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Developmental changes in reproductive parts of wild parsnip and optimal defense
Optimal defense theory is predicated on two fundamental assumptions:
1.defenses are beneficial--they enhance fitness relative to undefended organisms when predators, parasites, or pathogens are present.
2.defenses are costly--they reduce fitness relative to undefended organisms when predators, parasites, or pathogens are absent.
If defenses are both costly and beneficial, natural selection will produce defense systems with favorable (greater than one) benefit-cost ratios.  The allocation to defenses within a plant will depend on whether, and how often, the plant part is attacked and how valuable the plant part is to fitness
Reproduction in wild parsnip proceeds through a series of stages.  First, very small buds are produced.  These buds expand first into male flowers and, subsequently, females flowers.  If the ovules are successfully fertilized, fruits develop, which contain seeds.  Parsnips are temporally dioecious, which means their flowers pass through a male stage prior to becoming female.  This is the standard progression in the primary umbel (the first inflorescence to flower).  In later developing secondary and higher order umbels, a proportion of the flowers never become female and thus never produce fruit.
buds.JPG (77414 bytes)maleumbe.JPG (29541 bytes)female.JPG (56087 bytes)
             BUDS                         MALE FLOWERS         FEMALE FLOWERS
 
devel.JPG (33585 bytes)

As this process proceeds, more and more biomass is incorporated into the reproductive unit.  Concomitantly, the amount of chemical defense, in the form of furanocoumarins, increases as well, but at a faster rate.   Hendrix (1979) discovered that wild parsnips can compensate for flowers lost to herbivores by producing more inflorescences containing a higher percentage of flowers that have both male and female stages. In view of the plant's ability to compensate for lost flowers, the accelerated  investment in furanocoumarins during development was a mystery.
 
opdefflow.JPG (34771 bytes)The principal herbivore of parsnip reproductive parts is the parsnip webworm.  This insect feeds on all stages of reproductive development except late stage fruits. We reasoned that compensation might involve reallocation of resources that would have gone to structures, had they not been consumed, to production of new infloresences.  We further reasoned that this ability to reallocate would diminish as more and more resources were incorporated into the reproductive unit.  To test this hypothesis, we removed the primary umbel reproductive units of different sets of small  plants in the field at different developmental stages--female flowers or green fruits.  Other plants were permitted to complete development of seeds in the primary umbel.  Total number of seeds per plant was not significantly reduced if only female flowers were removed (252 seeds vs 358 seeds), suggesting that the compensation mechanism was successful.  However, when green fruits were removed, significantly fewer seeds were produced (190 compared to 358).   Thus, the enhanced defense of fruits may function to ensure survival of offspring that cannot be replaced.
last updated 1/11/2000