Biosynthesis ofPhenolic Compounds inFirst Internodes of Sorghum:Lignin andRelated Products'

Summslary. Ligninbiosynthesis in excisedtissues ofSorghumvulgare variety \Vheatland miloincubated inairwithandwithoutpriorinfiltration withHoOwas presumably limited by H202 production and was dependent upon an endogenous substrate, probably starch.Insolution cultuire withoutshaking, thisconversion of endogenous material was partially blocked atsome steppriortop-hydroxycinnamic acid.Thesynthesis was light independent andcontinued protein synthesis was not required. The accuimulation of lignin products was paralleled by an increase in dhurrin, alkaline sensitive estersofp-hydroxycinnamic, ferulic andsinapic acids, andflavin coenzymes, especially flavin-adenine dinucleotide. Therewas no detectable evidence ofcompetition forsubstrates withotherphenols suchas anthocyanins or withthegrowthofadventitious roots.Therewas evidence, however,ofmechanisms limiting lignification inthefirst internode intheintact seedling. Comparisons are madewithlignin production incomparable tissues ofPhideut. Themajorphenolic compoundssynthesized in thefirst internode ofSorghum,identified inthis andprevious papers (2,8, 9,13,14), can bedivided into4 majorgroups. In group I, containing dhurrin, lignin, andestersofC6-C3phenolic acids exceptcaffeic acid, thecompounds are presentin intact internodes after4 daysofgermination in thedark.Theycontinue to accumulate upon incubation oftheexcised internodes (2,9, 13).In group II,consisting oftheanthocyanins apigeninidinandluteolinidin, thecompounds are accuimulated inthedarkor light onlyafterexcision fromthe restoftheseedling (13).Ingroup III,including a thirdanthocyanin, a cyanidin glycoside, andan esterofcaffeic acid,accumulation occurred predominantly onlyafterlight treatmentofexcised internodes (13).Protein synthesis intheexcised internodes was necessary foraccumulation ofcompoundsingroup IIandIII,butnot forthosein group I (8,14). Thispaper is concerned withtheaccumulation ofcompounds ingroup I, withemphasis on lignin produiction in excisedinternodes. Relationships withotherbiosyntheses andgrowthprocesses are discussed.