Effects of Pollen Addition on Gender Expression and Fruiting in North Carolina Piedmont Populations of Horsenettle (Solanum carolinense), an Andromonoecious Herb

Katherine R. Okey, Kelly N. Kiefer, and Patricia A. Peroni

Department of Biology, Davidson College



Horsenettle (Solanum carolinense: Solanaceae) is an andromonoecious perennial herb. Andromonoecy is a breeding system in which an individual plant produces both male and hermaphroditic flowers. Steven, Peroni, and Rowell (1995) found evidence of pollen limitation in two Virginia horsenettle populations, and speculated that pollen limitation may serve as a selection pressure that favors male flower production in horsenettle. In order to further investigate the role of pollen limitation in the maintenance of andromonoecy in horsenettle, we designed a field experiment to address the following questions:
1. Are fruit and seed production in North Carolina piedmont horsenettle populations pollen limited?
2. Do pollen additions affect gender expression?
3. What are the means for flower production per plant and gender expression for horsenettle populations from the North Carolina piedmont?
We selected 50 plants in each population and randomly assigned half to a pollen addition treatment and the remainder to a control treatment.
Fruit set and percentage of male flowers were greater in the pollen addition treatment than the control for both populations. However, these trends were not significant. We cannot conclude that pollen limitation exists in our North Carolina populations this year. Our power to address this question was limited because only 36% of our plants flowered due to an extended drought and most of the plants produced less than 5 flowers. Longer term investigations will be required to determine if pollen limitation acts to maintain andromonoecy in horsenettle.


Horsenettle is a self-incompatible, perennial weed common to eastern North America. Horsenettle displays an andromonoecious breeding system which means that an individual plant produces both male and hermaphroditic flowers. Current theories for the evolution and maintenance of andromonoecy assert that this breeding system is maintained because it provides flexibility in resource allocation. Male horsenettle flowers contain less biomass and nitrogen than hermaphrodites. The ability to produce male flowers may increase fitness of horsenettle plants if these staminate flowers increase the number of offspring sired without reducing the number of fruits matured. Andromonoecy may be particularly effective in increasing fitness in pollen limited systems. Pollen limitation occurs when the amount of seed a plant can produce is limited by the quantity or quality of pollen it receives.
Steven, Peroni, and Rowell (1995) found clear evidence of pollen limitation in two Virginia populations of horsenettle. However, we have no data on pollen limitation in other regions of horsenettle's range. As part of a longterm investigation into the evolution of andromonoecy in horsenettle, we investigated whether pollen limitation exists in two North Carolina piedmont populations of the species.


Two horsenettle populations identified in Davidson, NC.

Transects constructed in each population, 50 plants in each. Selected horsenettle's with flowerbuds at 2 meter intervals.

Plants randomly assigned to pollen addition and control treatments (n=25 for each treatment).

Transects monitored every 1-2 days; sex and eventual fate of each flower recorded.

Hand-pollinated all hermaphroditic flowers in the pollen addition treatment group.

Seed production recorded as fruits mature.

Figure 2. The Barger site (top) and the Kelly site (bottom).


The results do not allow us to conclude that pollen limitation existed in our North Carolina populations.

However, the trends for fruit set and gender expression are consistent with Steven, Peroni, and Rowell's (1995) results with the Virginia populations of horsenettle.

Since the many of the plants in our populations failed to flower, and those that did produced less than 5 flowers, our power to detect any pollen limitation was severely compromised.

Pollen limitation may not be a universal or consistent phenomenon in horsenettle populations.


I thank Dr. Patricia Peroni for the incredible amount of assistance, support, and guidance she gave me throughout the course of this project. I also acknowledge Kelly Keifer, Wyatt Rivens, and Scott Addison for the generous amount of assistance they gave me.

© Copyright 2000 Department of Biology, Davidson College, Davidson, NC 28036
Send comments, questions, and suggestions to: macampbell@davidson.edu