|dc.description.abstract||The study was aimed at determining the metabolic fate of sucrose in intact
sugarcane internodal tissue. Three aspects of the fate of sucrose in storage
tissue of whole plants formed the main focus of the work. These were the rate of
sucrose accumulation in the developing culm, the characterisation of partitioning
of carbon into different cellular organic fractions in the developing culm and the
occurrence of sucrose turnover in both immature and mature stem tissues.
Specific attention was paid to confirming the occurrence of sucrose turnover in
both immature and mature internodal tissue. This sucrose turnover has been
described previously in both tissue slices and cell suspension cultures. However,
certain results from previous work at the whole plant level have indicated that
sucrose turnover does not occur in mature internodal tissue.
Radiolabeled carbon dioxide (14CO2) was fed to leaf 6 of sugarcane culms of a
high sucrose storing variety (Saccharum spp. hybrid cv. Nco376). All plants were
of similar age (12 months) and were grown under similar conditions. The
movement and metabolic fate of radiolabeled sucrose was determined at four
time points, (6 hours, 24 hours, 7 days and 6 weeks) during a 6 week period.
The metabolic fate of sucrose was determined in internodes number 3, number 6
and number 9. Internode 3 was found to have a relatively high hexose sugar
content of 42 mg glc&fruc fw g-1 and a low sucrose content of 14 mg suc fw g-1.
In contrast the sucrose content of internode 9 was much higher at 157 mg suc fw
g-1 and the hexose sugar content much lower at 4.3 mg glc&fruc fw g-1. Based
on previous work, the sugar content of internode 3 and internode 9 are
characteristic of immature and mature tissues respectively. Internode 6 occupies
an intermediary position between internode 3 and 6 with its sucrose content
higher than its hexose sugar content, but with the hexose sugar content still
being notable at 15 mg glc&fruc fw g-1.
Although the metabolic fate of sucrose within sink tissue was the focal point of
the study, the experimental design also allowed for certain aspects of sucrose
production in the source to be investigated. The average photosynthetic rate for
leaf 6 in full sunlight was estimated at 48 mg CO2 dm-2 s -1. During
photosynthesis, only 30% of the fixed carbon was partitioned into the storage
carbohydrate pool while the remaining 70% was partitioned into sucrose for
immediate export from the leaf. This high rate of carbon fixation combined with a
high rate of carbon export is characteristic of C4 plants such as sugarcane.
On entering the culm, translocation of radiolabeled sucrose was predominantly
basipetal with relatively little acropetal translocation. The majority of the
radiolabeled carbon was found to be stored in mature internodes. No significant
loss of radiolabeled carbon was observed in mature and elongating internodes
over the study period. A 22% loss of total radiolabeled carbon was observed in
immature internodes over the same period. This can probably be attributed to
the higher rates of cellular respiration known to occur in immature tissues.
There appear to be three phases of sucrose accumulation in the developing
culm. Initially, the accumulation rate in rapidly growing tissue, as internode 3
develops into internode 6, is relatively low. This is followed by a rapid increase in
the rate of sucrose accumulation during internode elongation, as internode 6
becomes internode 9. Finally, a decrease in the rate of sucrose accumulation is
observed during late maturation, as internode 9 becomes internode 12.
Determination of the sucrose content in internodes 3, 6 and 9 revealed that there
is a notable increase in sucrose content during internode maturation. It is
proposed that the higher sucrose content of mature tissue is not merely a
consequence of the longer growth period of mature tissue, but is due to the
increased rate of sucrose accumulation observed during internode elongation.
Short-term (24 hours) analysis of carbon partitioning revealed that intemodal
maturation was associated with a redirection of carbon from non-sucrose cellulal
organic fractions to sucrose storage. In immature internodes only 20% of the
total radiolabeled carbon was present in the sucrose pool 24 hours after feeding.
In elongating internodes the figure increased to 54% while in mature internodes
as much as 77% of the total radiolabeled carbon was retained in the sucrose
pool. Concomitant with the increased carbon partitioning into stored sucrose
down the developing culm is a decrease in carbon partitioning into the hexose
sugar pool. In immature tissue, 42 % of the total radiolabel is present in the
hexose sugar pool, while in mature tissue the percentage drops to 11%. This
decrease is probably indicative of decreased levels of carbon cycling between
the sucrose and hexose sugar pool as a result of internode maturation.
Internode maturation was also found to be associated with a decrease in the
amount of carbon in the water insoluble matter pool and the amino acid/ organic
acid/ sugar phosphate pool. Thus, internode maturation is associated with a
redirection of carbon from total respiration to sucrose storage. Long-term (6
weeks) analysis of carbon partitioning confirmed that sucrose storage in mature
tissue is greater than that in immature tissue. From the 6 hour time point to the 6
week time point, an 87% reduction in the stored radiolabeled sucrose content
was observed in immature internodes. During the same period only a 25%
reduction in the stored radiolabeled sucrose was observed in mature internodes.
Radiolabel loss from the radiolabeled sucrose pool in both mature and immature
internodes was accounted for by relative radiolabel gains in other cellular organic
At all time points during the study, and in all three tissues studied, radiolabel was
found in the sucrose pool, the hexose sugars pool, the ionic pool and the water
insoluble matter pool. The occurrence of radiolabel in the non-sucrose tissue
constituents suggests that sucrose turnover is occurring in both immature, and
mature internodal tissue.||en