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Stigma behavior in Mimulus aurantiacus (Scrophulariaceae)¹

²Department of Biology, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0116

The bilobed stigma of many species in the order Scrophulariales closes in response to touch by an animal pollinator. In hummingbird-pollinated bush monkey flower, Mimulus aurantiacus (Scrophulariaceae), closure is rapid, occurring within seconds of tactile stimulus. We investigated the proximate causes of stigma closure and subsequent reopening in M. aurantiacus, as well as potential costs and benefits of stigma closure for female fitness. Stigma closure is elicited by both touch and pollen, but closure in response to pollen is much slower, requiring 0.5–1.5 h. Stigmata reopen within 2.5–4.5 h if touch, but no pollen, is applied. Upon receipt of pollen, most stigmata remain closed for the remaining lifetime of the flower, even if less pollen is received than is needed for full seed set. Those stigmata that do reopen after pollination generally require between 20 and 28 h to do so, much longer than for unpollinated stigmata. Reopening after pollination appears to be a response to low seed set rather than to low pollen load. Natural pollination of stigmata manipulated to prevent closure shows that closure does not increase capture of pollen or seed set. In fact, closure reduces the average pollen load received by flowers. Despite this, there is no evidence that stigma closure has any negative effect on female fitness in terms of seed set or germinability. Hypotheses for the adaptive significance of stigma closure are discussed. Understanding proximate causes of stigma closure and reopening is essential in the evaluation of these hypotheses.

Key Words: behavior; hummingbirds • Mimulus • pollination • Scrophulariaceae • stigma sensitivity

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