Labile sex expression in andromonoecious Solanum hirtum: pattern of variation in floral structure

Comparison of hermaphrodite and staminate flowers of andromonoecious Solanum hirtum must consider two sources of morphological variation: variation due to differences in flower position and variation due to conditions that affect sex expression. In S. hirtum, positional variation occurs among hermaphrodite flowers of unpollinated plants; flowers vary quantitatively but not functionally with position on the inflorescence. On inflorescences of pollinated, fruit-bearing plants, flowers vary both quantitatively and functionally with position such that flowers borne distally are functionally staminate. Corolla, anther, style, and ovary length of staminate flowers are significantly smaller than those of hermaphrodite flowers and they have fewer, smaller ovules. Failure of fruit set in staminate flowers is due to a combination of factors. Stigmas and styles of staminate flowers are reduced and appear to lack the secretions necessary for normal pollen germination and growth. In the ovules, the integumentary tapetum is collapsed and appears senescent and most megagametophytes are abnormal. Lack of pollen tube growth and abnormal ovules could both contribute to gynoecial sterility in staminate flowers. Key words: andromonoecy, floral structure, reproductive anatomy, sex expression, Solanum.