II. CARBOHYDRATE FORMATION IN SOURCE LEAVES
Triose phosphates, the first products of the photosynthetic process, represent key metabolic intermediates,
for they are the immediate precursors of all carbohydrates synthesized in the source leaves. The type of
carbohydrates synthesized is, in turn, regulated by compartmentation of triose phosphates within chloro-
plastic and cytoplasmic pools within the source leaf. Because the major photosynthetic tissues are fully
matured leaves, cell wall synthesis (i.e., the synthesis of structural carbohydrate) is of little importance in
these tissues. Photosynthetic carbon is instead partitioned to nonstructural carbohydrates, which, in leaves
of higher plants, may take the form of insoluble polymers (starch), soluble polymers (fructans), and sol-
uble low-molecular-weight carbohydrates (sucrose, raffinose family oligosaccharides, simple monosac-
charides, polyols, and cyclitols).
A. Starch
Starch is an insoluble glucan polymer that exists as granules within the chloroplast where it is formed.
Starch consists of two molecular species: amylose, an essentially linear (-1,4)-glucose polymer, and
amylopectin, in which linear -1,4-glucans are linked via -1,6 linkages to form a highly branched struc-
ture. The proportion of amylose to amylopectin in starch grains varies in different plant species and in dif-
ferent cultivars of the same species [3–5].
In source leaves, starch is often called “assimilatory” starch because it is a major reserve of photo-
synthetically fixed carbon [4]. During nonphotosynthetic periods (i.e., at night), this starch is mobilized
and utilized to support growth and maintenance of the plant. Starch granules form by apposition of newly
formed polymers onto existing grains; they are degraded by the reverse process. Therefore, increases and
decreases in the size of starch grains are seen during photosynthetic and nonphotosynthetic periods, re-
spectively [3,6].
As indicated in Figure 1, starch formation in source leaves begins by the assimilation of CO 2 by pho-
tosynthesis, with the subsequent formation of triose phosphates (triose-P). Further operation of the pho-
tosynthetic carbon reduction (PCR) cycle results in the formation of fructose-6-phosphate (Fru-6-P). If
468 PATTANAGUL ET AL.Figure 1 Pathway of starch synthesis in the chloroplasts of source leaves. Reaction 1, fructose-1,6-bisphos-
phatase; reaction 2, phosphoglucomutase; reaction 3, ADPG pyrophosphorylase (ADPGPPase); reaction 4,
starch synthase.