some other physiological significance, for example, as a free radical scavenger imparting protection
against hydroxyl radicals generated during stress [33,34].
E. Raffinose Family Oligosaccharides
Like the polyols, raffinose family oligosaccharides are probably ubiquitous in the plant kingdom. There
are a large number of plant families in which these oligosaccharides are synthesized in leaves and used as
translocatable forms of carbon [35]. However, of the many plant species in which translocation of these
sugars may occur, only a few, such as the cucurbit vine crops, are of major agronomic importance. As a
result, this biochemical pathway of carbohydrate formation has been relatively neglected by crop physi-
ologists. This is truly unfortunate, for evidence now clearly indicates that the synthesis of raffinose fam-
ily oligosaccharides is quite different from that of other known soluble carbohydrates in a number of
ways.
The raffinose family oligosaccharides, of which raffinose and stachyose (Figure 2) are the most com-
mon examples, are all simple galactosides of sucrose. The addition to the sucrose molecule of the galac-
tose residues (which are linked by -1,6 linkages to the glucose moiety of sucrose) occurs, as was seen
with fructans, without the direct use of a sugar nucleotide. Instead, galactinol, a novel galactoside of myo-
inositol, is used as the galactose donor [36]. What is particularly unique about the raffinose family
CARBOHYDRATE SYNTHESIS AND CROP METABOLISM 475
Figure 6 Pathway of polyol (mannitol) synthesis in the cytoplasmic of photosynthetic leaf cells. Reaction 1,
mannose-6-phosphate isomerase; reaction 2, mannose-6-phosphate reductase (M6PR); reaction 3, mannitol
phosphate phosphatase.