Flowers that self‐shade reduce heat stress and pollen limitation

Abstract

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.

Refrigerated flowers in the torrid Mediterranean summer

“…The finding that flowerheads of C. corymbosa undergo a regular diel rhythm of internal refrigeration raises questions about the mechanism permitting these plants to periodically cool their flowers in the torrid weather that prevails during its flowering season, but also on the possible costs and benefits associated with the species' floral thermal biology. Regarding the possible cooling mechanism, reduced solar irradiance inside these flowerheads due to self‐shading (Karban et al, 2023) can be safely ruled out, since substantial thermal deficits persisted also for most of the night (Figure 2). The most parsimonious hypothesis is that periodic flowerhead cooling in C. corymbosa was achieved by temporarily enhanced transpiration in the florets and/or receptacle.…”

Floral thermal biology in relation to pollen thermal performance in an early spring flowering plant

“…At the reproductive stage in flowering plants, floral temperature can have important consequences for gamete development and viability (Thakur et al 2010;Zinn et al 2010;Hedhly 2011), pollination (Norgate et al 2010), and fertilization (Cerovi c et al 2000). Plants have evolved a variety of mechanisms that modify floral temperature, including heliotropism (Zhang et al 2010), thermogenesis (Watling et al 2008;Seymour et al 2009), evaporative cooling (Galen 2006), shading of reproductive structures (Karban et al 2023), and the focus or reflection of solar radiation (Kevan 1975;McKee & Richards 1998). While some of these mechanisms (e.g., heliotropism) are likely adaptive (Creux et al 2021), others, such as transpirational cooling, may result from plant-wide physiological responses to ambient temperature that incidentally impact floral temperature.…”

“…The effects of evaporative cooling may enhance both seed and pollen performance in some taxa (Patiño & Grace 2002). Recent work has shown that petal positioning over reproductive structures in a suite of Californian taxa can reduce floral temperature, with positive consequences for seed production (Karban et al 2023). Furthermore, some bees have been shown to 'overheat' within warm flowers and take cooling flights (Corbet & Huang 2016).…”

“…2009), evaporative cooling (Galen 2006), shading of reproductive structures (Karban et al . 2023), and the focus or reflection of solar radiation (Kevan 1975; McKee & Richards 1998). While some of these mechanisms (e.g., heliotropism) are likely adaptive (Creux et al .…”