Rubber Metabolism in Plant Life. Development of a New Theory

Systematic analysis of the latex of Hevea brasiliensis yielded results which induced the author to study the metabolism of rubber more closely. As it was realized that the latex picture alone would not lead to sound conclusions about the formation and breakdown of rubber, analysis of the whole Hevea tree, from the highest leaflets down to the very root tips, seemed desirable. For practical reasons, however, such a profound investigation had to be reduced to the analysis of only the most important parts of the tree. The various analytical results obtained have forced the author to make a radical revision of some of the current concepts. For several reasons the synthesis of rubber probably takes place mainly in the green parts of the plant, and principally directly from carbon dioxide. The possibility of the formation of rubber from precursors (secondary formation) is still uncertain. Such formation, for example, must proceed in etiolated germinating plants. Freshly synthesized rubber is present in the plant in the liquid state. This liquid rubber is translocated through the plant in the form of a highly dispersed oil-in-water emulsion, in which the rubber droplets are surrounded by a layer of a phospholipoid complex. As an instructive illustration of this hypothesis the formation of latex is discussed. The process of polymerization results in the transformation of the liquid rubber into the solid phase'; this solid rubber is deposited everywhere in the plant. It appears most probable that, in Hevea brasiliensis, polymerization takes place mainly in the latex vessels. The breakdown of rubber, finally, is proved to be an oxidation process. For several reasons there is cause for believing that oxidation of rubber in the plant leads to the formation of those incrusting substances which are built up by C5-groups, i.e., pentosans, hemicelluloses, lignins, etc.