Ray Peter M, Bret-Harte M Syndonia
Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA.
Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA.
PNAS Nexus. 2022 Jul 25;1(4):pgac131. doi: 10.1093/pnasnexus/pgac131. eCollection 2022 Sep.
We report a biophysical mechanism, termed cryocampsis (Greek , cold, + , bending), that helps northern shrubs bend downward under a snow load. Subfreezing temperatures substantially increase the downward bending of cantilever-loaded branches of these shrubs, while allowing them to recover their summer elevation after thawing and becoming unloaded. This is counterintuitive, because biological materials (including branches that show cryocampsis) generally become stiffer when frozen, so should flex less, rather than more, under a given bending load. Cryocampsis involves straining of the cell walls of a branch's xylem (wood), and depends upon the branch being hydrated. Among woody species tested, cryocampsis occurs in almost all Arctic, some boreal, only a few temperate and Mediterranean, and no tropical woody species that we have tested. It helps cold-winter climate shrubs reversibly get, and stay, below the snow surface, sheltering them from winter weather and predation hazards. This should be advantageous, because Arctic shrub bud winter mortality significantly increases if their shoots are forcibly kept above the snow surface. Our observations reveal a physically surprising behavior of biological materials at subfreezing temperatures, and a previously unrecognized mechanism of woody plant adaptation to cold-winter climates. We suggest that cryocampsis' mechanism involves the movement of water between cell wall matrix polymers and cell lumens during freezing, analogous to that of frost-heave in soils or rocks.
我们报告了一种生物物理机制，称为低温弯曲（希腊语，寒冷，+，弯曲），它有助于北方灌木在积雪负荷下向下弯曲。亚冰点温度会显著增加这些灌木悬臂加载树枝的向下弯曲程度，同时使它们在解冻并卸载后恢复到夏季的高度。这与直觉相反，因为生物材料（包括表现出低温弯曲的树枝）通常在冷冻时会变得更硬，所以在给定的弯曲负荷下应该弯曲得更少，而不是更多。低温弯曲涉及树枝木质部（木材）细胞壁的应变，并且取决于树枝是否含水。在测试的木本物种中，低温弯曲几乎发生在所有北极物种、一些北方物种、仅少数温带和地中海物种中，而我们测试的热带木本物种中没有出现。它有助于寒冷冬季气候下的灌木可逆地处于雪面以下并保持在那里，使它们免受冬季天气和捕食风险的影响。这应该是有利的，因为如果北极灌木的嫩枝被迫保持在雪面以上，其芽的冬季死亡率会显著增加。我们的观察揭示了生物材料在亚冰点温度下令人惊讶的物理行为，以及一种以前未被认识的木本植物适应寒冷冬季气候的机制。我们认为低温弯曲的机制涉及冷冻期间细胞壁基质聚合物和细胞腔之间的水分移动，类似于土壤或岩石中的冻胀现象。
