The occurrence of calcium in plant tissue as crystals of calcium oxalate

Abstract

Calcium oxalate crystals were described in plant cells as early as 1865. Numerous accounts are found in the literature indicating that crystals occur in some species more abundantly than others and none occur in some plant species. The crystals have been observed and analyzed by transmitted light microscopy, polarized light microscopy, electron microprobe and by mechanical and chemical means. Plants that accumulate large quantities of oxalic acid generally tend to contain large quantities of Ca‐oxalate crystals which are insoluble in water, alkalies and organic acids. In some species practically all of the oxalic acid is tied up as Ca‐oxalate. Calcium oxalate crystals occur as mono and trihydrate salts in many crystalline forms. Crystals occur as united molecules called druses and sphaerites or as elongated crystals called styloids and raphids. Solitary crystals of different shapes and sizes may also be present. Oxalic acid is a strong chelating agent and once reacted with Ca it becomes somewhat permanently immobilized in plant cells. However, some reports indicate that Ca can be reutilized from the Ca‐oxalate if plants are under extreme Ca stress. Crystal formation appears to be influenced by plant species, pH of cell constituents, temperature, light, oxalic acid concentration, Ca and other elemental contents and possibly cell turgidity. Plants, high in soluble oxalates, are poisonous to animals whereas plants having most of the oxalic acid tied up as Ca‐oxalate are not poisonous.

Notes

Contribution from the University of Georgia, College of Agriculture, Experiment Stations, Georgia Station, Experiment, Georgia 30212.

Ph.D and Assistant Professor of Agronomy.