Floral scent contributes to interaction specificity in coevolving plants and their insect pollinators.

Chemical defenses, repellents, and attractants are important shapers of species interactions. Chemical attractants could contribute to the divergence of coevolving plant-insect interactions, if pollinators are especially responsive to signals from the local plant species. We experimentally investigated patterns of daily floral scent production in three Lithophragma species (Saxifragaceae) that are geographically isolated and tested how scent divergence affects attraction of their major pollinator-the floral parasitic moth Greya politella (Prodoxidae). These moths oviposit through the corolla while simultaneously pollinating the flower with pollen adhering to the abdomen. The complex and species-specific floral scent profiles were emitted in higher amounts during the day, when these day-flying moths are active. There was minimal divergence found in petal color, which is another potential floral attractant. Female moths responded most strongly to scent from their local host species in olfactometer bioassays, and were more likely to oviposit in, and thereby pollinate, their local host species in no-choice trials. The results suggest that floral scent is an important attractant in this interaction. Local specialization in the pollinator response to a highly specific plant chemistry, thus, has the potential to contribute importantly to patterns of interaction specificity among coevolving plants and highly specialized pollinators.

The physiological implications of primary xylem organization in two ferns.

Xylem structure and function are well described in woody plants, but the implications of xylem organization in less-derived plants such as ferns are poorly understood. Here, two ferns with contrasting phenology and xylem organization were selected to investigate how xylem dysfunction affects hydraulic conductivity and stomatal conductance (g(s)). The drought-deciduous pioneer species, Pteridium aquilinum, exhibits fronds composed of 25 to 37 highly integrated vascular bundles with many connections, high g(s) and moderate cavitation resistance (P50 = -2.23 MPa). By contrast, the evergreen Woodwardia fimbriata exhibits sectored fronds with 3 to 5 vascular bundles and infrequent connections, low g(s) and high resistance to cavitation (P50 = -5.21 MPa). Xylem-specific conductivity was significantly higher in P. aqulinium in part due to its wide, efficient conduits that supply its rapidly transpiring pinnae. These trade-offs imply that the contrasting xylem organization of these ferns mirrors their divergent life history strategies. Greater hydraulic connectivity and g(s) promote rapid seasonal growth, but come with the risk of increased vulnerability to cavitation in P. aquilinum, while the conservative xylem organization of W. fimbriata leads to slower growth but greater drought tolerance and frond longevity.