Anton M. Clemmons* and David L. Robinson
Department of Biology, Bellarmine College, Louisville, KY, 40205

Dandelion (Taraxacum officinale Weber), an asexual species, produces achenes (seeds) apomictically. Three experiments were performed to explore the genetic variability and heritability of various morphological/ physiological traits that occur in natural populations. In the first study, achenes from a single dandelion population were subjected to a warm temperature treatment (37^oC) for different time periods (0, 3, 4, 6, 8, 10, 14, 16 days), followed by a 4-day incubation at 21^oC, and a final cold treatment (5^oC). Germination was assessed at the end of each temperature treatment. The experimental control (constant 21^oC) exhibited the highest percent germination. In the other treatments, a positive correlation was found between the percent of ungerminated achenes and the duration of exposure to high temperature. To examine the heritability of achene heat tolerance, germinated achenes exposed to 8 days at 37^oC, and then 21^oC, were collected, grown to maturity, allowed to flower, and set seed. Achenes from these heat tolerant plants (as well as control plants) were grown to maturity in order to evaluate the next generation. In the second study, achenes collected from dandelion populations occurring in 5 different U.S. states were germinated and grown to maturity in a controlled environment. Analysis of leaf morphology revealed more variability between the 5 populations than within them. In a third experiment, achenes from 19 different fasciated (deformed) plants were germinated, and grown in a controlled environment to examine the inheritance of their expressed fasciation. The forth experiment will evaluate the morphological features of dandelions growing naturally in Joe Creason Park. These experiments help to delineate the amount of genetic diversity in dandelion populations.

Introduction

One of the reasons that dandelion (Taraxacum officinale Weber) is a common weed in Kentucky (and the world) is its high reproductive capacity. Dandelion plants typically produce hundreds of achenes (seeds) each with their own parachute-like pappus to aid in dissemination.

Dandelion reproduces apomictically which is a type of reproduction where the embryos develop asexually. Hypothetically, the achenes produced by this type of reproduction are genetically identical to the parent. There are reports, however, of sexual reproduction in dandelion (Solbrig and Simpson, 1975). These authors also noted a high level of somatic mutation.

Experiments were performed to explore the genetic variability and heritability of various morphological/ physiological traits that occur in natural populations. Our objectives were to:

1) Examine dandelion germination response to three different temperature treatments (37^oC, 21^oC, 5^oC), and select achenes that germinated under these different conditions for later progeny testing. This latter experiment helps describe the level of genetic variability in a naturally-occurring dandelion population in terms of its response to the environment. (Experiment 1)

2) Investigate the amount of genetic diversity in dandelions by comparing the morphological characteristics of plants representing five different populations collected in Kentucky, Virginia, West Virginia, Ohio, and Minnesota. (Experiment 2)

3) Explore the heritability of fasciation in dandelion. Fasciation is defined in dandelion as the occurrence of "flattened, coalesced, and malformed" flower stalks (Dekker and Dekker, 1987). Currently, there are no conclusive reports in the scientific literature to indicate whether this deformity is genetically, biologically, or environmentally induced.

(Experiment 3)

4) Examine the genetic variability of a naturally occurring population of dandelion in Joe Creason Park. This information will be compared with data generated in the lab.

Materials and Methods

§ Achenes from a single dandelion population were subjected to a warm temperature treatment (37^oC) for different time periods (0, 3, 4, 6, 8, 10, 14, 16 days), followed by a 4-day incubation (21^oC), and a final cold treatment (5^oC). Germination was assessed at the end of each temperature treatment.

§ To examine the heritability of achene heat-tolerance, germinated achenes exposed to 8 days at 37^oC, and then 21^oC, were collected, grown, allowed to flower, and set seed. Experimental control plants (from achenes that germinated at 21^oC) were also allowed to flower and set seed. Achenes from both populations were planted and exposed to two treatments: Treatment 1 = 24^oC (for 19 days), Treatment 2 = 37^oC (for 8 days), and then placed back into 24^oC (for 11 days). Germination was assessed at 5, 8, 12, and 19 days after planting. Three replications per treatment.

§ Achenes (progeny) from the heat tolerant plants (as well as control plants) were collected and grown to maturity. These second-generation plants are currently being induced to flower so that the third-generation response to temperature can be assessed.

§ Achenes, collected from dandelion populations occurring in five different U.S. states, were germinated and grown to maturity in a controlled environment. The lengths and widths (across the widest portion of each leaf) from the five most mature leaves were measured.

§ Achenes from 19 different fasciated (deformed) plants were planted into individual containers, and grown in a controlled environment to examine the inheritance of their deformity. Two containers of achenes from a non-fasciated population were also planted as experimental controls. Seed emergence and seedling vigor were evaluated 9 days after planting. Various morphological characteristics were assessed in the plants at 34 days after planting.

§ Dandelion plant from Joe Creason Park were evaluated to study genetic variability.

§ The experimental control (0 days at 37^oC; constant 21^oC) exhibited the highest germination (column 1). Heat treatment generally caused germination to decline. There was no detectable pattern for germination in the heat treatments.

§ Except for the control, the greatest amount of germination occurred after the achenes were taken out of the heat treatment and allowed to sprout at 21^oC (column 2). These achenes could be considered "quiescent" meaning that achenes remained alive until the environment became favorable for germination.

§ Generally, the longer the heat treatment the greater the number of achenes that failed to germinate. Presumably, these achenes died due to heat stress (Column 5).

§ Up to 25% of the achenes didn't germinate until exposure to cold (column 4) indicating that dandelion achenes possess some ability to develop dormancy. This dormancy was greater in the heat treated achenes than in the control. There was no detectable pattern for germination after cold treatment.

§ Our overall conclusion from this experiment is that dandelion germination is sensitive to high temperatures, but that they have an ability to remain quiescent until placed in a more favorable temperature. Even after 16 days of high temperature, only a third of the achenes died. Dormancy was induced in as much as 25% of the achenes exposed to heat

§ At Day-8, the achenes kept at room temperature (21^oC) germinated significantly more than those incubated at the warm temperature (37^oC), regardless of what environment the achene's parents had been selected in (column 3).

§ At Day-19, however, the progeny of the plants selected for germination after heat treatment germinated significantly more than those not selected (column 4).

§ It appeared that the achenes that were most sensitive to the heat treatment were from the control group. The achenes from the heat tolerant parents showed twice as much cumulative germination as the control group (column 5).

§ Our overall conclusion is that we succeeded in genetically enhancing the ability of dandelion achenes to tolerate high temperatures.

§ These calculations show that more than a third of the achenes (progeny) from "heat tolerant" parents were tolerant to heat, whereas the unselected plants showed only 3% germination at high temperatures.

§ On average, there was more genetic variability for leaf width than leaf length or length:width ratio.

§ There was no obvious geographical pattern for dandelion leaf morphology. For instance, the Minnesota population was not radically different from the Virginia population.

§ Our overall conclusion is that there was generally more genetic variability for leaf morphology between the populations than within them.

§ Overall, about half of the progeny from the fasciated plants differed from the control plants ("wildtype") for any single characteristic.

§ The most dissimilar plants were produced from #2 and #7, which were both plants that produced flowering stalks with two inflorescences (instead of a single inflorescence, which is normal).

§ As of this writing, these plants are being induced to flower so that they can be evaluated for fasciated stalks.

§ (Table 7) The Morphological characteristics of the naturally occuring population of dandelion is similar to the controls. Therefore, these plants did not show any signs of fasication.

Summary

These experiments show that there are significant amounts of genetic variability in dandelions, in spite of the fact that they primarily reproduce asexually.

Acknowledgment

We would like to thank the following people for their assistance:

Ms. Maria Davis (Bellarmine College)

Ms. Joann Lau (Department of Pharmacology and Toxicology, University of Louisville, Louisville, KY)

Dr. Peter P. Rowell (Department of Pharmacology and Toxicology, University of Louisville, Louisville, KY)

References

Dekker, J and RG Dekker. 1987. Mutant weeds of Iowa: Fasciation in Taraxacum officinale. Phytologia 63:155-156.

Solbrig, OT and BB Simpson. 1975. Components of regulation of a population of dandelions in Michigan. Journal of Ecology 48: 473-486.