Sexual expression and maternal investment were investigated in the herbaceous perennial Solanum carolinense (Solanaceae). Field censuses, pollen germination studies, and the results of hand—pollination experiments in a greenhouse revealed that there are three flower types. Morphological males (M) are found near the tip of the inflorescence and have short styles; these flowers did not set fruit when pollinated. All other flowers have long styles and are morphological hermaphrodites (H); among these, those near the base of an inflorescence set fruit when pollinated (fruiting hermaphrodites, FH) while those in intermediate positions did not (functional males, FM). No morphological differences were apparent between FH and FM flowers. Male function of M and H flowers was indistinguishable in terms of pollen germination and quantity and quality of seed produced. The proportions of FM flowers decreased with later inflorescences, but the proportion of FM flowers increased correspondingly. Thus, the morphological sex ratio, M/(M + H), decreased with plant size, while the functional sex ratio, (M + FM)/(M + FM + FH) did not vary. These data do not support evolutionary theories that predict increased allocation to female versus male function with increasing plant size. In addition, experimental manipulations were performed on greenhouse plants. Morphological sex ratio changed in response to various plant hormones, which suggests hormone involvement. Sex ratio increased in response to increased shade and water stress, which supports those evolutionary theories that predict a shift to maleness in response to stress. However, increasing levels of fertilizer also increased the morphological sex ratio Pollination did not influence the morphological sex of subsequent flowers; rather, regulation of fruit—set after pollination occurred via changes in the proportion of H flowers that did (FH) or did not (FM) set fruit. The results are discussed in terms of three stages at which maternal investment can be regulated: flower initiation, flower maturation, and fruit maturation.

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Male and hermaphrodite flowers from the andromonoecious perennial Solanum carolinense were compared. Hermaphrodite flowers were 11–16% heavier than males, and this difference was consistent for two clones studied. Hermaphrodite flowers contained a greater amount of nitrogen than did male flowers, but males had higher percent nitrogen. Male and hermaphrodite flowers had equal investment in male structures: stamen mass, number of pollen grains, mass of pollen grains ejected by vibration, and pollen size were all similar for the two flower types. In contrast, male flowers had reduced masses of disfunctional female structures: ovaries of males were ⅓ as massive, and styles 1/7; as massive, as those of hermaphrodites. These results are consistent with the hypothesis that male flowers (which occur where fruit set is unlikely) increase male fitness through pollen donation, while saving resources that would otherwise be invested in hermaphrodite flowers that abscise.

Male and hermaphrodite flowers from the andromonoecious perennial Solanum carolinense were compared. Hermaphrodite flowers were 11–16% heavier than males, and this difference was consistent for two clones studied. Hermaphrodite flowers contained a greater amount of nitrogen than did male flowers, but males had higher percent nitrogen. Male and hermaphrodite flowers had equal investment in male structures: stamen mass, number of pollen grains, mass of pollen grains ejected by vibration, and pollen size were all similar for the two flower types. In contrast, male flowers had reduced masses of disfunctional female structures: ovaries of males were ⅓ as massive, and styles 1/7; as massive, as those of hermaphrodites. These results are consistent with the hypothesis that male flowers (which occur where fruit set is unlikely) increase male fitness through pollen donation, while saving resources that would otherwise be invested in hermaphrodite flowers that abscise.

Ovipositing females of the host—specific herbivorous moth Frumenta nundinella Zeller discriminated among horsenettle (Solanum carolinense L.) plants in the laboratory and at five field populations. In laboratory experiments, the number of larvae that developed in high—density plant groups was not significnatly greater than the number that developed in low—density groups. Consequently, individual plants in low—density groups were disproportionately affected; twice as many larvae developed on them as on individual plants in high—density groups. Moths also discriminated between groups of pure horsenettle (high relative abundance) and groups of horsenettle plus another naturally co—occurring species (low relative abundance). Individual plants in high relative abundance groups had a tenfold greater infestation than did plants in low relative abundance groups. Natural horsenettle populations varied in density, relative abundance, plant size, and fruit production. Plant size, fruit production, and population density were all positively correlated, so the populations with the greatest density had the largest plants, which produced the most fruits. Despite differences in these plant parameters among sites, the extent of infestation differed little, except for one site that lacked parasitoids that attack F. nundinella. Infestation within field sites was heterogeneous among plants. Virtually all large plants (those with @> 15 fruits), but only about half the small plants (those with < 15 fruits), became infested (one or more infested fruit per plant). However, a greater proportion of the fruit crop was infested on small plants than on large plants. Large plants remained in flower an average 2.5 wk longer than did small plants, so a smaller proportion of their flowers was available during the short period of high oviposition risk. Because of the short oviposition period, plants that flowered earlier or later than the mean flowering time of their populations appeared to have lower infestations than did plants that flowered close to the population means.