Growth and photosynthetic responses of four landscape shrub species to elevated ozone

Attention should be paid to ozone (O₃) sensitivity of greening plant since ground-level O₃ concentrations are increasing especially in urban and suburban area. We studied the ecophysiological responses to elevated O₃ of four shrub species [Euonymus bungeanus Maxim. (EB), Photinia × fraseri (PF), Chionanthus retusus Lindl. & Paxt. (CR) and Cornus alba L. (CA)], which are often used for garden greening in China. Saplings of those species were exposed to high O₃ concentration (70 nmol mol⁻¹, 7 h d⁻¹ for 65 d) in open-top growth chambers. Responses to O₃ were assessed by gas exchanges, chlorophyll (Chl) fluorescence and dry mass. We found that elevated O₃ significantly decreased lightsaturated net photosynthetic rate (P _Nsat), transpiration rate (E) and stomatal conductance (g _s). The ratio of intercellular CO₂ to ambient CO₂ concentration (C _i/C _a) did not reduce under O₃ fumigation which suggested that the O₃-induced depressions of P _Nsat under O₃ fumigation were probably due to limitation of mesophyll processes rather than stomatal limitation. High O₃ exposure also significantly depressed the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m) which meant the O3-induced photoinhibition. Both root dry mass and root/shoot ratios were significantly decreased under ozone fumigation, but the total mass was unchanged. The responses of gas exchange such as P _Nsat in these four shrubs to O₃ exposure were species-specific. Highest loss of P _Nsat was observed in EB (−49.6%), while the CR had the lowest loss (−36.5%). Moreover, the O₃-exposed CR showed similar g _s as CF, reflecting that its O₃ flux might be unchanged under elevated O₃ environment. Ozone drastically decreased actual quantum yield of PSII (Φ_PSII) and electron transport rate (ETR) in EB while increased Φ_PSII and ETR in CR. Furthermore, the relative losses in P _Nsat positively correlated with the relative decreases in Φ_PSII and ETR which indicated that the impairment of photosynthesis was probably affected by the light reaction process. The light reaction of EB was impaired most seriously but that of CR was not damaged. All results indicated that EB was probably the most sensitive shrub species to O₃ while CR the most tolerant one. Therefore, CR might be an ideal choice for greening in ozone-polluted areas.     

Growth responses of tropical forage plant species to vesicular-arbuscular mycorrhizae

Summary Growth and mineral uptake of twenty-four tropical forage legumes and grasses were compared under glasshouse conditions in a sterile low P oxisol, one part inoculated and the other not inoculated with mycorrhizal fungi. Shoot and root dry weights and total uptake of P, N, K, Ca, and Mg of all the test plants were significantly increased by mycorrhizal inoculation. Mycorrhizal inoculation, with few exceptions, decreased the root/shoot ratio. Non-mycorrhizal plants contained always lower quantities of mineral elements than mycorrhizal plants. Plant species showed differences in percentage mycorrhizal root length and there was no correlation between percentage mycorrhizal infection and plant growth parameters. A great variation in dependence on mycorrhiza was observed among forage species. Total uptake of all elements by non-mycorrhizal legumes and uptake of P, N and K by non-mycorrhizal grasses correlated inversely with mycorrhizal dependency. Mycorrhizal plants of all species used significantly greater quantities of soil P than the nonmycorrhizal plants. Utilization of soil P by non-mycorrhizal plants was correlated inversely with mycorrhizal dependency.     

Seed characteristics of plant species from riverine wetlands in Venezuela

Seed characteristics of 76 plant species from wetlands along four black-water and two white-water rivers in the Southeastern Venezuelan Llanos were examined. Weight, length, width, color, appendages, shape, and dispersal type of seeds were determined. Life form, leaf type and fruit type of plants also were described. Thirty-eight percent were perennial heloculms, 28% helosuffrutescents, 13% heloterophytes and 5% annual heloculms. Dominant leaf types were nanopylls and microphylls. Ninety-six percent of species showed dehiscent and indehiscent dry fruits (achene, capsule, nutlet, follicle). Sixty-five percent of species had seed dry weights between 32 and 315 μg. Lengths and widths of the seeds ranged from 0.2 to 6.2 mm and 0.1 to 4.1 mm, respectively. Seventy-six percent of species had seeds reddish brown, yellowish brown and amber in color, with 17% black and 7% white. Seventy-seven percent of species had seeds represented by pterochory, pogonochory, sclerochory, desmochory, and pleochory dispersal types, with 11% sarchochory, and 12% barochory. The seed size showed significant differences among families, life forms, geometric shapes and dispersal types. In general, species from wet and open environments (marshes) along black- and white-water rivers showed low variation in seed color and size, and were dispersed mainly by abiotic agents.     

Morphological responses of two plant species from different elevations in the Dongting Lake wetlands,China, to variation in water levels

The vegetation of wetlands show strong zonation patterns, but the mechanisms determining these patterns are not fully understood. In the present study, growth and morphological responses to a water level gradient (–20 cm (i.e. water level 20 cm below soil surface), –10 cm, 0 cm, 10 cm, 20 cm) were compared between a higher elevation plant (Imperata cylindrica) and a lower elevation plant (Carex brevicuspis) in the Dongting Lake wetlands of China. For both species, the aboveground, belowground, and total biomass were greater at –10 cm than at any other water level.. However, when the water level increased from –10 cm to 0 cm, there was a greater decrease in the biomass of I. cylindrica than in that of C. brevicuspis. Plant height, tiller number, leaf length, leaf width and leaf area showed greater variation along the water level gradient in I. cylindrica than in C. brevicuspis. Generally, with increasing water level, root length, rhizome number, and adventitious root biomass and number all decreased in I. cylindrica. However, in C. brevicuspis, neither the rhizome number nor the primary adventitious root biomass differed significantly among the five water levels. These results indicate that I. cylindrica have a lower tolerance for flooding and higher water sensitivity than C. brevicuspis and these differences may explain why I. cylindrica is found at relatively higher elevations that are not prone to flooding, while C. brevicuspis is found at comparatively lower elevations in the Dongting Lake wetlands.     

Growth and physiological responses to ozone and mild drought stress of tree species with different ecological requirements

An open-top chamber experiment was carried out in Curno (Northern Italy) in 2004 and 2005 on seedlings of Fagus sylvatica (FS), Quercus robur (QR), and an ozone-sensitive Populus (POP) clone, to investigate the role of two stress factors: tropospheric ozone and water shortage. Treatments were filtered air to achieve a 50% reduction in the environmental ozone concentrations (charcoal filtered, CF); and non-filtered air, with a 5% reduction in the environmental ozone concentrations (non-filtered, NF). Overall ozone exposure (AOT40) in open air (April–September) was 26,995 ppb h in 2004 and 25,166 ppb h in 2005. The plants were either watered (W) or not watered (dry, D). We investigated the above-ground biomass, tree-ring growth, stable carbon isotopes ratio, i.e. δ¹³C of tree rings, and the photosynthetic parameter Driving forces (DF_ABS), derived from chlorophyll a fluorescence analysis. Ozone-induced growth reduction (in terms of biomass) in POP, and that reduction was more pronounced in D plots. A synergistic effect of ozone and drought stress was evidenced by DF_ABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio δ¹³C. In drought-stressed seedlings, the increase in δ¹³C value was accompanied by the reduction in stomatal conductance and increased DF_ABS. Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.     

Factors influencing macroinvertebrate colonization of seasonal wetlands: responses to emergent plant cover

1. We conducted field experiments to examine factors influencing macroinvertebrate colonization of seasonally flooded marshes. Few macroinvertebrate species were found aestivating in soils within non-flooded wetlands indicating that most taxa colonize these marshes from other flooded habitats.
2. We manipulated amounts of salt grass ( Distichlis spicata ) to examine how emergent plant cover affects aerial colonization by macroinvertebrates. Areas mowed 3 weeks before flooding had low plant cover, areas mowed 5 and 9 weeks before flooding had medium and high plant cover, respectively, and non-mowed control areas had the most plant cover. Macroinvertebrate numbers and biomass were generally higher in mowed treatment areas than in control areas, but overall diversity was generally higher in high plant cover and control areas than in low plant cover areas.
3. Mosquitoes (Culicidae), brine flies (Ephydridae) and hover flies (Syrphidae) were positively correlated with amount of plant cover, and waterboatmen (Corixidae), midges (Chironomidae) and water scavenger beetles (Hydrophilidae) were negatively correlated with plant cover. Species assemblages changed seasonally among treatment areas because these taxa colonize wetlands at different times in the year.
4. These results demonstrate that invertebrate communities may be different within plant stands with heterogeneous amounts of emergent cover, and management practices that alter the structure of wetland vegetation can influence macroinvertebrate communities colonizing seasonal marshes.     

Induced responses in three alkaloid-containing plant species

In this paper we test three plant species for the inducibility of their alkaloid production. The plants were heavily damaged by cutting off 50% of their leaf surface using a pair of scissors. The cut-off leaf tips were used as controls for possible diurnal fluctuations. After 3, 6, 12, 24 and 48 h, respectively, the leaf bases of the damaged plants were harvested and the alkaloid concentration was measured. In Senecio jacobaea the pyrrolizidine alkaloid (Pa) concentration in damaged plants decreased within 6–12 h after damage. Within 24 h after damage the Pa concentration of Cynoglossum officinale doubled compared to control values. Indole alkaloid production in Catharanthus roseus was found not to be induced in this experiment. The responses are discussed in a functional context. We hypothesize that the nature of the response is not a feature of the type of secondary metabolite, but is related to whether the plants are damaged mainly by generalist or by specialist herbivores.     

Electroantennogram responses to plant volatiles in two species of Pieris butterflies

Electroantennogram (EAG) responses were recorded from females of two related butterfly species, Pieris brassicae L. and P. rapae L. (Lepidoptera: Pieridae) to 23 volatiles of plant origin, 19 of which have been reported to occur in the headspace of their preferred host plants, Brassica and Sinapis species (Cruciferae). In both species, selective responses were observed and the most effective compounds were the 6-carbon fatty acid derivatives trans-hex-2-enal, hexan-1-ol and hexanal, which are common green leaf volatiles. Of the 6 isoprenoids tested, myrcene and geraniol were most effective. Of the 4 crucifer-specific compounds tested, phenylacetonitrile was a distinctly stronger stimulant than the three isothiocyanates in both species. The rank order of mean stimulating effectiveness of all compounds tested was strongly correlated between the two species. Statistical analysis of dose-response relationships for 6 compounds revealed significant differences between compounds. The rank order of effectiveness changed with dose. Exclusive exposure to Sinapis arvensis during larval life and young adulthood resulted in quantitative changes in EAG responses to several compounds in both species, although the overall rank order of effectiveness was strongly correlated between the groups reared on the two host plants.     

Olfactory responses of two species of grasshoppers to plant odours

Electroantennogram (EAG) responses were recorded from two species of oedipodine grasshoppers, Oedaleus decorus asiaticus L. (graminivorous) and Angaracris barabensis Pall. (forbivorous), to volatiles emitted by chopped leaves of ten plant species. Male O. d. asiaticus showed much stronger EAG responses than conspecific females and both sexes of A. barabensis. Sexual differences in EAG responses correspond to different numbers of antennal sensilla of both sexes and to certain behavioural and morphological factors as well. The overall EAG response profiles of the two grasshopper species to the ten plant odours were similar. However, adaptation to host odour might have occurred because of differences in their feeding habits. Females of the graminivorous O. d. asiaticus possess a significantly higher olfactory sensitivity for poaceous plant species than A. barabensis, while the forbivorous A. barabensis showed significantly higher EAG responses to Allium senescens (Liliaceae), and a tendency of high responses to composite plant species in comparison with O. d. asiaticus.     

Growth responses of Rumex species in relation to submergence and ethylene

Abstract. Submergence stimulates growth of the petioles of Rumex palustris and Rumex crispus under field, greenhouse and laboratory conditions. Growth of Rumex acetosa petioles was hardly influenced by submergence. These growth responses under flooded conditions can be partially mimicked by exposing non-submerged Rumex plants to ethylene-air mixtures. Submergence of intact plants in a solution of AgNO₃ inhibited the elongation of all petioles of R. palustris and the youngest petiole of R. crispus and stimulated growth of the youngest petiole of R. acetosa , The ethylene-air mixture experiments, the effect of AgNO₃ and observed increase of the endogenous ethylene concentration during submergence suggest that ethylene plays a regulatory role in the growth responses of these Rumex species under submerged conditions. The three Rumex species showed a gradient in elongation responses to submergence, which correlates with the field distribution of the three species in a flooding gradient.     

Hydrologic similarity to reference wetlands does not lead to similar plant communities in restored wetlands

Few wetland restoration projects include long‐term hydrologic and floristic data collection, limiting our understanding of community assembly over restored hydrologic gradients. Although reference sites are commonly used to evaluate outcomes, it remains unclear whether restoring similar water levels to reference sites also leads to similar plant communities. We evaluated long‐term datasets from reference and restored wetlands 15 years after restoration to test whether similar water levels in reference and restored sites led to vegetation similarity. We compared the hydrologic regimes for three different wetland types, tested whether restored wetland water levels were different from reference water levels, and whether hydrologic similarity between reference and restored wetlands led to similarity in plant species composition. We found restored wetlands had similar water levels to references 15 years after restoration, and that species richness was higher in reference than restored wetlands. Vegetation composition was similar across all wetland types and was weakly correlated to wetland water levels overall. Contrary to our hypothesis, water table depth similarity between restored and reference wetlands did not lead to similar plant species composition. Our results highlight the importance of the initial planting following restoration and the importance of hydrologic monitoring. When the restoration goal is to create a specific wetland type, plant community composition may not be a suitable indicator of restoration progress in all wetland types.     

Heterogeneous responses to ozone and nitrogen alter the species composition of Mediterranean annual pastures

Air pollution represents a threat to biodiversity throughout the world and particularly in the Mediterranean area, where high tropospheric ozone (O₃) concentrations and atmospheric nitrogen (N) deposition are frequently recorded. Mediterranean annual pastures are among the most important ecosystems in southern Europe due to their high biodiversity and extension. Aiming to study the responses of these communities to the main atmospheric pollutants in the Mediterranean region, an experimental study was performed in an open-top chamber (OTC) facility. A mixture of six species representative of annual pastures was grown under field conditions inside the OTC. Plants were exposed for 39 days to four O₃ treatments and three doses of N. The species responded heterogeneously to both factors. Legumes did not react to N but were very sensitive to O₃: Trifolium species responded negatively, while Ornithopus responded positively, taking advantage of the greater sensitivity of clovers to O₃. The grasses and the herb were more tolerant of O₃ and grasses were the most responsive to N. Significant interactions between factors indicated a loss of effectiveness of N in O₃-polluted atmospheres and an ability of O₃ to counterbalance the damage induced by N input, but both effects were dependent on O₃ and N levels. The inclusion of plant competition in the experimental design was necessary to reveal results that would otherwise be missed, such as the positive growth responses under elevated O₃ levels. Surprisingly, competition within the legume family played the most important role in the overall response of the annual community to O₃. Both tropospheric O₃ and N deposition should be considered important drivers of the structure and biodiversity of Mediterranean annual pastures.     

Enzyme activity responses to nutrient loading in subtropical wetlands

Eutrophication caused by anthropogenic nutrient inputs is one of the greatest threats to the integrity of freshwater wetlands. The resultant changes in organic carbon cycling and nutrient mineralization may be expressed through increased decomposition rates, which are ultimately dependent on the metabolism of the resident microbial community. Specifically, microbial nutrient acquisition is controlled through the activity of enzymes, which are in turn influenced by local biogeochemical conditions. This study examines enzyme activities along distinct North-South P gradients within four distinct hydrologic units of the Florida Everglades. The results indicate that nutrient enriched sites exhibit lower N and P limitations on microbially constrained C mineralization, in addition to enhanced cellulose decomposition rates. Nutrient loading resulted in decreased microbial mobilization of resources for P mineralization, resulting in greater energetic allocation for C mineralization. Additionally, N appears to become less limiting to C mineralization in the enriched sites within Everglades National Park, the least P enriched area within the Everglades. A simple two component model, incorporating total P and the relationship between the enzymes involved in C and P mineralization accounted for between 46 and 92% of the variability in measured cellulose decomposition rates and thus demonstrates the significant influence that P loading plays in these systems. These results also suggest there is an environmental threshold TP concentration below which changes in enzyme-based resource allocation will not occur.     

Spatial heterogeneity in species composition constrains plant community responses to herbivory and fertilisation

Environmental change can result in substantial shifts in community composition. The associated immigration and extinction events are likely constrained by the spatial distribution of species. Still, studies on environmental change typically quantify biotic responses at single spatial (time series within a single plot) or temporal (spatial beta diversity at single time points) scales, ignoring their potential interdependence. Here, we use data from a global network of grassland experiments to determine how turnover responses to two major forms of environmental change – fertilisation and herbivore loss – are affected by species pool size and spatial compositional heterogeneity. Fertilisation led to higher rates of local extinction, whereas turnover in herbivore exclusion plots was driven by species replacement. Overall, sites with more spatially heterogeneous composition showed significantly higher rates of annual turnover, independent of species pool size and treatment. Taking into account spatial biodiversity aspects will therefore improve our understanding of consequences of global and anthropogenic change on community dynamics.