Jasmonic acid does not mediate root growth responses to wounding in Arabidopsis thaliana

Jasmonic acid (JA) is a crucial plant defence signalling substance that has recently been shown to mediate herbivory-induced root growth reduction in the ecological model species Nicotiana attenuata . To clarify whether JA-induced reduction of root growth might be a general response increasing plant fitness under biotic stress, a suite of experiments was performed with the model plant Arabidopsis thaliana . JA bursts were elicited in leaves of A. thaliana in different ways. Root growth reduction was neither induced by foliar application of herbivore oral secretions nor by direct application of methyl jasmonate to leaves. Root growth reduction was observed when leaves were infected with the pathogen Pseudomonas syringae pv. tomato, which persistently induces the JA signalling pathway. Yet, high resolution growth analyses of this effect in wild type and JA biosynthesis knock-out mutants showed that it was elicited by the bacterial toxin coronatine that suggests ethylene- but not JA-induced root growth reduction in A. thaliana . Overall, the results demonstrate that the reaction of root growth to herbivore-induced JA signalling differs among species, which is discussed in the context of different ecological defence strategies among species.     

Studies on the possible role of cattle nuisance flies, especially Hydrotaea irritans, in the transmission of summer mastitis in Denmark

The summer mastitis pathogens Actinomyces pyogenes, Peptococcus indolicus, Bacteroides melaninogenicus ss. levii, Fusobacterium necrophorum and Streptococcus dysgalactiae were isolated from the polyphagous symbovine dipterans Hydrotaea irritans (Fallén) and Morellia sp. caught around dairy heifers on pasture, but not from the haematophagous species Haematobia irritans (L.), Haematobosca stimulans (Meigen), Culicoides sp. and Simulium sp. Secretions from clinical cases of summer mastitis proved to be sources of summer mastitis bacteria for more than 3 weeks despite antibiotic treatment and teat amputation. Taking into account the seasonal activity pattern of Hydrotaea irritans and its topographical distribution on grazing cattle, it appears evident that this fly may play a central role in the establishment and maintenance of the bacterial contamination with summer mastitis pathogens on the teats of healthy cattle. In the present study the survival of A.pyogenes and P.indolicus for 7 days in experimentally infected Hydrotaea irritans, as demonstrated by the recovery of these microorganisms from agar plates exposed to live infected flies, is described. However, experimental transmission of summer mastitis from sick to healthy heifers by Hydrotaea irritans proved unsuccessful.     

Diel growth patterns of young soybean (Glycine max) leaflets are synchronous throughout different positions on a plant

Leaf growth is controlled by various internal and external factors. Leaves of dicotyledonous plants show pronounced diel (24 h) growth patterns that are controlled by the circadian clock. To date, it is still uncertain whether diel leaf growth patterns remain constant throughout the development of a plant. In this study, we followed growth from the primary leaves to leaflets of the seventh trifoliate leaf of soybean (Glycine max) on the same plants with a recently developed imaging‐based method under controlled conditions and at a high temporal resolution. We found that all leaflets displayed a consistent diel growth pattern with maximum growth towards the end of the night. In some leaves, growth maxima occurred somewhat later – at dawn – as long as the leaves were still in a very early developmental stage. Yet, overall, diel growth patterns of leaves from different positions within the canopy were highly synchronous. Therefore, the diel growth pattern of any leaf at a given point in time is representative for the overall diel growth pattern of the plant leaf canopy and a deviation from the normal diel growth pattern can indicate that the plant is currently facing stress.     

Consumer-specific responses to riverine subsidy pulses in a riparian arthropod assemblage

1. Aquatic resource fluxes from streams can provide significant subsidies for riparian consumers. Because aquatic resource fluxes can be highly variable in space and time, the subsidy efficiency (i.e. transfer to the recipient food web) is controlled by the short‐term aggregative response of riparian consumers. 2. Field manipulations of stream‐derived invertebrate prey subsidies were used to examine specific aggregative responses of ground‐dwelling arthropods to riverine subsidy pulses in a braided‐river (Tagliamento River, NE Italy). Subsidy manipulation comprised short‐term reductions of natural stream‐derived subsidies and increased subsidies of stream‐derived invertebrate prey during four seasons. 3. We hypothesised that specific aggregative responses of riparian arthropods depend on their specialisation on aquatic insects which was inferred from stable isotope analysis. Natural riverine subsidy sources including aquatic insect emergence and surface‐drifting organisms were quantified. 4. Arthropods responded significantly with a reduction in abundance by 51%, at reduced subsidies and an increase by 110% at increased subsidies, when averaged over all seasons. Different arthropod taxa responded differently to subsidy manipulations in relation to their specialisation on aquatic subsidies: ground beetles with a diet consisting predominantly of aquatic insects responded only to subsidy reductions, indicating that their local abundance was not limited by natural stream‐derived subsidies; lycosid spiders with a partly aquatic diet showed no significant response; and ants, although relying on a terrestrial diet, responded positively to added stream‐derived invertebrate prey, indicating that stranding of surface‐drifting terrestrial invertebrates represented an important subsidy pathway. 5. Ground beetles and lycosid spiders were seasonally separated in their use of aquatic subsidies. Results indicate that the life‐history characteristics of riparian consumers can control the subsidy efficiency for the recipient community. By the effective uptake of pulsed riverine‐derived subsidies, riparian arthropods can enhance the transfer of riverine food sources to the riparian food web.     

A strategy to assess river restoration success

1. Elaborate restoration attempts are underway worldwide to return human‐impacted rivers to more natural conditions. Assessing the outcome of river restoration projects is vital for adaptive management, evaluating project efficiency, optimising future programmes and gaining public acceptance. An important reason why assessment is often omitted is lack of appropriate guidelines. 2. Here we present guidelines for assessing river restoration success. They are based on a total of 49 indicators and 13 specific objectives elaborated for the restoration of low‐ to mid‐order rivers in Switzerland. Most of these objectives relate to ecological attributes of rivers, but socio‐economic aspects are also considered. 3. A strategy is proposed according to which a set of indicators is selected from the total of 49 indicators to ensure that indicators match restoration objectives and measures, and that the required effort for survey and analysis of indicators is appropriate to the project budget. 4. Indicator values are determined according to methods described in detailed method sheets. Restoration success is evaluated by comparing indicator values before and after restoration measures have been undertaken. To this end, values are first standardised on a dimensionless scale ranging from 0 to 1, then averaged across different indicators for a given project objective, and finally assigned to one of five overall success categories. 5. To illustrate the application of this scheme, a case study on the Thur River, Switzerland, is presented. Seven indicators were selected to meet a total of five project objectives. The project was successful in achieving ‘provision of high recreational value’, ‘lateral connectivity’ and ‘vertical connectivity’ but failed to meet the objectives ‘morphological and hydraulic variability’ and ‘near natural abundance and diversity of fauna’. Results from this assessment allowed us to identify potential deficits and gaps in the restoration project. To gain information on the sensitivity of the assessment scheme would require a set of complementary indicators for each restoration objective.     

Addressing the influence of instrument surface heat exchange on the measurements of CO2 flux from open-path gas analyzers

There is a growing concern in the flux community that using the eddy covariance method with open‐path CO₂ analyzers often leads to measurements of an apparent ecosystem CO₂ uptake during off‐season periods, especially in cold climates. Such uptake has not been observed when measurements were made with closed‐path analyzers, chambers, or profile methods, suggesting it is an artifact due in some way to the use of open‐path analyzers. In this study, a series of laboratory tests and field experiments were conducted to determine the magnitude of the instrument surface heat exchange in the open path and its relationship with the measured CO₂ flux. Results showed that (1) the surface of an open‐path instrument became substantially warmer than ambient due to electronics and radiation load during daytime, while at night, radiative cooling moderated temperature increases in the path; (2) high‐frequency temperature measurements inside the path were correlated with vertical wind speed producing sensible heat flux inside the instrument path exceeding the ambient heat flux by up to 14%; (3) enclosing the open‐path instrument eliminated the sensible heat flux in the path, and caused measured CO₂ flux to match a closed‐path reference; (4) using sensible heat flux measured directly inside the open path in the WPL term instead of the ambient sensible heat flux also led to a match in CO₂ flux between open‐path instrument and closed‐path reference; and (5) correcting previously collected open‐path CO₂ flux data was possible by estimating the instrument heating effect with a semi‐empirical model using standard weather variables. Results showed that all proposed techniques led to a significant reduction in apparent CO₂ uptake during off‐season periods and to a reduction of the underestimation of CO₂ release in other periods. Close agreement between the open‐path measurements and closed‐path references was achieved in all cases.     

Eddy-correlation system for long-term monitoring of fluxes of heat, water vapour and CO2

An eddy-correlation system is presented that was designed with special focus on long-term measurements of turbulent fluxes in the atmospheric boundary layer. It consists of a SOLENT sonic anemometer, a fast temperature sensor, and a LI-COR LI 6262 closed-path infrared gas analyser. The use of a fast temperature sensor turned out to be necessary because of errors in the sound virtual temperature measured by the sonic anemometer at high wind speeds. The components are combined with special attention paid to protection against lightning and other environmental stresses. The data acquisition program SOLCOM runs on standalone systems or in a network environment and performs ‘quasi on-line’ data processing, on-line graphical display of single data and fluxes, and on-line correction of the raw data. Raw data can be stored continuously on DAT tapes. All data handling can be done by remote access, thus only a minimum amount of m situ maintenance is required. Power spectra of vertical and longitudinal wind speed, air temperature, air humidity and carbon dioxide concentration showed to follow the -2/3 law quite well. There was some noise in the high frequency range of the carbon dioxide spectrum. However, the corresponding cross spectra with the vertical wind component showed less deviation from a straight line in the high frequency range. The sum of convective heat fluxes and soil heat flux showed good agreement with the measured net radiation for several months and it was concluded that the system described here constitute a good platform for long-term flux measurements over forest.