Twinned raphides of calcium oxalate in grape (Vitis): Implications for crystal stability and function

Among the higher plants that accumulate crystalline calcium oxalate, many t axa characteristically produce raphides, or needle-shaped crystals. Substan tial evidence has accumulated indicating that raphides function in plant de fenses against herbivory and that their acicular shape is a critical compon ent in proposed mechanisms for these defenses. Previous observations have s hown that raphides in members of the Vitaceae are twinned crystals. In this study, the fine structure of raphides in Vitis was examined in order to de termine the nature of twinning. Rotation of isolated raphides under cross-p olarized light revealed extinction patterns that indicated that raphides ar e twinned along their length. In cross sections of raphides, the twin plane extends across the raphides, parallel to their surfaces. The dissolution p atterns observed in etched crystals indicate that the type of twinning;is r otational. Parallels in other biomineralization systems indicate that the r otational nature of the twinning may increase mechanical strength. In addit ion, because twinning may affect crystal growth or enhance stability and pe rsistence of crystals, it could be an important factor in allowing plant ce lls to produce the raphide morphology.