Gradient of structural traits drives hygroscopic movements of scarious bracts surrounding Helichrysum bracteatum capitulum.
The capitulum of Helichrysum bracteatum is surrounded by scarious involucral bracts that perform hygroscopic movements leading to bract bending toward or away from the capitulum, depending on cell wall water status. The present investigation aimed at explaining the mechanism of these movements.



Surface strain and bract shape changes accompanying the movements were quantified using the replica method. Dissection experiments were used to assess the contribution of different tissues in bract deformation. Cell wall structure and composition were examined with the aid of light and electron microscopy as well as confocal Raman spectroscopy.



At the bract hinge (organ actuator) longitudinal strains at opposite surfaces differ profoundly. This results in changes of hinge curvature that drive passive displacement of distal bract portions. The distal portions in turn undergo nearly uniform strain on both surfaces and also minute shape changes. The hinge is built of sclerenchyma-like abaxial tissue, parenchyma and adaxial epidermis with thickened outer walls. Cell wall composition is rather uniform but tissue fraction occupied by cell walls, cell wall thickness, compactness and cellulose microfibril orientation change gradually from abaxial to adaxial hinge surface. Dissection experiments show that the presence of part of the hinge tissues is enough for movements.



Differential strain at the hinge is due to adaxial-abaxial gradient in structural traits of hinge tissues and cell walls. Thus, the bract hinge of H. bracteatum is a structure comprising gradually changing tissues, from highly resisting to highly active, rather than a bi-layered structure with distinct active and resistance parts, often ascribed for hygroscopically moving organs.
Department of Biophysics and Morphogenesis of Plants, Faculty of Biology and Environment Protection, University of Silesia in Katowice, Jagiellonska 28, 40-032 Katowice, Poland.
Dorota
D
Borowska-Wykret
Department of Biophysics and Morphogenesis of Plants, Faculty of Biology and Environment Protection, University of Silesia in Katowice, Jagiellonska 28, 40-032 Katowice, Poland.
Aleksandra
A
Rypien
Institute of Material Science, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland.|Silesian Center for Education and Interdisciplinary Research, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland.
Mateusz
M
Dulski
Department of Biophysics and Morphogenesis of Plants, Faculty of Biology and Environment Protection, University of Silesia in Katowice, Jagiellońska 28, 40-032 Katowice, Poland.|A. Chełkowski Institute of Physics, University of Silesia in Katowice, Uniwersytecka 4, 40-007 Katowice, Poland.
Michal
M
Grelowski
Silesian Center for Education and Interdisciplinary Research, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland.|A. Chełkowski Institute of Physics, University of Silesia in Katowice, Uniwersytecka 4, 40-007 Katowice, Poland.
Roman
R
Wrzalik
Department of Biophysics and Morphogenesis of Plants, Faculty of Biology and Environment Protection, University of Silesia in Katowice, Jagiellonska 28, 40-032 Katowice, Poland.
Dorota
D
Kwiatkowska
D002473:Cell Wall / Q000502:physiology*; D036562:Helichrysum / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology*; D014867:Water / Q000502:physiology*
D014867:Water
Helichrysum bracteatum; Raman spectroscopy; cell wall structure; hygroscopic movements; scarious involucral bracts; surface deformation