ducing oligosaccharides and sugar alcohols are translocated by this system. For many plants, sucrose ap-
pears to be the only carbohydrate translocated; others translocate a mixture of sugars and sugar alcohols,
but sucrose seems to be a component of all. Many different nitrogenous compounds are also translocated
through the phloem. These include most, if not all, of the protein amino acids and amides as well as sev-
eral ureides. Most common are aspartic and glutamic acid and their amides. Nitrate is of little, if any, sig-
nificance in nitrogen translocation through phloem.
Various inorganic materials are translocated through the phloem. By far the most concentrated, other
than water, is potassium. Together with weak organic anions and amino acids, inorganic cations form a
salt that buffers the pH of sieve tube sap at about 8.0.
It now appears that there are two pathways of phloem loading. In the apoplastic route, assimilates
pass into the cell wall before entering sieve tubes. The symplastic pathway follows the plasmodesmata
from cell to cell and finally into sieve tubes. Based on the anatomy of the vascular bundles, it appears that
many plants use primarily one or the other pathway, whereas other plants may to be able to use either or
both. Both mechanisms result in a higher concentration of sugars in the sieve tube–companion cell com-
plex than in other cells. The mechanism of concentrating the carbohydrates is understood for the apoplas-
tic pathway but not for the symplastic pathway.
Phloem unloading also follows either pathway. Because there is no symplastic connection between
a parent plant and the seed it is forming, transfer of assimilates into a developing seed must be apoplas-
tic, although the assimilates may not enter the apoplast directly from sieve tubes. When assimilates are
being transferred to cells within the same plant, they may follow either the symplastic or the apoplastic
pathway. In most cases, it appears that chemical modifications maintain free energy gradients for carbo-
hydrates along the unloading pathway. Judicious use of xenobiotics requires an understanding of trans-
port processes, including the special problems involved in loading and unloading of these substances.
Exploration of space requires the recycling of human wastes into materials used by humans. Efficient
use of plants is required in this system. Getting a high proportion of the photoassimilates partitioned into
edible parts is an important component of that efficiency.
ACKNOWLEDGMENTS
I wish to express my appreciation to all who contributed research cited in this chapter, as well as the many
others not cited whose work contributed to my background understanding. I wish, also, to thank those who
sent reprints for consideration and especially those with whom I discussed their research and who aided
me in interpretation of their work. Special thanks go to Dr. Cecil Stushmoff for reading the manuscript
and offering helpful comments.
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442 HENDRIX