Optimizing grain yields reduces CH4 emissions from rice paddy fields.

Microbial production in anoxic wetland rice soils is a major source of atmospheric CH4 the most important non-CO2 greenhouse gas. Much higher CH4 emissions from well managed irrigated rice fields in the wet than in the dry season could not be explained by seasonal differences in temperature. We hypothesized that high CH4 emissions in the wet season are caused by low grain to biomass ratios. In a screenhouse experiment, removing spikelets to reduce the plants' capacity to store photosynthetically fixed C in grains increased CH4 emissions, presumably via extra C inputs to the soil. Unfavorable conditions for spikelet formation in the wet season may similarly explain high methane emissions. The observed relationship between reduced grain filling and CH4 emission provides opportunities to mitigate CH4 emissions by optimizing rice productivity.

Coupling estimated effects of QTLs for physiological traits to a crop growth model: predicting yield variation among recombinant inbred lines in barley.

Advances in the use of molecular markers to elucidate the inheritance of quantitative traits enable the integration of genetic information on physiological traits into crop growth models. The objective of this study was to assess the ability of a crop growth model with QTL-based estimates of physiological input parameters to predict the yield of recombinant inbred lines (RILs) of barley. The model used predicts yield as spike biomass accumulated over the post-flowering period. We describe a two-stage procedure for predicting trait values from estimated additive and epistatic effects of QTLs. Values of physiological traits estimated by that procedure or measured in the field were used as input to the crop growth model. The output values (yield and shoot biomass) from the growth model using these two types of input values were highly correlated, indicating that QTL information can successfully replace measured input parameters. With the current crop growth model, however, both types of input values often resulted in large discrepancies between observed and predicted values. Improvement of performance may be achieved by incorporating physiological processes not yet included in the model. The prospects of using QTL-based predictions of model-input traits to identify new, high yielding barley genotypes are discussed.

Modelling field emergence patterns in arable weeds.

A model was developed to simulate weed emergence patterns after soil cultivation. In the model, the consecutive processes of dormancy release, germination and pre-emergence growth were modelled in separate modules. Input variables of the model were: date of soil cultivation, soil temperature and soil penetration resistance. Output variables of the model were: seedling density and timing of seedling emergence. The model was parameterized for Polygonum persicaria, Chenopodium album and Spergula arvensis with data from previous field and laboratory experiments. The model was evaluated with data from an experiment, in which emergence of P. persicaria, C. album and S. arvensis was monitored in field plots that were cultivated once only, at one of five dates in the spring. At the same time as the field observations on seedling emergence, seasonal changes in seed dormancy of the buried weed seeds were assessed by testing the germination of seed lots that were buried in envelopes. From a comparison between field observations and simulated data, it appeared that the model overestimated the rate of dormancy release in spring, whereas germination and pre-emergence growth were simulated well. In general, therefore, both the numbers of emerging seedlings and the timing of emergence could be predicted accurately, when dormancy was not simulated but introduced from experimental data. Improvement of predictions of field emergence of weeds should mainly focus on increasing the precision of the simulation of dormancy release. Close correlations were found between seedbed temperature and both the extent and rate of seedling emergence, but analysis with the simulation model revealed that they were only partly based on causal relationships, so that they have limited predictive value.

Modelling for precision weed management.

Recently, the need for the development of weed management systems with a reduced dependency on herbicides has increased because of concern about environmental side-effects and cost. The development of such systems requires new strategies based on improvements with respect to (1) prevention, (2) decision making and (3) weed control technology. For the development of improvements in all three aspects, quantitative understanding of weed population dynamics and crop-weed interactions is needed. Models that integrate the available quantitative knowledge can be used to design preventive measures, to develop long-term and short-term strategies for weed management, to assist in decision making to determine if, when, where and how weeds should be controlled and to identify new opportunities for weed control. Ecophysiological simulation models for crop-weed competition simulate growth and production of species in mixtures, based on ecophysiological processes in plants and their response to the environment. Such models help improve insight into the crop-weed system and can be used for purposes such as the development of simple predictive yield-loss models, threshold levels or the design of competitive crop plant types. For strategic weed management decisions, preventive measures and the identification of new opportunities for weed control, quantitative insight into the dynamics and spatial patterns of weed populations is also required. The complexity of the process and the long-term character of weed population dynamics make the use of models necessary. Different modelling approaches have been developed and are described briefly. Opportunities to use the available knowledge and models to improve weed management are discussed. Weeds occur in patches and their sensitivity to herbicides changes strongly with developmental stage, making precision techniques for herbicide application in time and space an option for reducing herbicide use. Limitations related to insight in biological processes as well as the state of technological development are discussed.

The effects of long-term open-air fumigation with SO2 on a field crop of broad bean (Vicia faba L.): III. Quantitative analysis of damage components.

Effects of SO2 on growth and production of broad bean, observed in three open-air fumigation experiments, were interpreted in terms of damage components with a mechanistic simulation model. The model consisted of an elementary model for crop growth, extended with submodels for the microclimate in the crop and with a submodel for uptake of SO2 by leaves and for effects on leaf photosynthesis. The major part of the observed reduction in total dry matter production could be explained by leaf injury observed in the oldest leaves of the fumigated plants at the end of the growing period. The effect consisted of dry matter loss through leaf abscission and a reduced growth rate at the end of the growth period due to reduction of the amount of absorbed radiation by the canopy. Direct effects of SO2 on leaf photosynthesis explained an extra 10% of the observed yield loss (losses ranged from 7 to 17% of control yield). This small effect was confirmed by field measurements which showed no detectable effects of SO2 on leaf and canopy photosynthesis. Increased leaf respiration, which was observed in the 1988 experiment, explained another 10% of the observed yield reduction. Total SO2 -sulphur uptake by the fumigated crop, which is an important component of dry deposition of SO2 , was accurately simulated by the model.

The effects of long-term open-air fumigation with SO2 on a field crop of broad bean (Vicia faba L.) II Effects on growth components, leaf area development and elemental composition.

Faba bean crops (Vicia faba L.) were exposed to elevated SO2 concentrations in three different years in an open-air field exposure system for the controlled release of air pollutants. The treated crops were exposed to an average SO, concentration of 165µg m-3 in 1985, 62µg m-3 in 1986 and 74µg m-3 in 1988. The ambient SO2 concentration was about 10 µg m-3 . Plant height, number of internodes and number of pods were not affected by SO2 . The specific leaf area was reduced in the plants exposed to SO2 at the end of the growing season. Leaf area development was strongly affected during the pod-filling period in 1985 and 1988 as a result of leaf injury and defoliation in the fumigated plots. In 1986 a similar trend in leaf area reduction was observed in the early reproductive phase. N and Mg content of the different organs was unaffected by SO2 . The S content was strongly elevated in the leaves and pods of the fumigated plants, and the Ca content of the leaves was reduced by SO2 . Chlorophyll content of different leaf numbers was unaffected by SO2 .

The effects of long-term open-air fumigation with SO2 on a field crop of broad bean (Vicia faba L.) I. Depression of growth and yield.

In an open-air Held exposure system for the controlled release of air pollutants, broad bean (Vida faba L.) crops were exposed to elevated SO2 concentrations in three growing seasons, in order to analyse the effects on crop growth under field conditions. The treated plots were exposed to a mean concentration of 165µgm-3 in 1985, 62µgm -3 in 1986 and 74µgm-3 in 1988. The background concentration was about 10µ m-3 . A reasonably uniform distribution of SO2 concentration was obtained over an area of 8m × 8m and concentrations exceeding the target concentration were rare. In 1985 and 1988, the growth rate of the crop was depressed at the end of the pod-filling period. This resulted in a reduction of total dry-matter production of 17% in 1985 and 9% in 1988, and a seed yield reduction of 23% in 1985 and 10% in 1988. In 1986, dry matter growth was not analysed up to the end of the growing season due to a severe infection of Botrytis fabae (Choclate spot disease) infection in the control plot in the middle of the pod-filling period. Slight B. fabae infections in the control plots only were also observed in 1985 and 1988. In all experiments no significant reductions of dry matter growth were observed in the vegetative and early reproductive phases.