Persistent Calyx Enhances Floral Thermoregulation and Reproductive Success in Brandisia hancei Hook. f. (Orobanchaceae)

While persistent calyces exhibit considerable functional diversity, this has not been fully substantiated by experiments, especially concerning their thermoregulatory function. This study investigates the thermoregulatory function of persistent calyx in winter-flowering <i>Brandisia hancei</i>. Changes in calyx dimensions throughout the flowering-to-fruiting developmental stages were measured. Differences between floral and ambient temperatures were measured when only calyxes were retained. Additionally, differences in floral temperature between calyx-removed treatments and intact controls were also measured. All measurements were taken at three developmental stages: pre-anthesis, anthesis, and post-anthesis. Furthermore, seed production after calyx manipulation was examined at both anthesis and post-anthesis stages. The calyx exhibits continuous size enlargement from flowering to fruiting stages. After either artificial corolla removal or natural corolla abscission, the calyx independently maintains thermoregulatory capacity, sustaining floral temperatures significantly above ambient levels. Consequently, calyx removal resulted in markedly diminished floral temperature at both pre- and post-anthesis stages. In line with the thermoregulation results, progressive removal of the calyx showed a strong negative correlation with seed production. In contrast, removal of only the calyx edge generally maintained seed production at a level comparable to that of the intact control. Collectively, our findings demonstrate that the persistent calyx plays a critical role in elevating reproductive temperature under winter conditions, enhancing reproductive success in <i>B. hancei</i> through the maintenance of a favorable thermal conditions for reproduction. This study provides direct evidence that plant reproductive structures can markedly adapt to winter low-temperature stress through such a thermoregulatory mechanism.