Positive Interactions： Crucial Organizers in a Plant Community

For more than a century, ecologists have concentrated on competition as a crucial process for community organization. However, more recent experimental investigations have uncovered the striking Influence of positive Interactions on the organization of plant communities. Complex combinations of competition and positive interactions operating simultaneously among plant species seem to be widespread In nature. In the present paper, we reviewed the mechanism and ecological importance of positive Interactions In plant communities, emphasizing the certainties and uncertainties that have made It an attractive area of research. Positive Interactions, or facilitation, occur when one species enhances the survival, growth, or richness of another. The Importance of facilitation in plant organization increases with ablotlc stress and the relative Importance of competition decreases. Only by combining plant interactions and the many fields of biology can we fully understand how and when the positive Interactions occur.     

Soil Carbon Changes Following Afforestation with Olga Bay Larch （Larix olgensis Henry） in Northeastern China

After converting cropland to forest, carbon Is sequestered in the aggradlng blomass of the new forests, but the question remains, to what extent will the former arable soil contribute as a sink for CO2？ Quantifying changes In soil carbon Is an Important consideration In the large-scale conversion of cropland to forest. Extensive field studies were undertaken to Identify a number of suitable sites for comparison of soil properties under pasture and forest. The present paper describes results from a study of the effects of first rotation larch on soil carbon In seven stands In an afforestation chronosequence compared with adjacent Korean pine, pasture, and cropland. An adjacent 250-year-old natural forest was Included to give Information on the possible long-term changes In soil carbon In northeast China In 2004. Soil carbon Initially decreased during the first 12 yr before a gradual recovery and accumulation of soil carbon occurred. The Initial （0-12 yr） decrease In soil carbon was an average 1.2% per year among case studies, whereas the Increase In soil carbon （12-33 yr） was 1.90% per year. Together with the carbon sequestration of forest floors, this led to total soil carbon stores of approximately 101.83 Mg/hm^2 over the 33-year chronosequence. Within the relatively short time span, carbon sequestration occurred mainly In tree blomees, whereas soil carbon stores were clearly higher In the 250-year-old plantation （184 Mg/hm^2）. The ongoing redistribution of mineral soil carbon In the young stands and the higher soil carbon contents In the 250-year-old afforested stand suggest that nutrient-rich afforestation soils may become greater sinks for carbon （C） In the long term.     

Leaf Epidermal Cells： A Trap for Lipophilic Xenobiotics

Plant surfaces are covered by a layer of cuticle, which functions as a natural barrier to protect plants from mechanical damage, desiccation, and microbial invasion. Results presented in this report show that the epicuticular wax and the cuticle of plant leaves also play an important role in resisting xenobiotic invasion. Although the epicuticular wax is impermeable to hydrophilic xenobiotics, the cuticle not only restricts the penetration of hydrophilic compounds into leaf cells, but also traps lipophilic ones. The role of the epidermal cells of plant leaves in resisting xenobiotic invasion has been neglected until now. The present study shows, for the first time, that the epidermal cells may reduce or retard the transport of lipophilic xenobiotics into the internal tissues through vacuolar sequestration. Although the guard cells appear to be an easy point of entry for xenobiotics, only a very small proportion of xenobiotics present on the leaf surface actually moves into leaf tissues via the guard cells .     

转基因(Cry1Ac)抗虫棉对土壤微生物的影响

对不同种植年限的转基因(Cry1Ac)抗虫棉田土壤中主要微生物数量变化进行了测定,结果表明：（1）种植转基因抗虫棉后对土壤微生物细菌、放线菌和真菌数量的影响趋势基本相似;（2）种植抗虫棉1年后,土壤中细菌、放线菌、真菌数量将有所增加,连续4年达到高峰,然后数量开始下降,连续种植7年后,棉田微生物数量接近于种植1年的棉田;（3）种植1年后又种植非转基因棉花的棉田,土壤微生物数量低于种植1年抗虫棉的棉田,与种植非转基因棉的数量无明显差异。由此可见,种植转基因抗虫棉对土壤微生物数量有一定的影响。     

ISSR Analysis of the Genetic Diversity of the Endangered Species Sinopodophyllum hexandrum （Royle） Ying from Western Sichuan Province, China

Slnopodophyllum hexandrum （Royle） Ying Is an Important medicinal and endangered species. Inter-simple sequence repeats （ISSR） analysis was conducted on seven natural populations from western Slchuan Province to Investigate the genetic diversity of S. hexandrum. Leaf samples of 140 Individuals were collected. Of the 139 discernible fragments generated by 12 selected primers （among 100 primers）, 54 appeared to be polymorphlc. The percentage of polymorphlc bands （PPB） was 38.85% at the species level, and PPB within a population ranged from 7.91% to 23.74%. Low levels of genetic variation （He = 0.092, Ho = 0.142） and high levels of genetic differentiation among the populations （Gst= 62.25%） was detected on the basis of results from POPGENE and analysis of molecular variance （AMOVA）, respectively. Furthermore, the limited gene flow （Nm = 0.361） may result from biological characteristics, such as self-pollination and short distance seed dispersal. Based on the genetic and ecological Information available for S. hexandrum, we propose some appropriate strategies for the conservation of the endangered medicinal species in this region, namely rescuing and conserving the core populations for in situ conservation and sampling and preserving more populations with fewer Individuals from each population for ex situ conservation.     

A Cytotoxic Neolignan from Schisandra propinqua （Wall.） Baill.

In the course of our study of bioactive natural products from Schisandra plants, we isolated a neolignan from an EtOAc extract of the stems of Schisandra propinqua （Wall.） Baill. The structure of the new com- pound was determined to be 4, 4-di （4-hydroxy-3-methoxyphenly）-2, 3-dimethylbutanol （compound 1） on the basis of 1H- and 13C-NMR spectra and 2D NMR methods. Eight known compounds, compounds 2-9, were also isolated and identified, of which compounds 3, 4, 6 and 9 were isolated for the first time from this plant. In addition, compounds 1-4 were evaluated for cytotoxicity by an 3-（4,5-dimethyl-2 thiazoyl）-2,5-diphenyl-2H- tetrazolium bromide （MTT） assay. Compound 1 showed significant potential cytotoxic ability in the bioassay.     

Vegetation and Environment History for the Past 14 000 yr BP from Dingnan, Jiangxi Province, South China

A Late Pleistocene-Holocene pollen, phosphorus, and charcoal record was reconstructed from a peatland in southern Jiangxi Province in southern China. The area today has a mountainous and rolling landscape with villages, small towns, and agriculture dominated by rice paddies, vegetable, and fruit gardens, as well as areas of secondary forest and pine re-afforestation. The record opens before 14 300 yr BP, with Alnus woodland dominating the wetland areas and with an open Quercus woodland on the surrounding slopes. The forest area becomes more complex from approximately 12 800 yr BP and further from 9 000 yr BP. At approximately 6 000 yr BP, there is evidence of clearing and, by 4 500-4 000 yr BP, a complete collapse in the wetland Alnus and terrestrial forest as the low-lying areas are converted to rice production. For much of the record, the occurrence of fire around the site was low, although there is evidence of regional fires. Fire was used as a tool in clearing and then used in the annual cycles of stubble burning after rice harvest. Nutrient levels, as reflected by total phosphorus in the sediment, seem to be closely related to forest changes and high values in the surface layers probably result from land-management techniques associated with agriculture. Therefore, human impact greatly altered forest cover, fire frequency, and nutrient dynamics; this has been evident for approximately 6 000 yr BP and then intensities towards the present day.     

Anthocyanin Accumulation Mediated by Blue Light and Cytokinin in Arabidopsis Seedlings

It has been reported that pigmentation In plants Is stimulated by light and cytoklnln （CTK）; however, the signaling pathways and the relationship between light and CTK Involved In the regulation of anthocyanln accumulation remain to be elucidated. We Investigated （i） the role of blue light （BL） and CTK In anthocyanln accumulation ; and （ii） the relationship between BL and CTK In wild type （WT） and by4 mutants of Arabidopsis thaiiana. Two-d-old seedlings grown on medium with or without klnetln （KT） or zeatln （ZT） In darkness were Irradiated using BL at different fluence rates for 3 d before the anthocyanln content was determined using a spectrophotometrlc method. Anthocyanln accumulation was strongly Induced by BL In WT seedlings but not In hy4 seedlings, which demonstrated that CRY1 Is the main photoreceptor for BL. Both KT and ZT enhanced the response of the WT seedlings to BL In a dose-dependent manner, whereas they were not sufficient to promote anthocyanln eccumulatlon In darkness. In addition, data from experiments using the hy4 mutant showed that the CTK effect of BL was also CRYl-dependent. The results from experiments with three different treatment programs showed that the relationship between BL and KT In anthocyanln accumulation of Arabidopsis seedlings seems neither muItlpllcatlve nor additive coactlon, but rather Interaction. BL Is necessary for anthocyanln accumulation, and KT might be Involved In the BL signaling pathway.