Giant flowers of Southern Magnolia (Magnolia grandiflora) are hydrated by the xylem

Taylor S. Feild ^*, David S. Chatelet , and Tim J. Brodribb

Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37919, USA; Department of Plant Science, University of Tasmania, Hobart, Tasmania, Australia

^* Corresponding author; email: tfeild{at}utk.edu.

Flowering depends upon long distance transport to supply waterfor reproductive mechanisms to function. Previous physiologicalstudies suggested that flowers operated uncoupled from stemxylem transport and received water primarily from the phloem.We demonstrate that the water balance of Magnolia grandifloraflowers is regulated in a manner opposite from that of previousexamined flowers. We show that flowers of M. grandiflora relyupon relatively efficient xylem hydraulic transport to supporthigh water-demand during anthesis. We measured rapid rates ofperianth transpiration ranging from twice to 100 times greaterthan previous studies. We found that relatively efficient xylempathways existed between the xylem and flower. Perianth hydraulicconductance and the amount of xylem to transpirational surfacearea ratios of flowers were both approximately one third thosemeasured for leafy shoots. Furthermore, we observed that perianthtissues underwent significant diurnal depressions in water statusduring transpiring conditions. Decreases in ${Psi}$ observed betweenflowers and vegetative tissues were consistent with water movingfrom the stem xylem and into the flower during anthesis. Xylemhydraulic coupling of flowers to the stem was supported by experimentsshowing that transpiring flowers were unaffected by bark girdling.As member of near basal evolutionary lineage, our results suggestthat flower water balance represents an important functionaldimension that influenced early flower evolution.