Effect of Seasonal Decrease in Temperature on the Content and Composition of Guayulins in Stems of Guayule ( Parthenium argentatum , Gray).

The guayulins are a family of sesquiterpene compounds that consist of an isoprenoid nucleus substituted either by trans -cinnamic or p -anisic acid, and are present only in the resinous fraction of the rubber plant guayule ( Parthenium argentatum , Gray). While the natural role of the guayulins remains enigmatic, they may serve as a defense function against other plants or herbivores by virtue of the accumulation of cinnamic acid. Prior research has suggested seasonal variation in guayulin content, which has been shown to decrease as winter arrives in two different varieties. In the present study, the effect of guayulins has been evaluated in 13 different accessions cultivated under the same conditions during autumn. A general reduction in guayulin content was found in the stems from all varieties between the September and November harvest, which was accompanied by an increase in the resin content. With respect to individual guayulins, while guayulin A was the most prominent member during most of the year, guayulin C had more prominence when temperature started to decrease. In this seasonal period, the production of each member of the guayulin family in the leaves was very balanced.