Identification and Characterization of Organic Calcium Crystals in the Plumeria pudica and Plumeria rubra Latexes

Latex is usually a milky fluid but can also be clear, brown, or orange. It consists of organic compounds (rubber particles and primary and secondary metabolites) and proteins dispersed in an aqueous phase. Four latex fluids were examined through optical microscopy, and organic crystals were found. Crystal structures were observed in Plumeria rubra and P. pudica, while no crystal was found in the latex of Calotropis procera and Himatanthus drasticus. Various sizes and shapes of crystals, including raphides, styloids, crystalline sands, and prismatic forms, were documented. The structures were stable for a 96 h period at 25 °C. The relative abundance of crystal forms varied based on whether the latex was crude or water-diluted. All crystal forms related to P. pudica were identified as hydrated calcium malonate [Ca­(C3H2O4)­(H2O)2]·2H2O, representing the first documentation of these structures in plants. Calcium malonate crystals also were identified in the latex of P. rubra, which were mainly in the forms of raphides and styloids. Nonetheless, the crystalline sands and prismatic forms found in P. rubra latex exhibited an additional carbon ring in their structure ([C20H34Ca2O19]) identified as cerberic acid B. This result also represents the first report of calcium-containing cerberic acid B. The results were discussed based on the possible functions of these molecules within laticifers.