M. van Oijen

Chlorophyll content of spring wheat flag leaves grown under elevated CO2 concentrations and other environmental stresses within the 'ESPACE-wheat' project

Spring wheat cv. Minaret was grown in open-top chambers at four sites across Europe. The effect of different treatments (CO2 enrichment, O3 fumigation, drought stress and temperature) on the chlorophyll content of the flag leaf was investigated using the MINOLTA SPAD-502 meter. Under optimum growth conditions the maximum chlorophyll content, which was reached at anthesis, was consistent among the sites ranging from 460 to 500 mg chlorophyll m−2. No significant effect of elevated CO2 or O3 was observed at anthesis. Leaf senescence, indicated by the chlorophyll breakdown after anthesis, was relatively constant in the control chambers. Under control conditions, thermal time until 50% chlorophyll loss was reached was 600°C day. Elevated CO2 caused a faster decline in chlorophyll content (thermal time until 50% chlorophyll loss was reduced to 500–580°C day) indicating a faster rate of plant development at two experimental sites. The effect of ozone on chlorophyll content depended on the time and dose of O3 exposure. During grain filling, high O3 concentrations induced premature senescence of the flag leaves (up to −130°C day). This deleterious effect was mitigated by elevated CO2. Drought stress led to faster chlorophyll breakdown irrespective of CO2 treatment.

Analysis of maize growth for different irrigation strategies in northeastern Spain

Water availability is the key factor determining maize yields in NE Spain. Irrigation is needed to obtain economic yields but it is costly and water supply is sometimes insufficient. The aim of this research was to test a simple simulation model for evaluating different irrigation strategies, especially under water-limited conditions. The LINTUL model was adapted and parameterized using experimental data from the 1995 season. Most parameters were obtained from experiments, although some were taken from the literature. This model is based on the concept of light use efficiency, incorporates a soil water balance and simulates phenology, crop leaf area, biomass accumulation and yield. It was tested on independent data from the 1995 and 1996 seasons under different irrigation treatments. The model predicted the flowering date within 9 5 days of the observed values. Leaf area index was predicted satisfactorily, except under extreme water-stress conditions, where it was overestimated. In general, soil moisture content and yield were accurately predicted. In the 1996 experiment measured yields ranged from 6.4 to 13.6 th a 1 and simulated yields from 6.5 to 12.2 t ha 1 . These results show that the LINTUL model can be used as a tool for exploring the consequences on maize yields of different irrigation strategies in NE Spain. Analysis of the model identified a process that strongly affects yield loss due to drought, but for which present understanding is still insufficient: the effects of drought on leaf senescence and canopy architecture.

Photosynthesis is not impaired in healthy tissue of blighted potato plants

The net photosynthetic rates of green leaf tissue of potato plants of different cultivars were measured in the field and in a controlled environment after infection of the plants byPhytophthora infestans.Infection had no significant effect on the net photosynthetic rate at light saturation, the efficiency of light use at low light intensities, or dark respiration. The reported effect ofP. infestans on tuber yield seems to be caused solely by a reduction in the green leaf area. Therefore, a high rate of photosynthesis in green leaf tissue of infected plants is not a good selection criterion for potato genotypes.SamenvattingBepalingen van netto fotosynthesesnelheden, bij verschillende aardappelrassen, werden uitgevoerd in het veld en onder geconditioneerde omstandigheden. De metingen werden gedaan aan groen blad van planten die in verschillende mate waren aangetast doorPhytophthora infestans.Infectie had geen significante invloed op de netto fotosynthesesnelheid bij lichtverzadiging, de efficiëntie van lichtbenutting bij lage lichtintensiteit, of de donkerademhaling. Het effect vanP. infestans op de knolopbrengst van aardappelrassen lijkt uitsluitend veroorzaakt te zijn door een vermindering van groen bladoppervlak. Daarom is selectie van aardappelgenotypen met superieure handhaving van fotosynthetische activiteit bij aantasting, geen kansrijk veredelingsdoel.

Photosynthetic responses in spring wheat grown under elevated CO2 concentrations and stress conditions in the European, multiple-site experiment 'ESPACE-wheat'

Spring wheat cv. Minaret crop stands were grown under ambient and elevated CO2 concentrations at seven sites in Germany, Ireland, the UK, Belgium and the Netherlands. Six of the sites used open-top chambers and one used a controlled environment mimicking field conditions. The effect of elevated CO2 for a range of N application regimes, O3 concentrations, and growth temperatures on flag leaf photosynthesis was studied. Before anthesis, flag leaf photosynthesis was stimulated about 50% by 650 compared with 350 mmol mol−1 CO2 at all sites, regardless of other treatments. Furthermore, there was no evidence of a decrease in photosynthetic capacity of flag leaves due to growth at elevated CO2 before anthesis, even for low N treatments. However, photosynthetic capacity, particularly carboxylation capacity, of flag leaves was usually decreased by growth at elevated CO2 after anthesis, especially in low N treatments. Acclimation of photosynthesis to elevated CO2 therefore appears to occur only slowly, consistent with a response to changes in sink–source relationships, rather than a direct response. Effect of elevated CO2 on stomatal conductance was much more variable between sites and treatments, but on average was decreased by ~10% at 650 compared with 350 mmol mol−1 CO2. Carboxylation capacity of flag leaves was decreased by growth at elevated O3 both before and after anthesis, regardless of CO2 concentration.

Level of threshold weed density does not affect the long-term frequency of weed control.

Abstract Weed control thresholds are often presented as a means to reduce unnecessary control measures, thereby increasing the effectiveness of weed management. While the threshold is a useful tool for cost-effective application of control on a single-year base, its role over the longer term is more complicated. It is shown that long-term application of thresholds results in a control frequency that is independent of threshold level, and in aperiodic dynamics of the weed population which may cause uncertainty about what control frequency and hence what costs of control are expected over a given period. We conclude that the economic underpinning of the threshold concept is deceptive and does not provide a base for rational use of weed control in the long term.

Evaluation of breeding strategies for resistance and tolerance to late blight in potato by means of simulation

A field experiment with three potato cultivars, where plants were inoculated withPhytophthora infestans, was used to parameterize a model of potato growth and blight population dynamics. The model was validated by accurately simulating a field experiment conducted in another year. Sensitivity analysis with the model showed that late cultivars are longer able to maintain a green canopy in the presence of disease, but still suffer more yield loss than early cultivars. The level of partial resistance of a cultivar was more important than its level of tolerance, and other plant characteristics. The model calculations showed that only between 4 and 15% of the yield loss in the experiments was due to accelerated leaf senescence caused by the disease; the major part of the loss was caused by lesion coverage of leaves.

Integrating remote sensing datasets into ecological modelling: a Bayesian approach

Process‐based models have been used to simulate 3‐dimensional complexities of forest ecosystems and their temporal changes, but their extensive data requirement and complex parameterisation have often limited their use for practical management applications. Increasingly, information retrieved using remote sensing techniques can help in model parameterisation and data collection by providing spatially and temporally resolved forest information. In this paper, we illustrate the potential of Bayesian calibration for integrating such data sources to simulate forest production. As an example, we use the 3‐PG model combined with hyperspectral, LiDAR, SAR and field‐based data to simulate the growth of UK Corsican pine stands. Hyperspectral, LiDAR and SAR data are used to estimate LAI dynamics, tree height and above ground biomass, respectively, while the Bayesian calibration provides estimates of uncertainties to model parameters and outputs. The Bayesian calibration contrasts with goodness‐of‐fit approaches, which do not provide uncertainties to parameters and model outputs. Parameters and the data used in the calibration process are presented in the form of probability distributions, reflecting our degree of certainty about them. After the calibration, the distributions are updated. To approximate posterior distributions (of outputs and parameters), a Markov Chain Monte Carlo sampling approach is used (25 000 steps). A sensitivity analysis is also conducted between parameters and outputs. Overall, the results illustrate the potential of a Bayesian framework for truly integrative work, both in the consideration of field‐based and remotely sensed datasets available and in estimating parameter and model output uncertainties.

Analysis of the experimental variability in wheat responses to elevated CO2 and temperature

Abstract For a reliable assessment of the impacts of climate change on future yields of arable crops the interpretation and extrapolation of the results from crop experiments has been a major problem. The reason is the large variation in observed crop responses to CO 2 enrichment and temperature change. This study aims at improved understanding of the causes of this variability. A large number of data sets from wheat experiments under CO 2 enrichment or temperature change were analysed, and time courses of crop growth processes have been simulated with a crop growth model. These model analyses explained the variability in observed yield responses on the basis of crop characteristics and climatic conditions. Simulation of wheat experiments with the new version of light interception and utilisation (LINTUL) for climate change studies (LINTULCC) growth model showed that the correspondence between observed and simulated yield responses to CO 2 enrichment or temperature rise was often poor. This was partly caused by the modelling approach in LINTULCC and partly by the experimental variability which cannot be reproduced by LINTULCC. This comparison of experimental with simulated results showed that their correspondence might be improved in two opposite ways. The morphological development of the crop and the limiting effects of available sinks (i.e. grains) on crop growth may be described in more detail. However, in many situations (i.e. without severe water or nutrient stress) crop variables in the experiments were more constant than simulated with LINTULCC. In such a situation, a simplified model approach may be applied and may yield better results.

Selection and use of a mathematical model to evaluate components of resistance to Phytophthora infestans in potato

Five models of general epidemics, spatially homogeneous, were all shown to fit well to disease progress data forPhytophthora infestans on a susceptible potato cultivar. The models were: the logistic equation, the paralogistic or Vanderplank equation, two models from medical epidemiology with similar complexity, and a slightly more complex model with explicit treatment of lesion expansion. The use of the models for analysing the sensitivity of disease progress to changes in resistance components is discussed. Sensitivity analysis of the most complex model, by varying components within their range of genetic variation, indicates lesion expansion and infection efficiency as the components offering the best perspectives for resistance breeding. Improving two components simultaneously reduces disease progress slightly more than additively, but not enough to add other components to the list of breeding objectives. Pitfalls in using models for component sensitivity analysis, in the form of erroneous model initializations, are discussed, including implications for the role of components in the development of natural epidemics and in resistance breeding trials.

Modelling the dependence of European potato yields on changes in climate and CO2

Results from potato trials at Dundee, UK were used to test a potato growth model. Next, the effects of future changes in climate on tuber yields of rainfed potato (cv. Bintje) in the EU were studied with this tested model. This was done first by modelling the sensitivity of rainfed tuber production to separately changed weather and air composition (CO2, O3) variables. Second, tuber yields of rainfed potato (cv. Bintje) were calculated for both historical climate conditions over the EU and future conditions (year 2050) as based on scenario climate changes. Yields generally increased by 1000–3000 kg/ha for scenario climate change in all regions of the EU. Third, regions in the EU that may become hazardous for potato production in the future, were identified. This showed that the risks of potato production did not significantly increase under scenario climate change.