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Carbohydrate Synthesis and Crop Metabolism
Wattana Pattanagul, Edmund R. Miranda, and Monica A. Madore
University of California, Riverside, California
467I. INTRODUCTION
Plants are capable of producing all organic materials required for growth, metabolism, and reproduction
from very simple inorganic molecules obtained from the atmosphere and the soil. Using light energy
trapped by chlorophyll in the process of photosynthesis, these inorganic molecules (principally CO 2 ,
phosphate, and nitrate or ammonia) are incorporated within the chloroplasts of mature leaves in a num-
ber of relatively simple biomolecules (e.g., triose phosphate, amino acids), which are then used elsewhere
in the cell for respiration or for the construction of the more complex biomolecules (e.g., complex sac-
charides, proteins, nucleic acids) required for growth and metabolism. In agronomic crop species, the in-
corporation of fixed carbon into carbohydrates is particularly important, for carbohydrate production
largely determines the yield of crop plants.
Plant carbohydrates can be classified into two forms: structural and nonstructural. The form of car-
bohydrate in a particular plant part will also to a large extent determine its agronomic usage. Structural
carbohydrates, as the name implies, are polymers that help to form the rigid plant cell wall and give sup-
port to the plant body. These carbohydrates are in the form of permanent, usually extracellular structures,
and carbon incorporated into structural elements is in general not available for further metabolism by the
plant. An exception may occur in some seeds, in which cell wall polysaccharides can be metabolized by
the germinating seedling [1,2].
Structural carbohydrates, of which cellulose is a prime example, have agronomic importance as com-
ponents of livestock feed and as sources of fiber for industrial purposes (e.g., cotton). In terms of human
nutrition, although they represent an important source of dietary fiber, they are of no direct nutritional
value, as the enzymes for their metabolism are lacking in humans. The same is also true of many of the
nonstructural carbohydrates. Indeed, of the many nonstructural carbohydrates characteristic of plant or-
gans, only two forms, sucrose and starch, are directly metabolizable by humans.
Both structural and nonstructural carbohydrates form a large proportion of the dry weight of plant or-
gans. The way in which carbon is partitioned among different carbohydrate types in a particular crop,
therefore, becomes an important determinant of the agronomic value of that crop, particularly from a nu-
tritional standpoint. In light of this, it is evident that a basic understanding of carbohydrate synthesis and
its control in crop plants is central to our understanding of crop physiology.