A long-standing paradigm in plant physiology proposes that cellular Ca2+ serves two primary functions: maintaining structural integrity and mediating intracellular signaling. Discovery of a morphologically similar congener of Asplenium ruta-muraria challenges this paradigm. The new species, A. yishuiensis from northern China, differs by its herbaceous to subsucculent and desiccation-sensitive laminae. Its mean mesophyll thickness is 232.8 μm, the number of vascular bundles at the petiole tips is 3–4, and the density of glandular hairs on the laminae averages 0.15 mm2/cm2. Conversely, A. ruta-muraria has a mesophyll thickness of 120–150 μm, two vascular bundles at the petiole tips, and glabrous laminae. Critically, A. yishuiensis lacks typical freezing adaptations common in temperate evergreens and does not exhibit poikilohydry. Its ability to remain evergreen at −20 °C reveals a previously unrecognized freezing tolerance mechanism. We hypothesize a two-tiered physiological strategy: (1) accumulation of Ca2+ complexes may indirectly lower cellular osmotic potential in some plants by promoting inorganic ions uptake; and (2) this mechanism varies among species, with A. yishuiensis using Ca2+ complexes more efficiently. Its total frond content of Ca + Mg reaches 12.66 g/kg dry weight, K reaches 22.7 g/kg, and V reaches 1.07 mg/kg. Specialized Ca2+-storage proteins and their accompanying ions likely enhance freezing tolerance in A. yishuiensis.
Wáng et al. (Mon,) studied this question.