Premise: Plants are facing increased risk of heat stress with global climate change. Reproductive tissues are particularly sensitive which can result in lower plant fitness. Floral shading and closure are possible mechanisms to limit heat stress although most previous work on petal orientation has considered adaptations to raise temperatures. We hypothesized that floral shading could reduce temperature and increase reproductive success.

Results: Individual flowers of all four species that shaded their pistils experienced temperatures 3 – 8 oC lower than those that remained open and unshaded. In our wiring experiment, unencumbered R. coulteri controls were 40% more likely to produce seeds than flowers that were either permanently open or closed. Without added pollen, control flowers produced 2x more seeds than flowers wired open and 8x more than those wired closed. However, pollen addition eliminated the effects of wiring and increased capsule weight and seed production. This suggests that pollen limitation was responsible for observed differences in the wiring treatments. Pollinators may prefer control flowers over those that were wired open or closed; petal shading may make flowers cooler and more attractive to pollinators.

Conclusions: Petal shading may be a behavior that allows flowers to reduce heat stress and increases their chances of pollination and seed set.

Flowers were selected soon after opening and randomly assigned to one of three wiring treatments: open such that the petals remained in a horizontal position that provided no shade, closed such that the petals remained in a vertical position that shaded the pistil, and a control that allowed the petals to assume their natural orientations. In year 3, these wiring treatments were crossed with a pollen addition treatment to half of the flowers. After the capsules matured, they were collected and the proportion of flowers that produced seeds as well the number of seeds per capsule were compared for these treatments.