Water relations of Calycanthus flowers: Hydraulic conductance, capacitance, and embolism resistance

Abstract

For most angiosperms, producing and maintaining flowers is critical to sexual reproduction, yet little is known about the physiological processes involved in maintaining flowers throughout anthesis. Among extant species, flowers of the genus Calycanthus have the highest hydraulic conductance and vein densities of species measured to date, yet they can wilt by late morning under hot conditions. Here, we combine diurnal measurements of gas exchange and water potential, pressure-volume relations, functional responses of gas exchange, and characterization of embolism formation using high resolution X-ray computed microtomography to determine drought responses of Calycanthus flowers. Transpiration from flowers frequently exceeded transpiration from leaves, and flowers were unable to limit transpiration under conditions of high vapour pressure deficit. As a result, they rely heavily on hydraulic capacitance to prevent water potential declines. Despite having high water potentials at turgor loss, flowers were very resistant to embolism formation, with no embolism apparent until tepal water potentials had declined to -2 MPa. Although Calycanthus flowers remain connected to the stem xylem and have high hydraulic capacitance, their inability to curtail transpiration leads to turgor loss. These results suggest that extreme climate events may cause flower failure, potentially preventing successful reproduction.

Similar geometric rules govern the distribution of veins and stomata in petals, sepals and leaves

“…Mass flow through xylem cells is the only means by which sufficient water can be carried internally to support transpiring leaves. However, because rates of water supply and water loss from petals can be low Roddy & Dawson, 2012;Roddy et al, 2018), the phloem contribution of water might be a relatively larger fraction of the total water needed by the flower. Variation of water transport in flowers suggests that different species may employ different hydraulic conductance strategies to remain turgid and to maintain function during flowering (Chapotin et al, 2003;Feild et al, 2009;Roddy et al, 2018).…”

“…However, because rates of water supply and water loss from petals can be low Roddy & Dawson, 2012;Roddy et al, 2018), the phloem contribution of water might be a relatively larger fraction of the total water needed by the flower. Variation of water transport in flowers suggests that different species may employ different hydraulic conductance strategies to remain turgid and to maintain function during flowering (Chapotin et al, 2003;Feild et al, 2009;Roddy et al, 2018). However, recent studies have suggested that flowers may be hydrated by xylem, at least in basal angiosperms (Chapotin et al, 2003;Roddy et al, 2018).…”

“…Variation of water transport in flowers suggests that different species may employ different hydraulic conductance strategies to remain turgid and to maintain function during flowering (Chapotin et al, 2003;Feild et al, 2009;Roddy et al, 2018). However, recent studies have suggested that flowers may be hydrated by xylem, at least in basal angiosperms (Chapotin et al, 2003;Roddy et al, 2018).…”

“…There is some evidence to suggest that the flowers from members of the early-divergent angiosperm clades Illicium, Magnolia and Calycanthus (basal ANA groups and magnoliids of angiosperms) are hydrated predominantly by the xylem (Trolinder et al, 1993;Chapotin et al, 2003;De la Barrera & Nobel, 2004;Galen, 2005;Roddy et al, 2018). Thus, xylem hydration of flowers may be the ancestral state in angiosperms .…”

“…Indeed, flowering plants may use a variety of mechanisms to keep their flowers turgid and functional during flowering. For example, some flowers have high hydraulic capacitance (Chapotin et al, 2003;Roddy et al, 2018), which can delay water potential declines and help isolate flower water status from changes in the water status of other parts of the plant.…”