Agriculture Reference
In-Depth Information
the formation of
-carbon acids are termed C
plants. Glycerate-
-P is phos-
phorylatedwhenATPispresenttoformglycerate
,
-Pwhichisreduced,when
NADPH is present, to give glyceraldehyde-
-P. This triose phosphate, together
with dihydroxyacetone phosphate (DHAP) can be combined to form fructose
-bisphosphate (fructose-P
) through the action of fructose bisphosphatase
(FBPase). Fructose
,
-phosphate can then be used to form starch.
The flow of carbon within this overall photosynthetic carbon reduction
(PCR) cycle is largely controlled by regulation of the enzymes within it. The
enzyme present in the highest concentration in the chloroplast stroma is
RuBPC/O, which may constitute up to
% of the total protein within the
chloroplast. The activities of several of the enzymes are regulated by light and,
in addition, changes in the stromal pH and magnesium concentration affect
the activities of both RuBPC/O and FBPase.
Both O
and CO
compete for RuBP at the catalytic site on the enzyme so
photosynthesis is directly inhibited by O
. The oxygenase reaction results in
the formation of phosphoglycolate at the start of the photosynthetic oxidative
carbon cycle which is the metabolic pathway for photorespiration. The release
of CO
from this pathway may be equivalent to
-
% of the rate of net CO
assimilation (Sharkey,
).
-carbon alcohol which plays several key roles in woody
Rosaceae
,
is formed by the reduction of
Sorbitol, a
-fructose and
-glucose. Glucose-P is reduced
to sorbitol-P by aldose
-P reductase and the P split from sorbitol-P by a
phosphatase. A glucitol
-P phosphatase has been found in apple leaves (Grant
and ap-Rees,
).
When
CO
is fed to apple leaves
% of the total activity in the sugar
fraction is found in sorbitol immediately after supply (Hansen,
-
a, b). The
rest is mainly in sucrose, with some in glucose and fructose. Similar results
are found with pear leaves (Bieleski,
). Sorbitol is the major end prod-
uct of apple and pear photosynthesis (Priestley,
; Oliveira and Priestley,
).
Transport of photosynthate from leaves to fruits appears to be mainly as
sorbitol.Aftersupplyof
CO
toappleleavestheconcentrationof
C-sorbitol
inthesefallsquickly,labelledsorbitolisfoundintheconductingtissuesbetween
the leaves and adjacent fruits and sorbitol appears as the main labelled 'sugar'
inthefruitwithinafewhours(Hansen,
).Sorbitolisthemaincarbohydrate
found in apple phloem (Bieleski,
). It is particularly suited to translocation
in the phloem because it is not metabolized there.
Sorbitol is the most abundant compound in the soluble fraction of carbo-
hydrate reserves in woody
Rosaceae
(Oliveira and Priestley,
). However,
sorbitol transported from leaves is quickly converted in the fruits into sucrose,
glucose and, especially, fructose (Hansen,
). This maintains the concen-
tration gradient which facilitates the leaf-to-fruit translocation. It is consistent
