Carola Pekrun

Cultural control of volunteer oilseed rape ( Brassica napus )

Laboratory studies on the biology of oilseed rape ( Brassica napus L.) showed that the induction of secondary dormancy is influenced by light environment, time of exposure to light and darkness, temperature regime and genotype. Seeds did not become dormant while exposed to light but were increasingly likely to become dormant the longer they were exposed to water stress and darkness. Dormancy was broken by alternating warm and cold temperatures. Conclusions from results obtained in Petri dishes have been tested in the field and hypotheses regarding the effects of post-harvest cultivation have been proposed. In July 1995, field experiments were initiated on a flinty silty clay loam and a sand to test the implications of post-harvest cultivation on the development of a persistent seedbank. The results largely confirmed assumptions made on the basis of laboratory findings. Seeds that had been exposed to water stress and darkness for longest, by cultivating the soil at the beginning of the experiment, immediately after seed distribution, exhibited the highest persistence rates. Seeds that were exposed to light for 4 weeks and then incorporated into the soil built up a much smaller seedbank. The seedbank was very small or nonexistent in plots that had not been cultivated at all.

Induction of secondary dormancy in rape seeds (Brassica napus L.) by prolonged imbibition under conditions of water stress or oxygen deficiency in darkness

Abstract Prolonged imbibition under conditions of water stress or oxygen deficiency can lead to the induction of secondary dormancy in rape seeds. During imbibition in darkness seeds develop light sensitivity. The percentage of seeds not germinating in the dark depends on various factors prevailing during and after the stress treatment, as well as on the intrinsic susceptibility of the seeds to these factors. In experiments on the effect of water stress it was found that the percentage of secondary dormant seeds could be increased in the range of 0–75%, by greater water suction and duration of imbibition of the seeds. Additionally, there were indications that increasing treatment temperature resulted in increasing percentages of dormant seeds. However, fewer secondary dormant seeds were produced when treatment temperature and test temperature differed greatly from each other. Experiments on the effect of oxygen deficiency also revealed a positive effect of treatment duration. The general degree of dormancy induction, however, in this series of experiments was low. At 20 °C almost no dormancy induction was observed, whilst at 12 °C there were between 0 and 30% dormant seeds. Genotypic differences previously assessed in model experiments with buried seeds were confirmed in both studies. Oilseed rape cultivars Bienvenu and Liglandor were shown to be able to build up high percentages of secondary dormant seeds, whereas cultivars Jet Neuf and Falcon had a rather low susceptibility towards the dormancy imposing factors tested. Cultivar Rubin was intermediate in response. There was often much variation between experiments with the same experimental design. The reasons for this are unclear.

The long-term persistence of seeds of oilseed rape ( Brassica napus ) in arable fields

The present paper reports on three sets of experiments exploring the persistence of seeds of oilseed rape ( Brassica napus ). The first, where known numbers of seeds were buried in September 1991 in two field experiments, demonstrated substantial initial losses of seeds, such that only 0·2 and 3·8% of seeds were still present after 4 months. In these experiments, which were not disturbed by mechanical cultivation, there was little evidence of further decline over the following 13 months. In the second of the two experiments, seeds were then left undisturbed for a further 136 months. A mean of 1·8% of seeds were still present after this period, providing further confirmation of the lack of decline in seed numbers in these undisturbed conditions. In the second pair of experiments, known numbers of seeds of three rape cultivars were broadcast onto plots and then either ploughed into the soil immediately after the start of the experiments, or were exposed to weekly shallow tine cultivation followed by ploughing after 4 weeks. The former created a larger seedbank than the latter. The experiments were then ploughed, annually (Expt 1) or at less frequent intervals (Expt 2); appreciable numbers of seeds survived for 65 months in both. Calculations based on exponential decline curves indicated that 95% seed loss would take 15–39 months, depending on the site, cultivar and initial post-harvest stubble treatment. The third part of the paper is based on more detailed studies of persistence of seeds of six cultivars in Petri dishes and buried in 25 cm pots. This work confirmed that cultivars differed in their persistence, as Apex was confirmed as highly persistent, whereas Rebel was short-lived. There were inconsistencies in the response of cultivar Synergy between the Petri-dish and pot experiment, which need further study. This experiment also reinforced the conclusion of the initial field experiments that little seed loss occurs in the absence of cultivations. Appreciable numbers of rape seeds will persist up to 4 years, in normal cropping conditions and in the absence of cultivation one experiment has confirmed persistence for over 11 years.

Seed persistence of oilseed rape (Brassica napus): variation in transgenic and conventionally bred cultivars

Seeds of oilseed rape ( Brassica napus L.) can persist in the soil over several years by becoming secondarily dormant and can then germinate to create volunteer plants in following crops. As well as agricultural impacts caused by volunteers, gene dispersal in time – particularly from genetically modified cultivars – can be another undesirable consequence. Conventionally bred and transgenic seeds were tested in 2001 and 2002 in laboratory experiments, and in a field experiment, by burying seeds in the soil to determine the variation in dormancy and persistence capacity. In the conventional group of cultivars tested in the laboratory, the level of dormancy was 13–76% in 2001, and 3–76% with an extended group in 2002. The transgenic group of cultivars was 1–31% dormant. In the burial experiments the number of viable seeds recovered in the conventionally bred cultivars ranged from 34–90% in 2001, and 7–68% in 2002. In the same studies the transgenic cultivars developed persistence levels from 12–79% in 2001, and 46–67% in 2002. Since dormancy levels of conventionally bred cultivars from 2 harvest years in the laboratory tests correlated significantly ( r =0·71), it appears that there is a genetic background to secondary dormancy. There was also a significant correlation ( r =0·61 in 2001 and 0·80 in 2002) between the results from laboratory and burial experiments. This indicates that the laboratory approach can simulate the situation in the field. Ageing over 6 months decreased the capacity for seed persistence to about a fifth of the level shown when freshly harvested. As a consequence of ageing and environmental impacts on persistence, only seeds from the same location and harvest year should be used for testing genetic variability. The high genetic variability among currently available rape seed cultivars gives breeding strategies a good chance of ideotyping low persistence genotypes and minimizing the risk of gene dispersal in time.

Population dynamics of volunteer oilseed rape (Brassica napus L.) affected by tillage

Volunteer plants of oilseed rape (Brassica napus L.) from persistent seeds in soil can affect subsequent crops. Apart from the agricultural disadvantages, the environment and the marketing of the seeds may also be affected, particularly if plants with special ingredients or genetically modified (gm) plants are grown. In order to investigate the influence of soil cultivation and genotype on seed persistence and gene flow via volunteers, a field experiment was set up testing four tillage treatments and two cultivars in a split-plot design. The cultivars tested were near-isogenic to two gm cultivars. To simulate harvesting losses, 10 000 seeds m −2 were broadcast on a soil in July. The subsequent tillage treatments were combinations of immediate or delayed stubble tillage by a rotary tiller, primary tillage with plough or cultivator, or zero tillage. Over the following year, the fate of the seeds was determined. Immediate stubble tillage with following cultivator or plough resulted in 586 resp. 246 seeds m −2 in the soil seed bank. After delayed stubble tillage with following plough, 76 seeds m −2 were found, and no soil seed bank was built up in the zero tillage treatment. Nevertheless, in the zero tillage treatment, several robust volunteer plants survived the herbicide application before the direct drilling in autumn until following spring. In the zero tillage treatment and in the cultivator treatment, 0.19 volunteers m −2 resp. 0.06 volunteers m −2 flowered simultaneously to ordinarily sown oilseed rape in the following crop of winter wheat and produced 73 resp. 18 seeds m −2 . Delayed stubble tillage reduced the risk of gene escape via the soil seed bank, while zero tillage resulted in the highest risk of gene escape by pollen and by production of a new generation of seeds. In terms of a labelling threshold for gm food this number of seeds would be below the threshold of 0.9% of transgenic parts in conventially bred food or feed.

Post-harvest gene escape and approaches for minimizing it

Data about gene escape by seeds and volunteers were compiled for the first time in one study for several crops, i.e. wheat (Triticum aestivum), sugar beet (Beta vulgaris), oilseed rape/canola (Brassica napus) and maize (Zea mays). These species represent important genetically modified (GM) crops with herbicide tolerance (HT) or insect resistance (Bt), show different levels of autogamy and allogamy and are grown in different climatic zones of the world. Post-harvest measures and strategies were identified for minimizing gene escape from these crops. All species were found to cause problems in terms of gene escape by seed and volunteers though there are important differences between species and climatic zones. Post-harvest tillage was identified as a key factor for reducing the soil seed bank and volunteers. Timing and intensity of tillage has to be specifically adapted to the dormancy characteristics of each species. Furthermore, there is a close interaction between gene escape and the cropping system. Rotations should avoid the same crop or other critical crops in temporal vicinity to the GM crop in order to keep volunteer populations below a critical density. In no-till systems with use of HT varieties, HT volunteers can reduce the efficiency of the whole system if additional herbicides have to be applied. Seed impurities and admixtures during seed production are another major source of gene escape. Since seed lots of certified growers present less adventitious presence of other varieties, these should be preferred to farm saved seeds. Education of farmers, cleaning of equipment, control measures and separate production and supply chains are additionally important to minimize gene escape.

Studies on the Persistence of Rape Seeds (Brassica napus L.), Emphasizing their Response to Light

Rape seed showed a range of responses to light. When tested under standard germination conditions, they were unresponsive to light but under unfavourable germination conditions they tended to be inhibited by light. After prolonged exposure to sub-optimal germination conditions in darkness rape seed exhibited light sensitivity. Their germinability in darkness was considerably reduced, whilst their germinability in light was high. This light sensitivity was subsequently lost. Seeds being transferred from darkness to light during the germination test were not able to react to light any more. So, during germination tests in darkness seeds developed skotodormancy. Generally, there was no change of germinability when rape seed were imbibed under sub-optimal germination conditions in light. In one cultivar light inhibition was imposed but seeds of this cultivar remained highly germinable in darkness as well. So, after imbibition in light rape seed were never dormant in darkness. A pot experiment carried out to test the conclusions of the previous laboratory experiments under a more natural environment confirmed the strong impact of the light environment on the ability of rape seed to persist in darkness. It also confirmed genotypic differences identified in the laboratory studies.

Is it necessary to split nitrogen fertilization for winter wheat? On-farm research on Luvisols in South-West Germany.

SUMMARY Mineral nitrogen (N) fertilization in cereals is commonly split into three or four applications. In order to simplify N fertilization, a single N application either broadcast or placed on the soil surface was compared to conventionally split fertilization for winter wheat (Triticum aestivum L.). The 4-year experiment (2007–2010) was performed using a participatory approach on farmers’ fields on deep loamy soils (Luvisols) in South-West Germany. Grain yield and crude protein contents differed only slightly or not at all between treatments including different N fertilizer types (calcium ammonium nitrate, urea ammonium nitrate solution, urea) and application techniques (broadcast, placed). Furthermore, no differences were found for the yield components ears/m2 and thousand grain weight. Inorganic N in the soil profile after harvest was generally below 40 kg N/ha and did not differ between treatments. In the area where N was placed, mineral N was depleted during the vegetation period. At the experimental sites a single N application in the period between tillering and stem elongation was sufficient to achieve high yield and quality of winter wheat without increased risk of nitrate leaching. This finding was independent of the method of application or the type of fertilizer.