Calcification in Higher Plants with Special Reference to Cystoliths

Abstract

The cystolith, a calcified body in the leaf of higher plants, was observed by soft X-ray microradiography using the mature leaves of nine species from five families. The microradiographs revealed very large cigar-shaped cystoliths (up to 500 μ m in length) in Pilea viridissima and Justicia procumbens, neighbor-cystoliths in Morus bombycis and Humulus scandens, and two to seven radially arranged cystoliths in Momordica charantia. The number of cystoliths (n/cm²) of various kinds of leaves vas estimated to be 850 to 4,200 by microradiography. The calcium carbonate content (mg/cm²) calculated was 0.3 to 1.1, suggesting a large reservoir of calcium or carbon dioxide. The cystoliths were isolated from leaves of nine species to analyze chemically, with electron probe, and with X-ray diffraction. The calcium carbonate content in cystoliths was about 75% on a dry weight basis. A small amount of magnesium was also found. Electron probe analysis revealed that calcium and magnesium were evenly distributed through the cystolith body except for the stalk and the basal part of the body to which the stalk is attached. In the latter parts, silicon was detected in high density, suggesting silicification of these parts. X-ray diffraction patterns showed amorphous calcium carbonate in all species tested. However, vaterite in Morus bontbycis and both vaterite and calcite in Ficus elastica were also detected in small amounts. The amorphous calcium carbonate in cystoliths changed rapidly into calcite in 0.05 M carbonate buffer (pH 9.2) or in distilled water. Then the smooth surface of the cystoliths was covered with small cubic calcite crystals. A tremendous number of cystoliths was contained in a single leaf, and the cystolith-bearing lithocyst was associated with many photosynthetic parenchyma cells in all species. These facts suggest a relationship between calcification in the cystolith and photosynthesis in the leaf. A possible mechanism of cystolith calcification coupled with bicarbonate utilization in photosynthesis is discussed.