Floral Development and Chemical Defense Allocation in Wild Parsnip (Pastinaca Sativa)

Abstract

Optimal defense theory predicts that within—plant allocation of defensive compounds should reflect the value of plant tissue in terms of its relative contribution to fitness. This theory was tested by examining the quantity of furanocoumarins in wild parsnip, Pastinaca sativa (Umbelliferae), in relation to the value of a floral unit as the unit develops. P. sativa is andromonoecious and protandrous, each hermaphroditic flower sequentially passing through bud, male, and female stages. Resource commitment to the primary umbel increases as flowers and seeds mature; if resources are limiting, the timing of primary umbel defloration due to herbivory can be expected to affect a plant's ability to compensate for lost floral parts. The degree of fitness reduction following defloration of the primary umbel and compensatory growth in higher order umbels was used as a measure of floral stage value. Primary umbels were artificially deflorated at various stages and resulting seed set was compared with undamaged control plants. In an experiment with small plants (with a mean total biomass of 5.838 g), the loss of developmentally more advanced floral units resulted in greater fitness reduction. Total seed production, both in terms of biomass and number, followed the pattern: control > female deflorated > fruit deflorated plants. A similar experiment using larger plants (mean total biomass = 21.78 g) and defloration at bud and female floral stages produced a parallel but statistically nonsignificant trend for seed biomass. Mean seed mass was significantly reduced by defloration in both experiments. The furanocoumarin content and composition of floral units were measured as primary umbels developed sequentially from bud stage through male, female, and mature seed (schizocarp) stages. Total and individual furanocoumarin levels increased with advancing floral development over the course of the season. In addition, co—occurring developmentally asynchronous floral units on the same primary umbel were compared for furanocoumarin content to control for passive allocation patterns or for differences in floral unit apparency. Within umbels containing developmentally asynchronous stages, advanced floral units had higher levels of furanocoumarins than less advanced floral units. These results were consistent with optimal defense theory: defensive compounds were allocated in higher quantities to floral units in developmental stages at which their loss would have a greater negative impact on fitness.