While climate change mainly applies to global warming and subsequent drought periods, freezing periods also are changing and may trigger genetic adaptation. However, there are few prospective, experimental demonstrations. This paper investigates how effectively genetic frost-resistant variants can respond differently to frost periods, especially in farmers' fields where various other selection pressures occur. An experiment using Avena fatua variants was set up in the field over 8 years of winter crop rotation. The frequency of each variant identified by their leaf isozymes was estimated every year. Six years were necessary to observe a weak trend towards an increased frequency of the frost-resistant phenotype, while the other variants had apparent erratic behaviour. Selection of the frost-resistant variant was challenged by irregular low temperature selection pressure, differential dormancy, germination and seed production, and possibly herbicide sensitivity that mitigated the expression of the selection response. This experiment shows how plant polymorphism and farmers' practices that superimpose habitat unpredictability diversify possible responses to selection pressures and delay adaptation. However, if climate change brings both higher mean temperatures and extreme values, changes of apparent plant phenology could happen for weeds species displaying the appropriate genetic variability within the time-frame of farmers' career, thus necessitating correlative adaptation of farming practices.

In order to assess the hybridization rate between oilseed rape and wild radish under normal agronomic conditions, three 1-ha field experiments were performed. In each case, wild radish plants were transplanted at different densities in the middle, the border, or the margin of the herbicide-tolerant oilseed rape field. Among the 189084 seedlings obtained from seeds harvested on wild radish plants, only one herbicide-tolerant interspecific hybrid (RrRrAC, 2n = 37) was characterized from seeds harvested on an isolated plant growing in the margin of the field. Thus, for the wild radish total harvest, with a 95% confidence limit, the frequency of interspecific hybrids was assessed to range from 10–7 to 3.10–5. Interspecific hybrids were detected in all cases among the smallest seeds with a diameter less than 1.6 mm harvested on oilseed rape, but the highest frequency was obtained from oilseed rape close to wild radish plants growing as clusters in the border or the margin of the field. Most hybrids had the expected triploid genomic structure (ACRr, 2n = 28) except for four amphidiploids (AACCRrRr, 2n = 56) and one hybrid from a wild radish unreduced gamete (ACRrRr, 2n = 37). Among the 73847 seedlings observed on the oilseed rape total harvest, the frequency of interspecific hybrids was assessed to range from 2.10–5to 5.10–4, with a 95% confidence limit. The results are discussed with regard to the type of oilseed rape variety used and the characteristics of the interspecific hybrids.

The occurence of spontaneous hybridization between Brassica napus (oilseed rape) and Raphanus raphanistrum (wild radish) was investigated under different density conditions in cages and open‐field experiments. Hybrids with wild radish as the seed parent were identified by screening for herbicide resistance belonging to rape. Small seed size and intermediate morphology were used to screen for hybrids with rape as the seed parent. Leaf isozyme patterns and flow cytometry provided confirmation of hybrids. Wild radish in an oilseed rape field produced as many as three interspecific hybrids per 100 plants. This is the first report of such a spontaneous event. The frequency of hybrids is expected to range from 0.006 to 0.2% of the total seed produced, at P = 0.05. Male‐sterile oilseed rape plants surrounded by wild radish can produce up to 37 hybrids per plant. Seed production of the F1 hybrids and their F2 descendants was up to 0.4% and 2%, respectively, of that of wild radish. Gene escape from transgenic oilseed rape to wild related species is discussed.

SummaryProgeny of several Sinapis arvensis L. plants sampled in a field were analysed for isozyme patterns at two highly polymorphic loci: esterasc and acid phosphatase. Allele frequencies of pollen subpopulations that effectively fertilized each separate sampled plant were compared. In spite of self‐incompatibility, large variations in allele frequencies were shown among pollen subpopulations. Non‐random mating owing to gametic selection and plant density effects were evidenced and could reduce gene flow within a population,

Inheritance of dinitroaniline resistance was investigated in F-2 and F-3 populations of crosses between a resistant green foxtail and three foxtail millet cultivars (Setaria viridis and Setaria italica, respectively), Resistance was assessed on the basis of inhibition of root growth of seedlings in trifluralin solution, Results indicated that resistance was recessive but did not give a monogenic segregation ratio in F-2. Distortion of segregation or two linked loci (r=.23) might account for only 15% resistant plants in F-2 progeny, Quantitative analysis showed that part of the response to trifluralin is under the control of further minor genes.

1. The growth of interspecific hybrids between oilseed rape (Brassica napus ssp. oleifera) and hoary mustard (Hirschfeldia incana) was studied in different conditions of temperature and photoperiod in individual pots and in competitive conditions with each of the two parental species.2. In the absence of competition, the hybrid plants were intermediate between parents for morphological and developmental traits. Under competitive conditions, the hybrids were closer to hoary mustard. The results of the studies indicated that hybrids and hoary mustard would not be better competitors against oilseed rape, but hybrids would be more competitive in a hoary mustard population.3. The relevance of these studies to the environmental impact of the release of transgenic crops is discussed. Hybrid survival would not limit gene escape through interspecific hybridization.