Carbohydrate metabolism in Parthenium argentatum Gray.
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Date
1991
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Abstract
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. [[14]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. [[14]C] Fructose was more readily translocated than [[14]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 [[14]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 [[14]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.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1991.
Keywords
Guayule., Carbohydrates--Metabolism., Theses--Botany.