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Title: Persistent supercooling of reproductive shoots is enabled by structural ice barriers being active despite an intact xylem connection

Author
item KUPRIAN, EDITH - University Of Innsbruck
item Tuong, Tan
item PFALLER, K - Innsbruck Medical University
item Livingston, David
item NEUNER, GILBERT - University Of Innsbruck

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2016
Publication Date: 9/15/2016
Citation: Kuprian, E., Tuong, T.D., Pfaller, K., Livingston, D.P., Neuner, G. 2016. Persistent supercooling of reproductive shoots is enabled by structural ice barriers being active despite an intact xylem connection. PLoS One. 11(9):e0163160.

Interpretive Summary: Ice formation in plants usually begins in lower portions of the plant when air temperatures fall below zero. In some plant species there are barriers to prevent ice from spreading into sensitive reproductive tissues such as flowers. Heather (Calluna vulagris) is a plant that grows in the Alpine region of the Alps and has a prominent barrier that is at the base of flower stems. This means that vegetative portions of the plant can freeze but ice will not proceed into flowers. Histological analysis revealed that the water conducting vessels at the base of the flower are much smaller than they are in other regions of the plant and explains why ice in vegetative stems does not spread into flowers until the temperature is from 5 to 8 degrees colder. This research can be applied to many other plant species to explain the considerable variability in plant tissues to survive freezing conditions.

Technical Abstract: Extracellular ice nucleation usually occurs at mild subzero temperatures in most plants. For persistent supercooling of certain plant parts ice barriers are necessary that prevent the entry of ice from otherwise already frozen tissues. The reproductive shoot of the evergreen woody dwarf shrub Calluna. vulgaris is able to supercool down to below -22 °C throughout all developmental stages (bolting, flowering, and fruiting) despite an established xylem conductivity. After exact localization of the persistent ice barrier between the reproductive and vegetative shoot of C. vulgaris at the base of the pedicel by IDTA (Infrared Differential Thermal Analysis), the currently unknown structural features of the ice barrier tissue were anatomically analyzed on cross and longitudinal sections. The ice barrier tissue was recognized as a 250 µm long constriction zone at the base of the pedicel that lacked pith tissue and intercellular spaces. Most cell walls were thickened and contained hydrophobic substances (lignin, suberin, cutin or chitin). Only a few cell walls had additionally increased cellulose inclusions. In the ice barrier tissue, the area of the xylem was at most 5.7 times smaller than in vegetative shoots. The mean number of vessels per cross section was reduced to 3.5 %, the vessel diameter to 70 % (4.983 µm) and the vessel length to 60 % (78,2 µm) as compared to the vegetative shoot. From vegetative shoots water transport in the ice barrier is forced to pass inter-vessel pit membranes that are likely impermeable to ice. Pit apertures were about 1.9 µm x 0.7 µm, which was significantly smaller than in the vegetative shoot. The peculiar anatomical features at the base of the pedicel particularly, that of the xylem, render the pore size of the pit membranes to be the critical constriction for ice propagation into the persistently supercooled reproductive shoots of C. vulgaris.