Library

Notes: Abstract Genetic variation in flowering time was studied in four natural populations of Arabidopsis thaliana, using greenhouse experiments. Two populations from ruderal sites flowered early, two others from river dykes late. However, the late flowering plants flowered almost as early as the others after cold treatment of the seeds. The observed differences in flowering time have consequences for the potential life cycles of plants in the field. Plants from ruderal sites can be summer or winter annual, perhaps even have two generations per year. The strong response to cold treatment of plants from the dyke vegetations makes their life cycle strictly annual. Segregation analyses of the F2-s from crosses between early and late plants suggests that this variation in flowering time was genetic, with a dominant major gene for late flowering. Lateness in combination with vernalization responsiveness appears to be mostly due to a single gene in A. thaliana. It is hypothesized that early flowering ecotypes have evolved from late flowering ecotypes in different habitats by a loss mutation in the inhibitory mechanisms that delays flowering until after a period of low temperatures.

Notes: Abstract The growth habit of the rosette plant Plantago lanceolata is highly variable, and many vegetative and reproductive traits co-vary. At one end of the range plants have relatively few but long and erect leaves, form few daughter rosettes, and produce a limited number of large spikes, with relatively heavy seeds. Plants at the other end of the range have the opposite characteristics. This suite of characters was shown to correlate with the height of the vegetation in mid-summer. The causes for this association between different traits were investigated in different experiments, with the following results:〈list xml:id="l1" style="custom"〉Plants from two contrasting habitats both react strongly to light intensity and the red to far-red (R/FR) ratio of the ambient light. Light intensity mainly affected plant size, whereas light quality affected the growth habit. Populations differ in their mean response rather than in the level of plasticity (i.e., slope of the reaction norms). Experiments show that genetic factors (population effects), R/FR ratio, and hormone treatments (GA or CCC) have similar effects on morphology, and are largely additive and interchangeable.Ten different populations were grown in a common garden, so that the genetic (clonal) correlation within populations, and their bivariate phenotypic means could be compared. Trait combinations which deviated in the same direction (both higher of both lower than the mean over all populations) on average had positive clonal correlations within populations, whereas combinations which deviated in opposite directions had negative correlations.Artificial selection on leaf length, performed under a high or a low R/FR ratio showed clear responses to selection, with heritabilities around 0.4. Correlated responses were found in many other traits, and genetic correlations were similar to the trait associations for the means of different natural populations. Correlated responses to selection depended on environmental circumstances. Under a high R/FR ratio (sun) evidence for a size/number trade-off was found for leaf length and leaf number. Under a low R/FR ratio, however, a trade-off between leaf length and leaf quality was found.In conclusion, the trait associations that are so characteristic for the growth habit in Plantago lanceolata are probably due to differences in hormone levels or activities. Genes and environmental factors affect growth habit in similar ways, by tapping into this regulatory mechanism. In the field, selection can lead to concerted changes in the mean of different traits, but changes in plasticity will be slow.