Studies of Lignin Biosynthesis Using Isotopic Carbon: Ii. Short-Term Experiments With C14o2

The formation of lignin in wheat plants from C14O2, during the period of rapid lignification, has been studied over the first few hours of photosynthesis by measuring the incorporation of carbon-14 into the guaiacyl and syringyl portions of the lignin molecule. Plants grown 62 days from seeding were exposed to 20 microcuries of C14O2 in a closed chamber for 20 min., and grown for periods of 1 to 24 hr. in a normal atmosphere before harvesting. Synthesis of lignin is most rapid four to six hours from activation, the syringyl residues apparently being formed more slowly than the guaiacyl. A slower rate of formation persists for the remainder of a one-day period, probably as a result of recycling in the carbon pool. Cellulose acquires carbon-14 more rapidly than lignin during the period immediately following administration of C14O2, but after three or four hours when synthesis of lignin has become rapid, the total carbon-14 content of both components reaches about the same value.