Giant Flowers of Southern Magnolia Are Hydrated by the Xylem

Abstract

Flowering depends upon long-distance transport to supply water for reproductive mechanisms to function. Previous physiological studies suggested that flowers operated uncoupled from stem xylem transport and received water primarily from the phloem. We demonstrate that the water balance of Southern magnolia (Magnolia grandiflora) flowers is regulated in a manner opposite from that of previously examined flowers. We show that flowers of Southern magnolia rely upon relatively efficient xylem hydraulic transport to support high water demand during anthesis. We measured rapid rates of perianth transpiration ranging from twice to 100 times greater than previous studies. We found that relatively efficient xylem pathways existed between the xylem and flower. Perianth hydraulic conductance and the amount of xylem to transpirational surface area ratios of flowers were both approximately one-third those measured for leafy shoots. Furthermore, we observed that perianth tissues underwent significant diurnal depressions in water status during transpiring conditions. Decreases in water potential observed between flowers and vegetative tissues were consistent with water moving from the stem xylem into the flower during anthesis. Xylem hydraulic coupling of flowers to the stem was supported by experiments showing that transpiring flowers were unaffected by bark girdling. With Southern magnolia being a member of a nearly basal evolutionary lineage, our results suggest that flower water balance represents an important functional dimension that influenced early flower evolution.Sexual reproduction is the primary function of the flower. Most investigations of how flowers orchestrate reproductive processes have focused on developmental biology and biochemical mechanisms operating inside flowers (Franklin-Tong and Franklin, 2003;Glover, 2007). Although such approaches have been successful in understanding the inner workings of flowers, the function of flowers is linked to processes operating in the rest of the plant (Galen, 2005;Lambrecht and Dawson, 2007). As such, the physiological mechanisms that make reproduction possible in the face of environmental stresses are best understood through studies that integrate reproductive and vegetative physiology.

Plant water status and hydraulic conductance during flowering in the southern California coastal sage shrub Salvia mellifera (Lamiaceae)

“…Maximum E corolla of S. mellifera was 2.5 mmol m -2 s -1 , a rate that was ~60% of maximum E leaf . This ratio of fl oral to leaf transpiration is similar to that of Magnolia grandifl ora L., which has the highest previously reported E corolla ( Feild et al, 2009 ) but has a lower ratio of fl oral to leaf transpiration than Persea americana Mill. ( Blanke and Lovatt, 1993 ).…”

“…The few reports of transpiration rates from fl owers range from about 0.006 to 2.2 mmol m -2 s -1 ( Blanke and Lovatt, 1993 ;Pati ñ o and Grace, 2002 ;Galen, 2005 ;Feild et al, 2009 ). Maximum E corolla of S. mellifera was 2.5 mmol m -2 s -1 , a rate that was ~60% of maximum E leaf .…”

“…Furthermore, although the petals of some species have stomata ( Lipayeva, 1989 ;Galen et al, 1993 ;Vemmos and Goldwin, 1993 ;van Doorn, 1997 ), these stomata appear to be nonfunctional and, thus, do not contribute to the control of water loss ( Watson, 1962 ;Hew et al, 1980 ;van Doorn, 1997 ;Pati ñ o and Grace, 2002 ). By contrast, Feild et al (2009) show large diurnal variation in the stomatal conductance of Magnolia tepals. Therefore, given the variation in these results, controls over fl oral water balance are not well categorized.…”

Variation in and adaptive plasticity of flower size and drought-coping traits

“…This floral plasticity may be functional for plants. Given that flowers transpire significant amounts of water (Patiño and Grace 2002;Feild et al 2009;Lambrecht et al 2011;Lambrecht 2013;Teixido and Valladares 2014) and this water loss can affect leaf functioning, particularly under dry conditions (Galen et al 1999;Lambrecht and Dawson 2007;Lambrecht 2013), variation and plasticity in floral size may be an important mechanism to control water loss when plants are faced with drought (Galen et al 1999;Caruso 2006;Lambrecht and Dawson 2007). Investigating the adaptive role of floral size variation and plasticity in conjunction with leaf morphology, physiology, and reproductive traits may provide insight into the evolution of plants in relation to moisture availability (Edwards et al 2012).…”

Are flowers vulnerable to xylem cavitation during drought?

“…A significant body of work studying the flow of water into flower petals shows considerable variation in the rates of water loss from flowers [11]. Traditionally, the maintenance of hydration in flowers has been attributed to water delivery from the phloem [4,12 -14], but recent evidence also indicates that in species with petals supporting significant transpiration (typically those possessing stomatal pores) the majority of water is supplied to the flowers by the xylem, in the same way as leaves [15]. Indeed, it could be expected that variation in the water-relations of petals should be high considering the multiple independent origins of floral structures in the angiosperm phylogeny [16,17].…”