Accumulation of arachidonic acid-rich triacylglycerols in the microalga Parietochloris incisa (Trebuxiophyceae,Chlorophyta)

The freshwater green microalga Parietochloris incisa is the richest known plant source of the polyunsaturated fatty acid (PUFA), arachidonic acid (20:4omega6, AA). While many microalgae accumulate triacylglycerols (TAG) in the stationary phase or under certain stress conditions, these TAG are generally made of saturated and monounsaturated fatty acids. In contrast, most cellular AA of P. incisa resides in TAG. Using various inhibitors, we have attempted to find out if the induction of the biosynthesis of AA and the accumulation of TAG are codependent. Salicylhydroxamic acid (SHAM) affected a growth reduction that was accompanied with an increase in the content of TAG from 3.0 to 6.2% of dry weight. The proportion of 18:1 increased sharply in all lipids while that of 18:2 and its down stream products, 18:3omega6, 20:3omega6 and AA, decreased, indicating an inhibition of the Delta12 desaturation of 18:1. Treatment with the herbicide SAN 9785 significantly reduced the proportion of TAG. However, the proportion of AA in TAG, as well as in the polar lipids, increased. These findings indicate that while there is a preference for AA as a building block of TAG, the latter can be produced using other fatty acids, when the production of AA is inhibited. On the other hand, inhibiting TAG construction did not affect the production of AA. In order to elucidate the possible role of AA in TAG we have labeled exponential cultures of P. incisa kept at 25 degrees C with [1-14C]arachidonic acid and cultivated the cultures for another 12 h at 25, 12 or 4 degrees C. At the lower temperatures, labeled AA was transferred from TAG to polar lipids, indicating that TAG of P. incisa may have a role as a depot of AA that can be incorporated into the membranes, enabling the organism to quickly respond to low temperature-induced stress.     

Adventitious root formation of kiwifruit in relation to sampling date, IBA and Agrobacterium rubi inoculation

During the winters of 2001 and 2002, the effects of three strain of Agrobacterium rubi (A1, A16 and A18), a range of IBA concentrations (0, 2000, 4000 and 6000 ppm) alone and in combination with three strains of Agrobacterium rubi and date of cutting collection on the rooting of hardwood stem cuttings of kiwifruit cv. Hayward were evaluated. Treatments of hardwood stem cuttings of kiwifruit cv. Hayward with the bacteria, IBA and IBA plus bacteria were found to promote rooting. Highest rooting percentage was obtained from cuttings treated with 4000 ppm IBA plus A18 in cv. Hayward in both years. Higher rooting percentages were observed when shoots were collected in February rather than in January.     

气候因子和非气候因子对白三叶草叶片生长的影响

本文采用裂区试验设计,研究了刈割频率（强度）和品种对永久性混播草地组分白三叶草叶片生长的影响,并通过回归分析探讨了气候因子与白三叶草叶片生长的相关性。刈割对叶片出生率和每个葡萄茎现存叶片数的处理效应具季节依赖性特性,在夏季生长期,高频率刈割 处理。三叶草品种对叶片出生率和每葡萄茎观存叶片数均无显著的处理效应,但刈割与品种对秋初生长期叶片的出生率有显著的互作效应,对秋季生长期每葡萄茎现存叶片数有微弱     

Age of A2 Horizon Charcoal and Forest Structure near Porto Trombetas,Pará, Brazil1

To study the structure and composition of old‐growth forest in the Saracá‐Taquera National Forest near Porto Trombetas, Brazil, we established 36 0.25 ha plots and described the vegetation. We collected charcoal from the A2 soil horizon of each plot for radiocarbon dating. Although fires have been very rare in this forest during historic times, the presence of charcoal in these soils indicates fire at some earlier period. The ages (conventional radiocarbon age adjusted to 1997) of the charcoal ranged from 177 to 1547 years. These ages, however, did not correlate significantly with any of several measures of biodiversity or stand characteristics. The relative uniformity of the current old‐growth forest indicates that either the prehistoric fires were of such low intensity that they had little long‐term effect on the vegetation or that the present stands have progressed to near steady state.     

Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century

Terra Preta soils of central Amazonia exhibit approximately three times more soil organic matter, nitrogen and phosphorus and 70 times more charcoal compared to adjacent infertile soils. The Terra Preta soils were generated by pre-Columbian native populations by chance or intentionally adding large amounts of charred residues (charcoal), organic wastes, excrements and bones. In this paper, it is argued that generating new Terra Preta sites ('Terra Preta nova') could be the basis for sustainable agriculture in the twenty-first century to produce food for billions of people, and could lead to attaining three Millennium Development Goals: (i) to combat desertification, (ii) to sequester atmospheric CO2 in the long term, and (iii) to maintain biodiversity hotspots such as tropical rainforests. Therefore, large-scale generation and utilization of Terra Preta soils would decrease the pressure on primary forests that are being extensively cleared for agricultural use with only limited fertility and sustainability and, hence, only providing a limited time for cropping. This would maintain biodiversity while mitigating both land degradation and climate change. However, it should not be overlooked that the infertility of most tropical soils (and associated low population density) is what could have prevented tropical forests undergoing large-scale clearance for agriculture. Increased fertility may increase the populations supported by shifting cultivation, thereby maintaining and increasing pressure on forests.     

入侵杂草五爪金龙在切割处理下的再生能力

对五爪金龙(Ipomoea cairica (Linn.) Sweet)进行切割试验,对其在不同光环境及不同季节的萌芽率、成活率、主茎长度和生物量等进行研究。结果表明:五爪金龙切条越短其萌芽越迟,其成活率、主茎长度和生物量下降;五爪金龙在光下和林下的成活率无显著差异,但林下生物量较低;切条越短,林下条件越不利于主茎的伸长;冬季五爪金龙的主茎长度和生物量最低;夏季在光下和林下不能再生的最长切条长度均为4 cm;秋季均为1 cm;而冬季光下为1 cm,林下为3 cm;春季光下则为4 cm,林下为5 cm。春季时将五爪金龙切割为5 cm并置于林下是最经济的防除方法;如果不考虑季节性及光条件,应将其切割至1 cm以下的片段,可以有效防止五爪金龙再生。     

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

Woody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities’ features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur.     

Cogongrass (Imperata cylindrica)—Biology,Ecology, and Management

Cogongrass is considered to be one of the ten most troublesome and problematic weedy species in the world. This species is found throughout tropical and subtropical regions, generally in areas disturbed by human activities. Over 100 common names have been associated with cogongrass, including japgrass, speargrass, alang-alang, and bladygrass. Although this species has several commercial uses, the problems associated with its weediness far outweigh most positive benefits. Cogongrass is a major impediment to reforestation efforts in southeast Asia, the number one weed in agronomic and vegetable production in many parts of Africa, and is responsible for thousands of hectares of lost native habitat in the southeastern U.S. Biologically, cogongrass possesses several features that foster its spread and persistence. Management efforts for cogongrass consist of an integrated approach with several control strategies. In agronomic production, the use of cover crops is widely successful, but incorporation into the overall production scheme is challenging. Success has been achieved with continuous deep tillage or chemical applications, but long-term eradication/suppression must employ sustainable revegetation strategies.     

刈割高度对草丛群体特征的影响的研究—典型变量分析在草地生态系统研究中的应用

典型变量分析（CVA）在农业研究中应用的条件已经具备,本文对CVA在草地生态系统研究中的应用作了例示与讨论,在控制光温的人工草地上,高羊茅草丛群体的3个特征－牧草产量,叶面积和分薛密度与钊割高度（3,6,10,14,18和24cm)的关系研究中采用CVA,结果表明,第一典型相关极显著,且第一典型变量能反映刈割高度问98％的变异,在用CVA,结果表明,第一典型相关极显著,且第一典型变量能反映刈割高度问98％的变异,在3－24xm范围内,刈割高度的作用是渐进式的,任两刈割高度间的群体之综合差异与其高谍差呈正比例,叶面积和分薛刈割高度间差异的解释能力基本持平,但作用方向相反,两都都大于牧草产量的作用,建议在应用CVA时,应该适当增加重复次数,考虑附加线性变量,剔除分布不对称的指标变量等等。     

放牧、刈割及摘顶对亚热带人工草地牧草种群的影响

通过比较放牧、刈割及摘顶3种干扰发现,适度放牧、刈割提高了亚热带高山人工草地鸭茅、黑麦草种群的适应性,部分原因在于2种干扰均对牧草进行了摘顶．摘顶解除了牧草的顶端优势及生殖生长。放牧、刈割对牧草产生的生态影响在某种程度上可通过摘顶而实现。从而提高种群密度、热值及能量积累．单因子的人工摘顶试验表明,牧草顶端优势的维持降低了牧草种群密度,抑制了能量积累。从抑制了种间竞争．持续摘顶解除了牧草的顶端优势。促进了牧草密度及能量的增长,从而提高了牧草种群的竞争力．