Biocrystallization of mineral material on forage plant cell walls

Abstract

Seventeen mature roughage plant materials (10 grasses, 7 legumes) were chemically analysed for cell wall content and other fractions. The nature and distribution of the less soluble mineral depositions found on structural elements of the mature plants was investigated by means of X-ray diffraction and electron optical techniques. Energy-dispersive X-ray analysis of the ash fractions of the dry matter (DM), cell wall (cw) and acid detergent matter (ADM) of five grasses, and of the DM of three legumes, the cw of five legumes and the ADM of two legumes, showed the plant structures examined to be highly mineralized. DM, cw and ADM fractions showed different patterns of mineralization. Within each chemical fraction fibres had elemental compositions similar to those found in general fields of the same fraction. This suggests that a general pattern of mineral deposition occurs during plant growth. Cell wall ash fractions of both grasses and legumes were shown to be relatively high in calcium and phosphorus and, in the case of grasses, silicon. Following acid detergent extraction, there was a major increase in the amount of silicon underlying the calcium and phosphorus-rich (and, in grasses, silicon-rich) hemicellulose fraction. Examination of the cellulose fibre matter of lucerne and of wheat straw after potassium permanganate extraction but not ashing showed silicon to be the dominant element in the external layer. Eight plant roughages, yielding 20 specimen fractions (cw, ADM, lignin) were examined by X-ray diffraction. Four plant cw materials were examined by transmission electron microscopy. At least two crystalline forms of calcium and phosphorus, hydroxylapatite and Whitlockite, were shown to exist in cw ash. α-Quartz, in rod and tube-like structures, was also identified in cw ash and ADM ash. These findings are discussed in terms of the concept of the plant as a solar still.