FTIR-based quantification of calcium oxalate monohydrate in almond bark reveals a relationship between crystal concentrations and aridity

Calcium oxalate monohydrate (whewellite) is a widespread plant biomineral implicated in calcium regulation, structural reinforcement and stress responses. The quantitative distribution of these crystals in woody tissues and whether crystal abundance is influenced by environmental conditions, remain poorly understood. Here, we developed a Fourier-transform infrared spectroscopy (FTIR) assay for quantifying whewellite concentrations in plant woody tissues and validated the approach using independent micro-computed tomography measurements across a wide concentration range. We applied this method to bark samples of multiple almond species collected along climatic gradients spanning arid to mesic environments. Bark whewellite concentrations ranged from about 5 wt% to ∼32 wt% and showed a strong negative relationship with mean annual precipitation. This shows enhanced calcium oxalate crystal formation under arid conditions. In contrast, lithological calcium availability did not significantly explain whewellite concentrations once precipitation was accounted for, suggesting that calcium oxalate crystal formation is more closely associated with physiological responses to water availability than with substrate calcium content. This strong correlation between amounts of crystals formed in bark and the mean annual precipitation raises questions about bark physiology, as well as the intriguing possibility that this property of bark can be used to reconstruct ancient terrestrial climate changes.

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