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Phloem Transport of Solutes in Crop Plants
Edmund R. Miranda, Wattana Pattanagul, and Monica A. Madore
University of California, Riverside, California
449I. INTRODUCTION
Many plant parts, including many flowers, fruits, and seeds, do not contain chlorophyll and are therefore
not photosynthetically competent. Other plant parts, particularly underground roots, rhizomes, and tubers,
are located on the plant in areas where light reception is insufficient to drive photosynthesis. In plant parts
such as meristems, stems, and developing leaves, modification, incomplete development, or insufficient
number of plastids also limits photosynthetic competence. Photosynthetic activity is therefore found to be
largely confined to organs located in areas of maximal light interception and containing fully functional
chloloplasts. In higher plants, these organs are represented by mature, fully expanded leaves.
The consequence of this separation of the plant body into photosynthetically competent and non-
photosynthetic organs is that photosynthesizing leaves become the sole “source” of photosynthetically
produced biomolecules (photoassimilates) for the rest of the plant. Thus, to supply the demands of non-
photosynthetic plant parts, which act as competing “sinks” for photosynthetic products, leaves must pro-
duce photoassimilates in amounts far in excess of what is required simply for maintenance of leaf
metabolism. In higher plants, the delivery of photoassimilates from source to sink regions within the plant
body is accomplished by translocation in the phloem tissues.
In crop plants, phloem transport is a particularly important physiological process, for with very few
exceptions, the agronomically important plant parts that are harvested from our major agricultural crop
plants are sink tissues. From a physiological standpoint, what this ultimately means is that the ability of
a particular crop plant to carry out photosynthesis during a growth season will only partly determine the
final harvestable yield of that crop. The phloem transport process will be of equal importance, for it is this
process that determines just how efficiently photosynthetically produced nutrients are made available to
the plant part to be harvested. A complete understanding of phloem transport and its regulation is there-
fore basic to our understanding of crop physiology.
II. PHLOEM STRUCTURE
It is beyond the scope of this chapter to provide more than a general description of the anatomy of the
phloem transport system. Readers should consult a general plant anatomy textbook (e.g., Ref. 1) or re-
views of phloem structure [2–4] for more details regarding the anatomy, morphology, and differentiation