Flowers that self-shade reduce heat stress and pollen limitation

Premise: Plants are facing increased risk of heat stress with global climate change. Reproductive tissues are particularly heat-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.

Methods: We measured floral temperatures of four species that exhibited intraspecific variation in flower closure (Opuntia ficus-indica, Oenothera elata, Convolvulus arvensis, and Romneya coulteri). We also wired newly opened R. coulteri flowers so that they were either permanently open or permanently closed; controls were not wired.

Results: Individual flowers of all four species that shaded their pistils were exposed to temperatures 3-8°C 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 2× more seeds than flowers wired open and 8× more than those wired closed. However, pollen addition eliminated the effects of wiring and increased capsule mass and seed production. This effect of pollen addition 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.