Calcium oxalate in the bark of stone birch (Betula ermanii): morphology, age trends, and biomineralization dynamics under salt stress in mud volcanic environments

Abstract

The patterns of crystalline Сa oxalate (whewellite) accumulation can be a proxy of tolerance in plants exposed to stress from drought, high salinity, climate changes, pollution, etc. Age-dependent variations in the distribution and morphology of Сa oxalate in the bark of Betula ermanii growing in salt stress conditions in the Yuzhno-Sakhalinsky mud volcano were investigated for the first time and compared with the respective data from a typical environment (southern Sakhalin Island, northeastern Asia). The samples of bark were analyzed by: inductively coupled plasma optical emission and mass spectrometry, energy-dispersive X-ray spectroscopy, light microscopy. The specific number of Сa oxalate in the bark was found out to decrease systematically with age (1–147-150 year), being the highest in the parenchyma of young (1–5 year) crown branches. The decreasing age-trend of Сa oxalate in conducting phloem follows the logarithmic law and correlates with carbon flows along the phloem. The bark of trees growing in the mud volcano accumulates many elements which have higher enrichment than in the typical area: 1.2–1.6 times for K, Mg, Zn, Na, and S, 2–3 times for Ba, Ca, Sr, B, and Sb, 10 times for Li. The Сa oxalate show morphological diversity: single crystals, contact twins, spherulites nucleated around organic clots. Spherulites mainly occur in parenchyma near apical and lateral meristems. They may represent a dynamic system of emergency storage/release of C and Ca which the plants can use for metabolism and growth as a prompt response to salt stress associated with mud volcanic activity.