equivalent, volemitol, is less common and appears to be restricted to certain section of the genus Primula
[57]. In this species, the polyol is a product of photosynthesis and appears to be phloem mobile. An en-
zyme that catalyzes the reduction of sedoheptulose to volemitol has been characterized [57].
Mannoheptulose and its polyol form, perseitol, are found in all parts of the avocado (Persea ameri-
canaMill.) tree [58,59]. In avocado, these C7 sugars are apparently important in metabolic processes as-
sociated with fruit development as well as respiratory processes associated with postharvest physiology
and fruit ripening [59]. Because of their localized distribution in the plant kingdom, the biosynthesis and
metabolic pathways of these sugars have not yet received much attention. Mannoheptulose, a very potent
inhibitor of hexokinase reactions in respiration, does hold some promise as an antitumor agent, so further
study of these sugars is again warranted and needed.
III. CARBOHYDRATE FORMATION IN NONPHOTOSYNTHETIC (SINK)
TISSUESThe soluble and insoluble forms of carbohydrate that have been listed are all used as temporary storage
reserves in the leaf. However, certain of the soluble forms, such as sucrose, the raffinose family oligosac-
charides, and the polyols, are also phloem mobile and can be delivered to nonphotosynthetic tissues to
support growth and development of these plant parts [35]. It is commonly found, therefore, that even non-
photosynthetic tissues will contain some or all of the same carbohydrates that commonly occur in phloem
sap. However, because carbohydrates are also required for growth processes such as respiration or cell
wall synthesis, sink tissues are also equipped with enzymes for breakdown, interconversion, and
metabolism of whatever phloem-mobile carbohydrates are supplied to them. As a result, it is also not un-
common to find carbohydrates that are in fact quite different from those supplied to the sink by phloem
transport.
Carbohydrates formed in sink tissues may act as storage reserves and, as occurs in source leaves, they
are found to be compartmentalized in specialized cells or cellular compartments such as the plastids or vac-
uoles. In most agronomic crops, it is these storage reserves that are of economic importance—for exam-
ple, the yield of seeds, grains, and storage roots or tubers is dictated principally by the size of their carbo-
hydrate reserves at harvest. The enzymes for synthesis of common storage carbohydrates, including
soluble and polymeric forms, are therefore found in a range of plant tissues, not just the mature leaves. Re-
search into carbohydrate metabolism in nonphotosynthetic tissues is showing that the controlling factors
in the regulation of carbohydrate synthesis are often surprisingly similar to those in photosynthetic cells.
A. Starch
Starch synthesis by sink tissue is probably one of the most important plant biochemical reactions in terms
of human nutrition because starch, particularly from grain crops (where it can make up 70% of the dry
weight), is a major provider of nutritional calories in the human diet everywhere [15]. Despite the im-
portance of starch biosynthesis in crop plants, we actually know very little concerning the biochemical
details of starch deposition in sink tissues.
Starch deposition in sink tissues occurs at the expense of imported assimilates and appears to require
the conversion of phloem-delivered solutes into a usable hexose phosphate form [60]. For phloem-derived
sucrose, there are at least two pathways by which this conversion occurs: by invertase hydrolysis to hex-
ose followed by phosphorylation to hexose phosphate:
Invertase:Sucrose→glucosefructoseand by reversal reaction of sucrose synthase to provide fructose and UDPG:
Sucrose synthase:SucroseUDP→UDPGfructoseDepending on the sink tissues, there is evidence for operation of both these pathways in sinks. For-
mation of hexose-P takes places by direct phosphorylation by hexokinase reactions or, in the case of
UDPG, by reversal of the UDPG pyrophosphorylase reaction:
UDPPase:UDPGPPi→Glu-1-PUTPCARBOHYDRATE SYNTHESIS AND CROP METABOLISM 479