BERTIN, ROBERT I.* AND GREGORY M. GWISC. Biology Department. Holy Cross College, Worcester, MA 01610. - Sex expression and gynomonoecy in Solidago (Asteraceae).
Roughly 3% of flowering plants are gynomonoecious, with individual
plants bearing both female and bisexual flowers. Little attention has
been paid to the adaptive significance of this sexual system, which is
particularly prevalent in the Asteraceae. Here we investigated one
hypothesized advantage of having two flower types, namely that the
arrangement permits flexibility in allocation of resources to male and
female reproductive functions. We examined several perennial,
gynomonoecious members of the genus Solidago. These plants
produce small heads consisting of several female flowers, each bearing
a single small petal, surrounding several bisexual disk flowers. We
carried out greenhouse experiments in which we varied one or more of
three environmental variables: light, nutrients and water, and/or
examined heads in different positions on the plants or produced on
different dates. The effects of these variables on the proportion of
female flowers were modest to none. Specifically, significant effects
were found for light in 0 of 3 species, for nutrients in 2 of 4
species, for water in 0 of 1 species, for position in 1 of 3 species
and for date in 1 of 1 species. Because of the small number of
significant effects and their modest magnitude, we conclude that the
presence of two flower types in goldenrods is probably not
advantageous in allowing flexibility in allocation of resources to
male and female functions. It seems likely that this sexual system
has been more important either in providing for pollinator attraction
of in reducing pollen-pistil interference.
Key words: capitulum, goldenrod, gynomonoecy, sex expression, sex ratio, Solidago