the composition of commercial clones as expected, our results concerning the species S. barberi and S. sinense indicated them to be much less related to the main Saccharum germplasm pool than previous molecular marker investigations, suggesting that introgression from other genera may exist in their ancestry. The differing results of our analysis of S. barberi and S. sinense are likely attributable to more equal and larger numbers of clones studied in each species. Given the extremely high ploidy, the use of only 15 SSR markers enabled clear distinction of Saccharum species and clones, and also identified likely mislabeled accessions. In addition, the replication of DNA extraction, polymerase chain reactions, and fragment electrophoresis increased the robustness of our analysis.     

Toxic effect of landfill leachate on microalgae

Leachates from two landfills, namely Junk Bay (JB) and Gin Drinkers' Bay (GDB), were assessed for their acute toxicity using four green algal species, Chlorella pyrenoidosa, C. vulgaris, Scenedesmus sp. and Dunaliella tertiolecta. JB leachate was more toxic to the four algal species tested than leachate from GDB landfill. The growth rates of all four species in 50% JB leachate, and two species (C. vulgaris and D. tertiolecta) in 50% GDB leachate were significantly lower (P<0.05) than those in the control (Bristol Medium only). Values of 96h-EC50 of JB leachate for all four species were lower than those of GDB. The high contents of ammoniacal-nitrogen and organic compounds (such as volatile fatty acids) seemed to be the factors governing the toxicity of leachate on algae. There were differential sensitivities to leachate exhibited by the tested algal species. Susceptibility to leachates in terms of cell number were in the ascending order of C. pyrenoidosa, Scenedesmus sp., C. vulgaris and D. tertiolecta.     

Karyotype analysis and chromosomal evolution in Asian species of Corchorus (Malvaceae s. l.)

Here, we report the karyotypes and mean haploid idiograms of the ten Asian Corchorus species (2n = 2x = 14). Chromosomes were small, with a mean chromosome length of 2.30 μm. The largest chromosome was recorded in C. pseudo-olitorius (3.50 μm) and the shortest in C. pseudocapsularis (1.60 μm). The karyotypes of the two cultivated species (C. capsularis and C. olitorius) and C. pseudo-olitorius were the most diverse and specialized, whereas those of C. depressus and C. trilocularis were the least diverse. C. fascicularis had the most asymmetrical and C. urticifolius the most symmetrical karyotypes. An increase in genome size was accompanied by increasing karyotype diversity in terms of morphologically distinct chromosome types and interchromosomal asymmetry, with uneven distribution of additional DNA throughout the karyotype. A positive correlation between interchromosomal asymmetry and dispersion index suggested that size differences between chromosomes were mainly associated with karyotype asymmetry. Karyotypes of the Corchorus species became progressively asymmetrical in the course of evolution. Relationships among the ten Corchorus species were defined by using a neighbor-joining tree inferred from inter-simple sequence repeat data. C. fascicularis and C. pseudocapsularis, with shorter karyotypes and smaller genomes, were closely related to C. pseudo-olitorius and C. capsularis, respectively, which were characterized by relatively longer karyotypes and larger genomes. However, the two cultivated species with different levels of interchromosomal asymmetries, dispersion indices and genome sizes were distantly related. Taking this molecular evidence into consideration, we have discussed chromosomal evolution in relation to karyological data including genome size.     

Karyotype characterization reveals an up and down of 45S and 5S rDNA sites in <Emphasis Type="Italic">Crotalaria</Emphasis> (Leguminosae-Papilionoideae) species of the section Hedriocarpae subsection Macrostachyae

The genus Crotalaria is one of the largest within the family Leguminosae-Papilionoideae, with more than 600 species. However, few karyotypes have been described. In the present paper, five species belonging to the section Hedriocarpae were studied (subsection Machrostachyae), in order to better understand chromosomal evolution in Crotalaria. The results reveals that all species presented 2n = 2x = 16 with symmetrical karyotypes, and slight differences in the chromosome morphology. A secondary constriction was identified at short arm of the pair 1. The 45S rDNA was mapped in the secondary constriction and adjacent heterochromatin (NOR-heterochromatin) and a minor site was identified in C. ochroleuca. The 5S rDNA was mapped linked to 45S rDNA at chromosome 1 short arm in all species. Additional sites for 5S rDNA were identified in C. pallida, C. striata and C. mucronata. Heterochromatin blocks around the centromeres are not CMA⁺ neither DAPI⁺. The karyotypes of the subsection Macrostachyae are characterized by an inversion at chromosome pair one in relation to previous specialized floral species analyzed. Additional sites of 45S and 5S rDNA were assumed to be a result of transposition events by different ways. The results suggest heterochromatin differentiation and the position of ribosomal genes indicates chromosomal rearrangements during evolution. Karyotype characteristics corroborate the morphological infrageneric classification.     

Nitrogen uptake pattern of herbaceous plants: coping strategies in altered neighbor species

The mechanisms for maintaining the species diversity of plant communities under conditions of resource limitation is an important subject in ecology. How interspecific relationships influence the pattern of nutrient absorption by coexisting species in N-limited ecosystems is still disputed. We investigated the effect of neighbor species on the uptake of inorganic and organic N by three common plant species using ¹⁵N tracer techniques in a semi-arid alpine steppe on the northern Tibet. The results showed that the plant species varied in their capacity to absorb NO₃ ^?-N, NH₄ ⁺-N, and glycine-N with or without neighbor species. Carex moorcroftii and Leontopodium nanum showed much more plasticity in resource utilization than Stipa purpurea when neighbor species were present. When C. moorcroftii and S. purpurea coexisted, they all increased their ¹⁵N uptake for the NO₃ ^?-N (C. moorcroftii 2.2-fold increase and S. purpurea 2.2-fold increase) and glycine-N treatments (C. moorcroftii 2.9-fold increase and S. purpurea 3.4-fold increase), which indicated that neighborhood had a positive effect for N absorption between the two species. However, L. nanum was a less effective competitor for N utilization than the neighbor species across almost all treatments. The dominant species appeared to have an inhibitory effect on N absorption by the accompanying species in this alpine steppe environment. Thus, interspecific neighbor pairs may result in both a mutually beneficial cooperative relationship and a competitive relationship among neighbors in resource use patterns in extreme environments. Resource use plasticity in altered neighbor species may be due to phenotypic plasticity based on the conditions of the realized niche, offering a valuable insight into niche complementarity and providing a general and important mechanism for resource partitioning in an alpine area.     

The 18S rDNA gene discriminates between red-listed and unexplored ethnomedicinal species of Myristicaceae restricted to humid tropics of India

A potentially diagnostic 18S rDNA (ribosomal DNA) gene was amplified reliably from red-listed ethnomedicinal species of Myristica and its wild and related genera. Individuals from nine species of Myristicaceae were utilized for the study. The sequences ranged from 1,767 to 1,794 nucleotide (nt) in length. The GC content (%) varied from 52.77 to 51.04. The frequencies (%) of nt were A (23.31), T (23.82), C (24.48) and G (28.39). The alignment of all sequences produced 195/1,516 variable sites and 1,257/1,516 conserved sites. Total numbers of single nucleotide polymorphism (SNP) sites found in the alignment were 146/1,516. Knema andamanica (Warb.) W.J. de Wilde was the most distinct that included 18 variable regions and 15 InDel with 27 SNP sites, specific to this species. The identified regions from nine species of Myristica and its wild and closely related genera were deposited in the GenBank Database (Accession numbers JN228257-JN228265). Comparison of morphological identifications and phylogenetic analysis indicated that the specimens were correctly assigned on the basis of a short stretch of 18S rDNA (~1,600 bp) making this a potentially useful marker for the rapid molecular assignment of an unknown related species also. Significant sequence homology ranging from 72 to 99 % was observed on comparison with 18S rDNA genes of other plants in the public domain. A comparison of intraspecific data information of nine 18S with that of 73 matK and 86 rbcL sequences from GenBank revealed that polymorphism, divergence and conservation is higher in 18S locus for Myristicaceae. Hence these markers may be utilized for phylogenetic analysis, evaluation of species richness during ecological surveys or for environmental assessments. These molecular markers are especially important due to the fact that the species studied are mostly vulnerable and red-listed with limited availability in endangered ecological niches.     

Contrasting mitochondrial DNA diversity estimates in Austrian Scutovertex minutus and S. sculptus (Acari, Oribatida, Brachypylina, Scutoverticidae)

Important insights into the evolutionary and demographic history of species can be obtained from inter-specific comparisons of patterns and from determing the degree of genetic diversity. Analysis of sequences of the mitochondrial cytochrome oxidase I (COI) gene revealed remarkable differences in mtDNA diversity estimates and the distribution of mitochondrial lineages between the two closely related oribatid mite species Scutovertex sculptus and S. minutus in Austria. Divergence time estimates revealed an age of approximately 48-74 Myr for the split between the two Scutovertex species and age estimates of about 2-3 and 8-12 Myr for the most recent common ancestors of Austrian S. minutus and S. sculptus, respectively. Genetic diversity was considerably lower in S. minutus than in S. sculptus. A clear geographic sub-division into samples originating from north and south of the Central Alps became evident in S. minutus, whereas no phylogeographic structure was found in S. sculptus. Together with a high genetic diversity this is suggestive of a panmictic population and exceptional dispersal ability, most probably facilitated by phoresy on birds. The lack of sub-structure with regard to habitat types in S. sculptus suggests that this species can cope with a wide range of environmental conditions.     

A seeding experiment for testing tree-community recruitment under variable environments: Implications for forest regeneration and conservation in Mediterranean habitats

As a result of historical and ecological factors, woody-plant communities are composed of species with different biogeographic origins that presumably diverge in their ability to recruit under variable environments. The aim of this study is to evaluate the conservation implications of the differences in recruitment of six tree species of different biogeographical origin coexisting in a Mediterranean mountain forest. Under field conditions, we experimentally sowed a total of 16,650 seeds belonging to two Mediterranean (Quercus ilex and Q. pyrenaica), two Sub-Mediterranean (Acer opalus subsp. granatense and Sorbus aria), and two boreo-alpine (Pinus sylvestris and Taxus baccata) species. We selected the three most common successional scenarios where seeds are dispersed (woodland, shrubland, and open areas), and half of the seeds were irrigated simulating a rainy summer in terms of Mediterranean ecosystems. Seedling survival was strongly affected by the type of habitat as well as irrigation. The recruitment of the six species was possible only in wooded areas under wet conditions (20-60%). In shrubland, only Mediterranean and Sub-Mediterranean species recruited in typical dry years (10-50%), but irrigation increased their survival (25-65%) and allowed P. sylvestris to recruit (32%). Open areas constituted the most adverse scenario for seedling recruitment, especially for non-watered seedlings (0% for boreo-alpine species, 1-24% for the rest). Overall, the benefit of a wet summer was more critical for the recruitment of boreo-alpine species (P. sylvestris and T. baccata) than for the rest, although a rainy summer is also necessary for the recruitment of A. granatense and S. aria in shrubland habitats. Our experimental results indicate that the maintenance of current species composition in Mediterranean mountain forests is critically dependent on the periodic, scattered occurrence of wet summers. From a conservational standpoint, the predictions of less rainy summers and greater aridity in the future imply a change in the relative abundance of species in the mature forest, with Mediterranean species becoming dominant and boreo-alpine species becoming even rarer and, consequently, a serious threat for the persistence of boreo-alpine species at their southern distribution limit.     

Ethnobotanical investigation and diversity of Gbolo (Crassocephalum rubens (Juss. ex Jacq.) S. Moore and Crassocephalum crepidioides (Benth.) S. Moore), a traditional leafy vegetable under domestication in Benin

Thirty five villages randomly selected across ethnic and agro-ecological zones of Benin were surveyed to document the indigenous knowledge associated with the domestication (bringing into cultivation), production and utilization of vegetable Gbolo (Crassocephalum spp.). The study revealed the existence of different morphotypes (1–4 per village) corresponding to two botanical species (Crassocephalum rubens (Juss. ex Jacq.) S. Moore) and (Crassocephalum crepidioides (Benth.) S. Moore) found unequally distributed throughout the country. The domestication (bringing into cultivation from the wild) of Gbolo was started in many villages and with some tribes in southern Benin, the vegetable is already being produced in sole cropping for commercial purposes. Regular consumption (49.90 % of responses) and scarcity (20.30 % of responses) were the two main reasons attributed by the interviewees for domestication of the vegetable. The cultural practices recorded were of five categories. The commonest cropping system found was mixed or inter-row cropping (92.31 % of producers) with either vegetable crops (53.85 % of producers) or non-vegetable crops (38.46 % of users). Sauce prepared with either fresh or dried leaves was the main type of preparation of Gbolo and the vegetable was reported to possess special nutritional and medicinal values. For further promotion, important researches directions are suggested.     

Soil moisture effects on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei,S. glaseri,Heterorhabditis zealandica,and H. bacteriophora

We tested the effect of soil moisture on the performance of four entomopathogenic nematodes species that have recently shown promise for the control of white grubs, i.e., Heterorhabditis bacteriophora, H. zealandica, Steinernema scarabaei, and S. glaseri. Experiments for all four nematodes were conducted in sandy loam, for S. scarabaei also in loamy sand and silt loam. Infectivity was tested by exposing third-instar Japanese beetle, Popillia japonica, to nematodes in laboratory experiments and determining nematode establishment in the larvae and larval mortality. Nematode infectivity was the highest at moderate soil moistures (−10 to −100 kPa), and tended to be lower in wet (−1 kPa) and moderately dry (−1000 kPa) soil. In dry soil (−3000 kPa), only S. scarabaei showed some activity. S. scarabaei was active from −1 to −3000 kPa in all soil types but the range of highest activity was wider in loamy sand (−1 to −1000 kPa) than in loamy sand and silt loam (−10 to −100 kPa). Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with larvae of the greater wax moth, Galleria mellonella. For both Heterorhabditis spp. persistence was short at −10 kPa, improved slightly at −100 kPa, significantly at −1000 kPa, and was the highest at −3000 kPa. Both Steinernema spp. persisted very well at −10 kPa. However, S. glaseri persistence was the shortest at −10 kPa but did not differ significantly at −100 to −3000 kPa, whereas S. scarabaei persistence was not affected by soil moisture. Our observations concur with previous observations on the effect of soil moisture on entomopathogenic nematodes but also show that moisture ranges for infectivity and persistence vary among species. Differences among species may be based on differences in size and behavioral and physiological adaptations.     

Genetic composition of contemporary proprietary U.S. lettuce (Lactuca sativa L.) cultivars

Pedigree history of 146 lettuce (Lactuca sativa L.) cultivars registered in the U.S. by Plant Variety Protection and/or utility patent of the era from 2000 through 2010 facilitates determination of coefficient of parentage among these cultivars, identification of ancestral parental lines, and their genetic contribution. Principal ancestors of leaf lettuce developed in this era are the cultivars ‘Malibu’, ‘Waldmann’s Green’, and ‘Salad Bowl’ contributing 6.4, 6.1, and 3.5% of the genes, respectively. The cultivars ‘Parris Island Cos’ and ‘Tall Guzmaine’ are major ancestors of romaine lettuce, contributing 25.9 and 23.4% of the genes, respectively. Three crisphead lettuce ancestors identified are the cultivars ‘Vanguard’, ‘Salinas’, and ‘Calmar’, the former two descend from interspecific crosses of L. sativa with Lactuca virosa L. and Lactuca serriola L. Among these three, ‘Vanguard’ is the major ancestor contributing 23.8% of the genes to crisphead lettuce. The crisphead cultivar ‘Salinas’ was frequently crossed with romaine lettuce types and the romaine parental cultivar ‘Parris Island Cos’ was crossed with leaf types contributing to romaine and leaf lettuce genetic diversity, respectively. Genetic similarity was less within leaf cultivars (coefficient of parentage = 0.02) than found within romaine (0.15) and crisphead (0.13) cultivars registered in the U.S. during this era.     

The Effect of Temperature on the Accumulation of Cadmium, Copper, Zinc, and Lead by Scirpus acutus and Typha latifolia: A Comparative Analysis

Wetland plants have a capacity for metal sequestration and have been used to remediate such environmental contaminants. How this capacity will be affected by a warming environment is not well-known. Our objective was to identify the effect of small environmentally realistic increases in temperature on metal (cadmium, zinc, lead, and copper) accumulation in Typha latifolia as compared with Scirpus acutus. These common wetland plant species were grown in metal-contaminated sediments at 13??C, 16??C, and 18??C for 3?months to determine the effect of environmentally realistic temperatures on metal accumulation. Cd¹⁰⁹ was used as a radiotracer to study the effect of temperature on uptake kinetics. Growth of the two wetland plants differed markedly; S. acutus displayed linear growth reaching a maximum height of ca. 100?cm; by contrast, T. latifolia grew to ca. 40?cm by day?60 with no further growth occurring over the remainder of the 105-day growth period. S. acutus accumulated more cadmium, lead, and zinc from contaminated sediments than T. latifolia, but only within roots and rhizomes. Although not significant, uptake of cadmium, lead, and zinc by both plants was enhanced under warmer conditions and was most pronounced in S. acutus. This was supported by the radiotracer studies which indicated that under the higher temperatures, there was increased rates of Cd¹⁰⁹ uptake by shoots of S. acutus. By contrast, temperature did not affect Cd¹⁰⁹ uptake rate constants in T. latifolia. S. acutus appears to be more effective at sequestering metals from contaminated sediments; this species as compared to T. latifolia may also be more affected by a warming climate. In the design of wetlands for metal remediation, differences in how these two plants sequester metals from their environment should be considered.     

Fat but slim: Criteria of seed attractiveness for earthworms

Earthworms were shown to significantly affect seeds and seedlings survival via their ingestion and digestion for nutritive purposes. Such selective feeding of earthworms on plant seeds is likely to favour certain plant species and to affect seed bank composition, plant recruitment and plant community structure. Relationships between earthworms and seeds, particularly seed traits that determine attractiveness of seeds for earthworms, are yet to be determined. In this study, the influence of six seed traits was tested on the ingestion, digestion and germination of seeds by two earthworm species (Lumbricus terrestris, anecic and Satchellius mammalis, epigeic). The seed traits tested were their length, width, weight, shape, oil content and the presence of trichomes on their surface. Each earthworm species was introduced into a microcosm with eleven seed species from a chalk grassland that represented those different traits. Ingested, digested and germinated seeds were counted after voiding the guts of the earthworms. Univariate and multivariate analyses showed that seed length, width, weight and seed oil content could significantly affect the ingestion of seeds for both earthworm species. Seed width and seed oil content were the two traits that influenced the digestion of seeds the most, but only for L. terrestris. We also found that seed ingestion was earthworm species-specific but we found no correlation between earthworm traits and number of ingested or digested seeds. Few seeds germinated from L. terrestris casts and no seeds germinated from S. mammalis casts. Implications in terms of plant evolution strategies are further discussed.     

Genetic diversity and origin of cultivated potatoes based on plastid microsatellite polymorphism

The origin of cultivated potatoes has remaining questions. In this study, 237 accessions of all cultivated species and 155 accessions of wild species closely related to cultivated potatoes, including their putative ancestors, were analyzed using 15 plastid microsatellites (SSRs) to investigate genetic diversity and their relationships with the wild species. We here used polymorphic plastid SSRs we developed from potato plastid genome sequences as well as already known plastid SSR markers. All 15 loci were polymorphic and identified a total of 127 haplotypes. Dramatic decreases in levels of genetic diversity were revealed in landraces in comparison with wild ancestor species. The plastid SSR results showed a decrease in haplotype number and diversity from Peru to both north and south. Phylogenetic analysis revealed two distinct groups. One of them, group A, contained the majority of accessions of cultivated species of the Solanum tuberosum Andigenum group including all accessions of cultivated diploid and triploid cytotypes of this group (S. chaucha, S. phureja, and S. stenotomum by a former taxonomic system) and most of tetraploid accessions of the S. tuberosum Andigenum group (S. tuberosum subsp. andigenum), and the majority of accessions of wild ancestors from the northern members of the S. brevicaule complex. Another group B comprised most of the wild species accessions and almost exclusively hybrid cultivated species which have introgressed plastid genomes from the other wild gene pools. Lack of clustering of traditional cultivated species (as used above) support a revised group classification of S. tuberosum.     

Inter-MITE polymorphisms of a newly identified MITE show relationships among sugarcane (Saccharum) species

Sugarcane, one of the most important tropical crops, belongs the genus Saccharum. This genus consists of six species, four cultivated and two wild. The domestication histories of the four cultivated Saccharum species is an interesting and important topic of study. Previous studies have categorized the four cultivated species into two groups, one consisting only of S. edule and the other comprising S. officinarum, S. sinense and S. barberi. All four species have inherited the genomic DNA of S. robustum, one of two wild relative species. Saccharum species have large genomes with complex structures, as evidenced by chromosomes with a high degree of polyploidy, alloploidy and aneuploidy. Miniature inverted-repeat transposable elements (MITEs) are class II (DNA) transposons that disperse throughout the plant genome. In this study, a Tourist family MITE sequence with 18/19-bp terminal inverted repeats and a 2-bp target site duplication was newly identified from genomic DNA of S. robustum. The abundant accumulation of this MITE sequence in the sugarcane genome enabled the application of inter-MITE polymorphism (IMP) analysis to Saccharum. IMP analysis revealed the genetic relationships among all six Saccharum species and the domestication histories of the four cultivated species.     

Investigation of genome polymorphism and seed coat anatomy of species of section Adenolinum from the genus Linum

Twelve species of section Adenolinum from genus Linum have been studied using cytogenetic methods (DAPI/C-banding, Ag-NOR staining, FISH with 5S and 26S rDNA probes), RAPD analysis, and seed surface ultrastructure and seed coat anatomy investigation. Karyotype analysis revealed that these species have a chromosome number of 2n = 18 or 2n = 36. The DAPI/C-banding patterns enabled the identification of all chromosomes and revealed the similarities in chromosome patterns in all of the studied species. It was found that 18-chromosome plants are diploids, whereas 36-chromosome forms are tetraploids. In diploid species, 5S and 26S rRNA genes are located in similar positions on a single pair of chromosomes or on two chromosome pairs in tetraploids. Ag-NOR staining showed NOR activity in both genomes of tetraploids. RAPD analysis and seed coat anatomy investigation revealed some interspecies differences. Species from nearby areas have more similar RAPD characteristics compared with more geographically distant species. The results of this study may serve as an additional argument in favour of a monophyletic origination of the Adenolinum species and its strong isolation from other Linum species.     

Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method

Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N₂ fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the ¹⁵N dilution method. The two Sesbania species as N₂-fixing reference plants and four aquatic weed species as non-N₂-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with ¹⁵N at 0.26 atom % ¹⁵N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % ¹⁵N excess. The results showed that ¹⁵N enrichment of soil NH ₄ ⁺ -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in ¹⁵N enrichment, in both N₂-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in ¹⁵N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N₂-fixing and non-fixing species was estimated by the technique of varying the ¹⁵N level. In the second crop, this ratio in non-fixing species was higher than that in N₂-fixing species. Comparable estimates of % Ndfa were obtained by using ¹⁵N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using ¹⁵N enrichment of non-fixing species and those by using soil NH ₄ ⁺ -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N₂ due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the ¹⁵N dilution method are discussed with reference to these results.