Ground vegetation in the Mongolian taiga forest‐steppe ecotone does not offer evidence for the human origin of grasslands

Question: Is the vegetation of meadow and mountain steppes distinct from the ground vegetation of light taiga forests in the transitional zone between these biomes? Location: Western Khentey Mountains, northern Mongolia. Methods: Vegetation was recorded from 100‐m² plots from all dominant types of light taiga forest and dry grassland. Distinctness of ground vegetation was studied with Detrended Correspondence Analysis (DCA). Results: Ground vegetation in the light taiga was significantly different from the herbal vegetation of meadow and mountain steppes. Clear separation was only absent for the Car ex amgunensis meadow steppes that occur in a narrow strip along the forest edge and are partly shaded by trees. Forest and steppe communities followed a moisture gradient according to the DCA ordination with light taiga forests at the moistest sites and steppe communities at the driest sites. Ulmus pumila open woodlands diverged from this pattern, because of their close spatial and phytosociological relationship to mountain steppes. Conclusions: The present results do not support the assumption that grasslands in Mongolia's transitional zone between forest and steppe would generally resemble the ground vegetation of light taiga forests. This contradicts a published hypothesis stating that the vegetation of meadow and mountain steppes would not clearly differ from ground vegetation of light taiga forests in the forest‐steppe transitional zone of Mongolia.     

Epiphytic lichen diversity and its dependence on bark chemistry in the northern Mongolian dark taiga

Epiphytic lichen diversity was studied in a dark taiga forest of Pinus sibirica, Abies sibirica and Picea obovata in the western Khentey Mountains, northern Mongolia. Though most lichen species occurred on all three tree species, lichen diversity was higher on Abies and Picea than on Pinus. On branches, lichen vegetation differed less between tree species than on the trunk. The occurrence of many Parmeliaceae species with a hydrophilic surface and of many species producing the dibenzofuran usnic acid gives evidence of the low deposition of acidic pollutants in the study area. The Mn content of bark, which is known to limit at high values the abundance of epiphytic lichens in coniferous forests of Europe and North America, is apparently not controlling the spatial distribution of epiphytic lichens in the dark taiga of Mongolia. This is attributed to the dry and cold winters in Mongolia, as high Mn is especially leached from the surface of trees under moist conditions at temperatures around the freezing point, when the contact between water droplets and the tree surface is particularly intensive. Such moist and cold weather conditions are frequent in most parts of the northern coniferous forests of Europe and North America, but are rare events in the most continental parts of Asia, i.e. in Mongolia and eastern Siberia.     

Insect and small mammal herbivores limit tree establishment in northern Mongolian steppe

The extent to which human activities expanded the range of central and north-eastern Asian steppes into the forest is controversial. Natural versus anthropogenic causes of the inhibition of tree growth were investigated in a case study in the western Khentey Mountains in montane meadow steppe on sun-exposed southern slopes, representing treeless outposts within the northern Mongolian mountain taiga. Sowing and planting experiments with Mongolia’s most common tree species, Larix sibirica, did not result in the successful establishment of any tree during one growing season, despite the exclusion of grazing by large herbivores from the sample plots by fencing. Less than 1% of the seeds germinated 4 weeks after sowing in spring. All seedlings died within a further 3 weeks due to drought and heat. Two-year-old seedlings suffered from drought as well as from feeding by of gypsy moth larvae (Lymantria dispar), grasshoppers and rodents. While the gypsy moth was effective at damaging L. sibirica only during few weeks at the beginning of the growing season, feeding by grasshoppers and rodents persisted through the entire growing season. At the end of the growing season two thirds of the seedlings died due to insect and small mammal herbivory and one third due to drought-related damage. Herbivores attacked larch seedlings more rapidly on the open meadow steppe than at the forest edge. Drought and high temperature at the soil–air interface are crucial for inhibiting tree encroachment on the studied steppe slopes. Competition by non-arboreal plants is of subordinate significance for the establishment of L. sibirica. Trees can only establish at especially favorable microsites under humid weather conditions. Such trees, however, are exposed to a high risk of mortality from insect or small mammal herbivory.     

Late Holocene vegetation history suggests natural origin of steppes in the northern Mongolian mountain taiga

Insular occurrences of steppe vegetation are a common feature of the northern Mongolian mountain taiga. Steppe vegetation is limited here to southern slopes, whereas northern slopes and valley bottoms are principally wooded with light and dark taiga forests. In a case study in the valley of the river Eroo at Khonin Nuga Research Station in the western Khentey Mountains, we searched for evidence of an anthropogenic versus natural origin of steppe vegetation on the southern slopes. Pollen data of three profiles covering the last 2500 years showed continuous presence of steppe throughout the late Holocene with human influence restricted to the recent past. Virtual absence of charcoal in the soil on and beneath three steppe slopes suggested that the present steppe grasslands are not replacing former forests burnt by humans or lightning. The floodplains in the center of the Eroo valley were recently deforested. This is suggested by the pollen analysis and by interviews with local people on landuse history. Steppe grasslands of the study area have probably never been used as pastures. Pastoral nomads traditionally avoided the Eroo valley near Khonin Nuga because of difficult access and high densities of wolves and bears. All our data suggest that the present vegetation pattern of the western Khentey with steppes (and single small Ulmus pumila trees) on south-facing slopes occurring as islands in the mountain taiga is driven by climate and relief and is not the result of human activities as suggested for other regions of Asia.     

Aboveground biomass and litterfall ofPinus pumila scrubs growing on the Kiso mountain range in central Japan

Aboveground biomass and litterfall ofPinus pumila scrubs, growing on the Kiso mountain range in central Japan, were investigated from 1984 to 1985. The biomass of two research plots (P1 and P2) with different scrub heights was estimated by two methods, the stratified clip technique and the allometric method. Aboveground total biomass estimated by the latter method reached 181 ton d.w. ha⁻¹ in P1 and 132 ton d.w. ha⁻¹ in P2. Creeping stems contributed to about half of the total biomass. Although estimates of woody organs differed between the two plots, leaf biomass estimates were almost the same at 15.5 ton d.w. ha⁻¹. The canopies of the twoP. pumila scrubs were characterized by a large mean leaf area density of 5.0 m² m⁻³. Despite this large area density, relatively moderate attenuation of light intensity was observed. Specific leaf area generally increased with reduced leaf height. Annual total litterfall was estimated to be 3.60 ton d.w. ha⁻¹ yr⁻¹ in P1 and 2.39 ton d.w. ha⁻¹ yr⁻¹ in P2. Annual leaf fall in both plots was approximately 2.0 ton d.w. ha⁻¹ yr⁻¹. Leaves fell mainly in early autumn. Annual loss rates of branches, estimated as the sum of annual branch litterfall and the amount of newly formed attached dead branches, were 0.29 ton d.w. ha⁻¹ yr⁻¹ in P1 and 0.37 ton d.w. ha⁻¹ yr⁻¹ in P2.     

Dynamics and dry matter production of belowground woody organs ofPinus pumila trees growing on the Kiso mountain range in central Japan

Size, biomass and spatial distribution patterns of belowground woody organs ofPinus pumila trees were investigated. Dry weight estimates of five sample trees were between 14 and 36 kg tree⁻¹. Belowground stems accounted for about 32% of the total tree weight. However, the belowground stems had extensive barkless portions, indicating that the decomposition of dead belowground stems was an important source of organic matter in the soil. The, basal diameters of adventitious roots tended to become smaller as their orginating positions neared the ground surface. It was suggested thatP. pumila trees regenerate by successively producing adventitious roots from their buried stems and moving down the slope.     

平茬密度对荒漠草原人工柠条林间生境的影响

合理平茬对柠条林更新复壮及科学利用具有重要意义。以宁夏荒漠草原6 m带距人工柠条林(中间锦鸡儿,Caragana intermedia)林间草原为研究对象,研究了全平(QP)、隔一带平茬两带(G1P2)、隔一带平茬一带(G1P1)、隔两带平茬一带(G2P1)、未平茬(WP)五种密度(间距)平茬后柠条林间植被和土壤性状、小气候和土壤风蚀特征等变化。结果表明:(1)G2P1处理下林间植被物种总数、密度、高度和地上生物量最高,物种丰富度、多样性指数以G1P1、G2P1较高。(2)0—40 cm土壤容重以G1P1、G2P1较低,但机械组成无明显变化规律;土壤有机质、速效钾、全氮和速效氮以G2P1最高,但速效磷G1P1最高;土壤平均含水量以G2P1较高,达到8.33%,G1P2和WP最低,WP水分垂直变异系数最大。(3)处理间气温和风速无显著差异;风蚀量以QP、G1P2和WP较高,G1P1与G2P1较小。研究认为,适宜的密度平茬对人工柠条林间生境有改善作用,宁夏荒漠草原人工柠条林平茬时可采取隔两带平茬一带的方式。     

载畜率对荒漠草原冷蒿植物种群密度及其空间分布的影响

载畜率是衡量草原生产能力的指标之一,研究荒漠草原冷蒿(Artemisia frigida)植物种群在不同载畜率下的耐牧程度和空间分布特征,对退化草原的恢复具有重要的指导意义。以短花针茅荒漠草原优势种冷蒿为研究对象,在4个不同载畜率放牧试验设计(对照区CK(0只/hm~2)、轻度放牧区LG(0.93只/hm~2)、中度放牧区MG(1.82只/hm~2)和重度放牧区HG(2.71只/hm~2))小区中选择代表性样地(面积为40m×40m),采用机械取样法进行取样,记录冷蒿植物种群的密度,使用SAS 9.4软件对其进行描述性统计和方差分析,并用地统计学软件GS+9.0进行空间异质性统计分析。结果表明：荒漠草原优势种冷蒿密度、出现频率和单位均值上的变异值随载畜率增大都明显下降,其植物种群空间分布的决定性因素随载畜率增大由结构性因素转变为随机性因素。LG和HG处理区的空间自相关距离较大,但这两者表现结果的影响因素存在差异。在CK和HG区,冷蒿空间分布斑块化较为明显,LG区的冷蒿空间分布的特征表现为带状分布,MG区冷蒿空间分布形成了一处大的条带和零星的几块斑块。基于上述研究结果可以得出,随着载畜率...     

荒漠草原土壤相对湿度对猪毛蒿表型可塑性的影响

猪毛蒿(Artemisia scoparia)为菊科蒿属草本植物,是一种适应性较强的广幅种。研究荒漠草原不同土壤相对湿度条件下猪毛蒿的表型可塑性,对认识异质生境下猪毛蒿的生存适应策略具有重要的生态学意义。结果表明:株高、茎粗、根长、根重和单株生物量均表现出随土壤相对湿度的增大而增加的趋势,对异质生境具有较强的可塑性,而根冠比则表现出相对的稳定性。植株不同部位生物量大小排序为:上部中部下部,且植株下部显著大于上部生物量(P0.05)。土壤相对湿度40%生境下的头状花序数量和重量显著高于土壤相对湿度30%和30%—40%生境。繁殖器官绝对投入量(lg R)随着个体大小(lg V)的增大呈极显著的增加(P0.001),繁殖阈值介于1.868—2.006 g。随着土壤相对湿度的增加,繁殖分配比例极显著增大(P0.001)。营养器官和繁殖器官生物量、头状花序重量和数量、地下生物量和地上生物量均呈极显著线性正相关关系(P0.001),存在正向权衡。单个头状花序重量并不随个体大小和头状花序数量的增加而发生显著变化(P0.05),且在不同土壤相对湿度和不同部位间均无显著差异(P0.05)。由此可见,猪毛蒿在异质生境下产生的可塑性是其生存繁殖的重要反应机制之一。     

Aboveground net production and dry matter allocation ofPinus pumila forests in the Kiso mountain range,central Japan

Aboveground net production rates of the subalpine stone pine (Pinus pumila) forests in central Japan were estimated by the summation method; net production was defined as the sum of annual biomass increment and annual loss due to death. In the two pine stands of different scrub heights, P1 (200 cm) and P2 (140 cm), aboveground biomass reached 177 and 126 ton ha⁻¹, respectively. Leaf biomass was about 14 ton ha⁻¹ in each stand. The estimates of aboveground net production during the 2 year period (1987–1989) averaged 4.1 and 3.7 ton ha⁻¹ y⁻¹ in P1 and P2, respectively, which were at the lowest among the pine forests in the world. Two indices of efficiency of energy fixation, that is, the ratio of net production to the total radiation during a growing season and the ratio of net production to total radiation per unit of leaf weight, were evaluated. Both efficiency indices for the twoP. pumila stands fell in the range obtained for other Japanese evergreen conifer forests. This suggested that the low annual net production of the stone pine stands were mainly due to a limitation in the length of the growing season. The pine forests were also characterized by a small allocation (about 17%) of aboveground net production into biomass increment, in comparison with other evergreen conifer forest types. Annual net carbon gain in theP. pumila forests was suggested to be largely invested in leaf production at the expense of the growth of woody parts.     

不同刈割频度对大针茅草原群落特征及土壤元素含量的影响

在草原生态系统中,土壤是植物生长发育最重要的基质,对植物群落动态有显著的影响。刈割是内蒙古典型草原区除放牧外最重要的利用方式。研究不同刈割频度对典型草原区群落特征和若干土壤性质的影响及土壤因子与群落特征的相关性,旨在得出对群落多样性最有利并最适宜生产利用的刈割频度。研究区设置在内蒙古锡林浩特市东部以大针茅建群的典型草原;在2009—2013年进行不同频度的刈割处理,共设置处理,一年两次刈割、一年一次刈割、两年一次(割一年休一年)刈割、围封,在2013年8月采集测定土壤养分的样品。刈割对群落多样性影响显著,割一年休一年Shannon-Wiener多样性指数、Pielou均匀度指数、丰富度指数均高于其他刈割处理(P0.05);不同频度刈割下,一年两次刈割处理下土壤20—30cm和30—40cm的有机质含量显著高于其他处理(P0.05);不同频度刈割处理下土壤中的速效磷和速效氮的含量没有显著差异;一年两次处理的土壤全氮含量显著高于其他处理(P0.05)。Shannon-Wiener多样性指数与有机质含量、全氮含量呈显著负相关;Pielou均匀度指数与土壤元素间无显著相关关系;Richness指数与有机质含量呈显著负相关,与全氮含量为极显著负相关关系。割一年休一年刈割处理下土壤元素含量呈现更好的状态,结合群落物种多样性及生产利用的角度,割一年休一年为最合理的刈割频度。     

模拟氮沉降和降雨变化对贝加尔针茅草原土壤细菌群落结构的影响

研究氮沉降和降雨变化对土壤细菌群落结构的影响,对未来预测多个气候变化因子对草地生态系统影响的交互作用具有重要意义。以施氮和灌溉分别模拟氮沉降和降雨增加,采用高通量测序技术,研究8个氮添加水平(0、15、30、50、100、150、200、300kg N hm-2a-1)和2个水分添加水平(不灌溉、模拟夏季增雨100 mm灌溉)对土壤细菌群落结构的影响。结果表明,氮素和水分输入增加后,土壤细菌群落组成、丰度均显著变化(P0.05)。在群落中占主导的细菌门类有疣微菌门Verrucomicrobia(30.61%—48.51%)、变形菌门Proteobacteria(21.37%—29.97%)、酸杆菌门Acidobacteria(9.54%—20.67%)和拟杆菌门Bacteroidetes(4.96%—9.74%)。在常规降雨和水分添加两种条件下,随着氮添加水平的增加,占主导的细菌门类(相对丰度1%)表现出不同的变化趋势。疣微菌门相对丰度在常规降雨N100—N300条件下显著降低,但在氮素和水分同时添加条件下随氮添加水平升高而逐渐升高,在N200—N300时显著升高。变形菌门和拟杆菌门相对丰度在常规降雨高氮添加条件下呈升高趋势,但在水分添加时却无明显变化。酸杆菌门相对丰度在常规降雨高氮添加条件下升高,但在水分添加后呈明显下降趋势。放线菌门Actinobacteria相对丰度在常规降雨N100—N300条件下显著升高,但在水分添加后高氮添加时显著降低。厚壁菌门Firmicutes相对丰度在常规降雨条件下无显著变化,但在水分和高氮添加条件下降低。浮霉菌门Planctomycetes相对丰度在两种不同的水分添加条件下均呈先升高后降低的趋势。氮素和水分添加对土壤细菌群落结构的变化存在明显的互作效应(P0.0001)。在不同氮素和水分输入条件下共有19个土壤细菌门类相对丰度有显著差异。土壤细菌群落结构的变化主要来自于疣微菌门和酸杆菌门的相对丰度变化,两者可作为土壤细菌群落结构变化的指示种。综上,氮素和水分添加显著改变了土壤细菌群落结构,氮素和水分对土壤细菌不同门类相对丰度变化存在明显的互作效应。     

不同生育期榆树净光合速率对生态因子和生理因子的响应

利用Li-6400便携式光合作用系统,观测了内蒙古浑善达克沙地榆树(Ulmus pumila)的净光合速率(Pn)、上表皮气孔导度(Gs-u)、下表皮气孔导度(Gs-l)等生理因子及大气温度(Ta)、光合有效辐射(PAR)、大气二氧化碳浓度(Ca)和空气相对湿度(RH)等生态因子,应用相关分析、通径分析、决策系数分析及灰色关联分析,系统的研究了榆树Pn在不同生育期的日变化特征及其与生态因子和生理因子间的关系,为深入揭示其生理特征及浑善达克沙地榆树规范化种植提供理论依据。研究表明:1榆树Pn日变化在各生育期均呈双峰型曲线,表现出不同程度的光合"午休"。2榆树Pn在各生育期大小顺序为8月7月9月,其值分别为8.18、7.92μmol CO2m-2s-1和6.52μmol CO2m-2s-1。37、8、9月份PAR、Gs-u与榆树Pn呈显著正相关,且PAR的直接作用和决策系数均最大,Gs-u的直接作用大于其他因子的间接作用,且决策系数仅次于PAR,其他因子在各生育期也表现出对榆树Pn不同程度的影响。4不同生育期榆树对各生态因子和生理因子的响应有所不同,7月份Ta为Pn主要限制变量,8月份RH为Pn主要限制变量,9月份各因子均起增进作用。5不同生育期榆树Pn的变化主要受PAR和Gs-u的影响,但不存在简单的线性相关关系,是所有因子综合影响的结果。     

Glyphosate-induced changes in the attack success and development of the mountain pine beetle and impact of its natural enemies

Roundup (active ingredient glyphosate) administered into the sapwood around the root collar of lodgepole pine trees, Pinus contorta var. latifolia Engelm., was investigated as a tool for creating trap trees for the mountain pine beetle (MPB), Dendroctonus ponderosae Hopk. MPB's attacked semiochemical-baited, glyphosate-treat trees before attacking baited control trees. Bark samples disclosed an increase in survival of MPB eggs and early instar larvae, enhanced larval development and increased attacks by insect parasites and predators in treated trees. The braconid wasp, Coeloides dendroctoni Cush., parasitized MPB larvae found at high densities in treated trees two months following attack. The dipteran predator, Medetera aldrichii Wh., was only found in treated trees. Increased competition by Ips pini (Say) had a negative impact on MPB survivorship. The enhanced impact of insectan agents apparently offset any glyphosate-induced gains in MPB survival or development, but did not cause a significant reduction in MPB emergence.     

Altitudinal change in soil and foliar nutrient concentrations and in microclimate across the tree line on the subtropical island mountain Mt. Teide (Canary Islands)

Mt. Teide (Tenerife, Spain) is a high volcanic island mountain with an unusually low tree line elevation (2000–2100 m). While searching for the causes of this tree line depression, we analysed the concentrations of total N, available P, and salt-exchangeable Ca, K and Mg in three soil horizons, and the foliar N, P, Ca, K and Mg concentrations in six abundant plant species (trees, shrubs, forbs) along a transect from 1400 (1600) to 3100 m a.s.l. The objective of the study was to detect altitudinal trends in soil and plant nutrient status below and above the tree line. For characterising elevational changes in microclimate and hydrology, we also conducted measurements of air and soil temperatures (T_a and T_s), atmospheric water vapour saturation deficit (D), potential evaporation (E) and soil moisture (θ) on 3–4 occasions along the transect.T_a and T_s did not linearly decrease with elevation but were highest at or immediately below the tree line and were relatively low in the closed Pinus canariensis forest at 1600 m. T_s reached maxima at about 70 °C near the tree line. The absence of a linear temperature decrease with elevation was caused by a stable temperature inversion at about 2000 m and by canopy shading in the forest canopy below the tree line. In June, the topsoil (10 cm) dried out almost completely between 1800 and 3100 m, but remained moist in the pine forest at 1600 m. This gradient reflects the transition from the montane cloud belt to the dry alpine belt higher upslope. The subsoil (30 cm) contained >30 vol% of soil water at all elevations even in June except for the uppermost site (3100 m). Potential evaporation increased with elevation despite a decrease in D. We assume that this is mainly due to the air pressure-dependent increase with elevation in the diffusion coefficient for water vapour in air.The concentrations of N and ‘available’ P (after Olsen) in the mineral topsoil were by far smaller than in alpine soils of other humid mountains which is thought to be a consequence of a very dry and biologically inactive topsoil in the semi-arid alpine belt of Mt. Teide. In contrast to many other mountains, foliar N, P and cation concentrations in the plants did not increase with elevation but either remained unchanged, or decreased as in the tree line species P. canariensis. Nevertheless, P. canariensis probably is not limited by nutrient deficiency at the tree line despite rather low N and P needle concentrations. Rather, drought and heat stress effects on seedling establishment are thought to be the causes of the tree line depression.     

一氧化氮对家榆种子老化的影响及其作用机制研究

以家榆种子为试材,采用种子活力检测技术、激光共聚焦显微镜技术、蛋白质S-亚硝基化检测技术,结合多种相关抑制剂的使用,研究了NO对种子老化的影响及其作用机制。结果表明:(1)外源NO可显著提升老化处理后种子的活力,NO清除剂cPTIO可降低老化处理后种子的活力,且此影响可被NO供体硝普钠所恢复。(2)硝酸还原酶底物亚硝酸钠、类一氧化氮合酶底物L-精氨酸(L-Arg)均可提高老化处理后种子的活力,2种酶的抑制剂可降低种子活力,且此影响可被NO供体硝普钠所恢复,即硝酸还原酶与类一氧化氮合酶可参与种子老化过程中NO的产生。(3)种子老化过程中NO首先在子叶中合成,随后在胚根尖部、生长点与下胚轴等部位出现,蛋白质S-亚硝基化水平与NO在种子中产生的时间特点一致。研究认为,NO可提高种子抗老化能力,种子内NO可通过硝酸还原酶途径和类一氧化氮合酶途径产生,且与种子蛋白质S-亚硝基化水平相关。     

荒漠草原区土壤粒径组成对柠条根际土壤微生物数量及酶活性的影响

柠条(Caragana korshinskii)是荒漠草原区主要的造林绿化树种,研究其根际土壤微生物和酶活性与不同土壤类型土壤粒径组成的关系有重要意义,然而土壤粒径对荒漠草原柠条根际土壤微生物数量和酶活性的影响知之甚少,探讨土壤颗粒组分与微生物数量、土壤酶活性之间的关系,以及土壤颗粒组成对荒漠草原区固沙灌木植物柠条根际土壤微生物数量及酶活性的影响,可为揭示荒漠草原土壤退化及生态修复提供参考。以宁夏荒漠草原区土壤粒径组成差异显著的灰钙土、红黏土、风沙土环境下栽植的柠条为研究对象,研究不同土壤颗粒组成对根际土壤微生物数量及酶活性的相互关系与影响。结果表明:土壤微生物的数量表现为细菌放线菌真菌。根际土壤中的细菌、真菌数量显著高于非根际,且在3种不同类型的土壤中随着细砂粒的增多,真菌和放线菌数量逐渐降低,而细菌数量呈先增大后减小的趋势;根际与非根际土壤的蔗糖酶、碱性磷酸酶及过氧化氢酶活性均呈现出灰钙土红黏土风沙土的趋势,红黏土根际土壤中的脲酶活性显著高于灰钙土与风沙土;除过氧化氢酶外,土壤酶活性表现为根际高于非根际,在3种不同类型的土壤中随着细砂含量的增加,土壤酶活性均呈递减趋势。土壤颗粒组成与微生物数量之间没有明显的相关性,而与土壤酶活性之间显著相关,土壤酶活性与黏粒、粉粒呈正相关,与细砂、中砂呈负相关关系,根际土壤中酶活性更高,能够为植物及微生物提供更多的营养。     

Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.     

Effects of UV-B radiation and additional irrigation on the Mediterranean evergreen sclerophyll Ceratonia siliqua L. under field conditions

Seedlings of Ceratonia siliqua L. were grown for 1 year in the field under ambient or ambient plus supplemental UV-B radiation (corresponding to 15% ozone depletion over Patras) and received two levels of additional irrigation during the summer dry period. The experiment was started during February 1998 and two major samplings were performed, the first at the end of the dry period (September 1998) and the second at the end of the experiment (January 1999). Plants receiving additional irrigation showed significantly higher leaf number, plant height and chlorophyll content at the end of the summer, but these differences were abolished at the final harvest. Plants growing under enhanced UV-B radiation had significantly fewer leaves and less nitrogen content at the end of the dry period, but these effects were also abolished at the final harvest, during which significant UV-B induced increases in stem dry mass were observed. None of the other measured parameters (mean leaf area, leaf dry mass, leaf thickness, UV-B absorbing compounds, phenolics, tannins and photochemical efficiency of PSII) were affected by either treatment. Combined UV-B / water effects were not significant. We may conclude that although some minor responses to enhanced UV-B radiation were evident, C. siliqua is resistant against UV-B radiation damage at the level applied.     

Fluence rate and wavelength dependence of photobleaching in the cyanobacterium Anabaena variabilis

The fluence rate dependence of the photobleaching in the cyanobacterium Anabaena variabilis was studied under physiological conditions. According to the in-vivo absorption spectra measured every day during the 5 d exposition the phycobiliproteins are more sensitive to high fluence rates than chlorophyll a. The carotenoids are least sensitive, so that a relative, but not an absolute increase in the carotenoid content occurred. At very high fluence rates exceeding about 50 Wm^-2 white light the organisms were photokilled after 5 d of irradiation. Measurements of the nitrate concentrations during the experiments have shown that nitrate was not the limiting factor in these experiments. Analysis of the photobleaching kinetics at 13.5 Wm^-2 white light revealed that after about 8 d the contents of all the pigments studied have reached a new, constant level. After exposure of the photobleached cyanobacteria to low irradiances repigmentation occurred. Thus, photobleaching is a light adaptation process and not simply a photodamage phenomenon. Studying the wavelength dependence of photobleaching at a constant photon fluence rate of 4·10^-8 mol cm^-2 s^-1 we found that the photobleaching of both phycobiliproteins and chlorophyll a was exclusively caused by wavelengths absorbed by the phycobiliproteins, mainly phycoerythrocaynin, and red light absorbed by short wavelength chlorophyll. Wavelengths <520 nm were ineffective.