Allocation shifts favor asexual reproduction: Response to damage and pollination in a clonal flowering plant (Eichhornia crassipes)

Background/Question/Methods

Plants that reproduce both sexually and asexually may benefit from the allocation of resources to the mode of reproduction that will be most successful in current environmental conditions. Successful pollination or herbivore damage to clonal buds might favor increased allocation to flowers, while lack of pollination or herbivore damage to apical meristems might favor increased allocation to clones. Herbivore damage reducing overall resources might reveal which mode of reproduction is favored when resources are limited. I used Eichhornia crassipes (water hyacinth) and its specialist herbivores Neochetina spp. (water hyacinth weevils) to examine reproductive responses to damage and hand pollination. Three damage types were intended to simulate larval and adult weevil damage: apical meristem damage, axillary bud damage, and leaf damage. Plants were surveyed for leaf, stolon, and inflorescence production, and insect bioassays tested for induced resistance to manual damage. Additionally, I surveyed three north Florida E. crassipes populations to assess differences in allocation pattern, herbivore damage, and herbivore resistance, and conducted a common garden experiment to determine if differences in allocation pattern or resistance persisted in a common environment.

Results/Conclusions

Manually imposed damage does not induce defenses (although bioassays indicate that E. crassipes can induce resistance to real herbivore damage), suggesting that responses to damage were due to changes in allocation to growth or reproduction rather than to defenses. Apical damage limited leaf and flower production and increased subsequent stolon production; axillary damage limited stolon production and increased subsequent leaf production; no damage types altered flower or inflorescence production. Since increased leaf production will enable subsequent stolon production, these results suggest that allocation shifts generally favor asexual reproduction over sexual reproduction. Neither leaf damage nor pollination affected growth or reproduction. Overall, meristem damage and asexual responses appear more important to plant success than seed set, even when pollination is available. This may explain why E. crassipes is a successful invader despite an apparent lack of pollinators in its north Florida range. Common garden experiments indicate that differences in size, reproduction and damage among populations are not genetic.