Stigma receptivity and effects of prior self-pollination on seed set in tristylous Lythrum salicaria (Lythraceae)

• Stigma position is a key aspect of flower morphology that may influence pollination success and seed production. In the self-incompatible, tristylous herb, Lythrum salicaria, the stigma is positioned inside the corolla tube in the short-styled morph, but outside the corolla in the mid- and long-styled morphs. We performed controlled pollinations to determine whether style morphs differ in duration of stigma receptivity and in interference caused by self-pollen deposition. As predicted, flowers of the short-styled morph remained receptive for longer if not pollinated than did flowers of the other two style morphs. When deposition of self-pollen preceded compatible cross-pollination, seed set was higher in the mid-styled morph than in the other style morphs. Interference caused by self-pollen deposition was overall strongest in the short-styled morph, but the magnitude of differences among style morphs was contingent on the relative timing of self and compatible pollen receipt. The results indicate that both differences in duration of stigma receptivity and interference from self-pollen deposition may contribute to observed variation in seed production and pollen limitation among style morphs in natural populations of L. salicaria. An association between stigma position and duration of stigma receptivity should constrain the evolution of flower morphology.

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