Petal-shading in Romneya coulteri affects seed set and interactions with floral visitors

Abstract

Heat stress will be an increasing problem for plants and pollinators. Some Romneya coulteri flowers self-shade reproductive parts with vertically oriented petals; this keeps pistils 3–8 °C cooler. A previous study conducted outside of the plant’s native range found that self-shading flowers experienced higher seed set than flowers with petals that were experimentally forced to all be horizontal (open) or vertical (shaded). Hand-pollinated flowers had higher seed set which negated the effect of petal orientation, suggesting that insect pollination may have driven seed set. In this study, we observed insect visitors and their effects on seed set in R coulteri’s native range. Plant performance was again limited by pollen such that experimentally adding outcross pollen increased seed set. Native and introduced bees showed no proclivity toward either petal orientation. However, the most common visitor, a native beetle (Lytta auriculata), was strongly biased toward approaching and ultimately accepting flowers with vertical petals that provided shade. Beetles were covered with R. coulteri pollen. Flowers bagged with only beetles for 24 h produced as many seeds as controls left open to all visitors. However, beetles also consume pollen and chew petals. Flowers with vertical petals received more beetle damage and damaged flowers produced 35% fewer seeds than those without chewed petals. Despite the potential role of beetles as pollen vectors, flowers with vertical petals did not experience higher seed production. The outcome of beetle visitation on seed set is likely context-dependent. Petals that affected microclimate were attractive to at least one potential pollinator.