Carbohydrate metabolism in Parthenium argentatum Gray.
The metabolism of carbohydrates in guayule is a subject which has not been considered with respect to its role in cis-polyisoprene synthesis, It has been suggested that acetate or sucrose act as the distal, and isopentenylpyrophosphate as the immediate precursor of the isoprenoid biosynthetic pathway. Application of radioactive precursors to the leaves of guayule plants in Winter and Summer showed that the fate of the carbohydrate depends on the chemical structure of the carbohydrate and the time of application. [C] Sucrose was incorporated into the acetone (resin) fraction during the Summer and petroleum ether (rubber) fraction during the Winter. The amount of radioactivity that was translocated in Winter and Summer was similar. The loss of leaves during Winter reduced the area for photosynthesis, while the loss of carbon from the leaves during Summer, probably due to photorespiration, decreased the amount of available photosynthates. These two phenomena did not disadvantage the plant as far as the allocation of carbon was concerned. No plant components were acting as sinks during the Winter. The pith of the crown area incorporated the most radioactivity in Summer. [C] Fructose was more readily translocated than [C] sucrose during a 12 hour experiment. When fructose was applied and plants were left for 48 hours, more radioactivity was translocated to the stems and roots during the Summer. The [C] from fructose was incorporated into the acetone (resins) rather than the petroleum ether (rubber) fraction in Winter therefore apparently having a different fate to [C] sucrose. The principal reserve carbohydrates in guayule are fructans. Two types of fructans were detected and are referred to as water-soluble or ethanol-soluble fructans. The ethanol-soluble fructan polymers apparently played an active role in metabolism of guayule and showed cyclic patterns of accumulation. The water-soluble fructans seem to be true reserve carbohydrates, depolymerizing when the carbon supply decreased at the end of Winter, and the demand for carbon increased at the inception of bud break. Fructans provide carbon for budbreak and exposure of plants to longer days and higher temperatures did not seem to alter this role. It is proposed that fructans are providing carbon for budbreak and renewed growth and are utilized for flowering when required. Starch production occurs during the warmer months in the leaves and young stems. Starch is synthesized from the immediate photosynthetic supply and it is this source of carbon which is utilized for the synthesis of cis-polyisoprene (rubber). Sucrose in the cytosol is sequestered for cis-polyisoprene synthesis while fructose, which can enter the plastid, is providing carbon for the synthesis of isoprenoids. Compartmentation of resin and rubber production ensures that the supply of carbon is adequate for both processes. As cis-polyisoprene synthesis occurs at a time when the plant is not disadvantaged by insufficient carbon , induction of rubber transferase enzymes would not depend on excess substrate, but would use a more reliable cue like temperature or daylength. Any attempt therefore to increase the carbon supply in guayule during the winter months would not necessarily lead to partitioning into cis-polyisoprene, but may be stored as fructan to ensure that, at bud break, the plant has an adequate and utilizable carbon supply.