Dimorphotheca pluvialis is a herbaceous annual native to South West Africa (Norlindh 1977). As is common in the Asteraceae, the capitulum (flower head) bears two types of florets. The species Dimorphotheca is characterized by hermaphrodite disc florets and female-fertile (male-sterile) ray florets. Both types of florets produce distinctly different types of seeds (achenes). Seeds produced by ray florets are small, angular, while those of the disc florets are flattened and have winged margins (Barclay and Earle 1965). The ray florets have one large white petal, which is often colored purple at the base, giving the appearance of a "ring" in the inflorescence.
The species is well adapted to the maritime climate of Northern and Western Europe, and fits in a rotation system with annual crops. Although it is known as a garden ornamental, Dimorphotheca is considered an undomesticated species showing many primitive characteristics. Populations collected from the natural habitat in general have a long, unsynchronized flowering period and show poor seed retention. These factors together account for severe yield losses prior to and during harvest. Realized yields at trial fields range 500-1500 kg/ha, with potential yields of at least 2000-2500 kg/ha. As it is sensitive to frost, in the Netherlands, Dimorphotheca is grown as a summer annual. It is sown in April, flowers in July and can be harvested in August (van Dijk et al. 1993). The average oil content of collected populations is 21%, which is too low for mechanical expelling. At present, oil recovery should be done by solvent extraction or preferably with supercritical carbon dioxide extraction (Muuse et al. 1992).
Dimorphotheca was first introduced in the Netherlands by the Dutch Gene Bank (CGN) in 1986, and since 1990 has been studied extensively in the framework of three large multidisciplinary projects (the Dutch National Oilseeds Program, and the EC-projects VOICI and VOSFA). In these projects expertise from the whole production chain was brought together including germplasm collection, evaluation, cultivation, breeding, crop physiology, pathology, harvest techniques, oil recovery, processing of the oil, application, and market research (van Soest and Mulder 1993).
At the Centre for Plant Breeding and Reproduction Research (CPRO-DLO), research in Dimorphotheca has been focusing on improvement of synchronization of flowering and seed ripening, oil content, and plant architecture. Furthermore, research is carried out to study pollination and mating system.
For synchronization of flowering two distinct characteristics were
To establish the variation for synchronization of flowering (both between and within plants) and oil content, two experiments were carried out on loam soil at location Lelystad in the Netherlands. Heritabilities were estimated by means of parent-offspring regression in the following year.
To estimate the heritability of this character in this population, 40 plants were selected and their seeds collected. D. pluvialis is considered a predominantly outcrossing species, and the progenies of the selected plants are considered to be half-sib families. In 1993, a trial field was sown with 24 (three rows of eight) plants of each of the 40 families, and time of flowering was scored.
The relationship between selected plants and the mean of their progenies for time of flowering is presented in Fig. 1. It is clear that selected plants and their corresponding families showed considerable resemblance, this despite difference in weather conditions in 1992 and 1993.
Narrow sense heritability (h 2n) is described as the ratio between genotypic and phenotypic effects. High heritabilities indicate that the genetic component in the observed phenotype is relatively important to the environmental component. This means that high heritabilities for a character usually result in a quick response to selection. With low heritabilities the response to selection is not necessarily lower, but more time consuming. From the linear regression of offspring (Y) on female parents (X), expressed as Y = a + bX, the (narrow sense) heritability can be estimated by h 2n = 2b (Falconer 1989). The estimated heritability for time of flowering in this experiment was 0.94, which is very high.
The cumulative numbers of open flowers per plant plotted against time fitted a logistic curve (Y = c/[1 + e-b(X - m)]). In this curve, c represents the upper asymptote (being the total number of heads, TNH), b the "slope parameter," and m the inflexion point of the curve, which is also the date at which the maximum number of open flowers was counted: peak bloom. The flowering of each individual plant was characterized by these three parameters. In this experiment this model on average accounted for 99.6% of the observed variation, indicating that it described the flowering of individual plants well.
Using this model, the period in which the plants produced 90% of their total number of flowers (the duration of flowering) could be calculated. The 90% interval, and not 100%, was chosen because slight deviations from the model occurred at beginning and end of flowering, accounting for relatively large aberrations in estimates of duration of flowering when using the 100% interval.
Duration of flowering ranged from 11 to 63 days, with a mean of 27 days. Since plants did not start flowering at the same time, environmental factors may have had a considerable effect. Therefore, also for this character heritability was estimated by means of parent-offspring regression.
From the population grown in 1992, 20 plants were selected showing much variation for duration of flowering. In 1993, ten plants per progeny were sown in a complete randomized block design, and flower counts were made in the same way as before. The explained variation for the fit of the logistic model on the data was 99.8%.
The relationship between female parents and the mean of their corresponding families is shown in Fig. 2. The calculated regression line explained only 13% of the variation. This means that it leaves 87% of the variation still to be accounted for, and seems drawn rather arbitrarily through a cloud of data points. The estimated heritability (based on the slope of this regression line) of 0.27 can therefore be regarded as unreliable.
Analyses of variance revealed no difference between treatments 1 and 2 for crop development, seed set and thousand seed weight. Apparently the light shading did not effect these characters (Table 1). Population 879585 flowered slightly earlier than the other two. No population x treatment interaction was found for any of these characters.
Seed yield of plots with open cages was lower than yield of open fields. This could not be explained by a lower seed set, lower number of flowers, or lower thousand seed weight. In the harvest bags of this treatment moths were found, which might have caused severe damage.
Exclusion of insects led to a prolonged flowering of the crop, and a severely reduced seed set and yield. Thousand seed weight was higher.
For these experiments it was assumed that random mating and complete cross pollination has taken place. Furthermore, interaction effects (epistasis, genotype-year, year-location, and genotype-location) were considered negligible. It is likely that some of these assumptions were incorrect, and may have affected the outcome. Year and location effects can only be studied when experiments are carried out at several locations in several years. The presented results on heritability estimates are therefore preliminary, but nevertheless give an indication of what can be expected from selection.
Presence of insects during flowering is essential for a good seed set. Exclusion of insects may result in yield losses up to 75%.
Dimorphotheca pluvialis is as yet not ready for commercialization. Several agronomic constraints are recognized, but most can be overcome given time. Other problems still lay in the area of oil recovery and purification. However, the unique structure of dimorphecolic acid justifies further studies.
|No. of open flowers/ |
|Thousand seed weight|
|Variable||June 30||July 7||July 14||July 21||July 28||Seed yield (g/m2)||No. unwinged seeds/flower||No. winged seeds/flower||unwinged seeds (g)||winged seeds (g)|
|Fig. 1. Parent-offspring relationship for beginning of flowering in a population of Dimorphotheca pluvialis (days after sowing).|
|Fig. 2. Parent-offspring relationship for duration of flowering in a population of Dimorphotheca pluvialis (days).|
|Fig. 3. Parent-offspring relationship for oil content in a population of Dimorphotheca pluvialis (% oil in the seed).|