favored sucrose. As with sugars, the concentrations and ratios of concentrations of various nitrogenous
compounds in sieve tubes vary markedly from the values for the same compounds in mesophyll cells
[159]. Other nitrogenous compounds such as ureides and alkaloids are translocated through the phloem
in certain species [23].
During development, the ratio of amino acids to sugar in the translocation stream increased
[23,24,157]. This would be expected, for as seeds developed, more nitrogenous compounds would be re-
quired by those sinks. A significant portion of the nitrogenous compounds in phloem was derived from
xylem, presumably transferred in stems and minor veins of leaves. This system appears to provide con-
trol for partitioning of nitrogen compounds required for seed development. Amino acid concentration of
sieve tube exudate increased from 14 g mL^1 1 week after anthesis to 21 g mL^1 6 weeks later, while
the sucrose concentration fell from 112 to 80 g mL^1 [23,157]. Environment also influences what is
transported; for example, water-deficient alfalfa markedly increased the concentration of proline in
phloem sap [160].
Ziegler [156] reviewed several studies that found large amounts of protein in sieve tube exudate of
members of the Cucurbitaceae. In the same volume, Eschrich and Heyser [161] concluded that protein in
the exudate resulted from surging induced by a sudden release of turgor when cuts were made. That con-
clusion is supported by^14 C labeling studies of selected members of this family indicating that most of the
carbon is carried as sucrose, raffinose, and stachyose [139,140]. More recently, a study using^35 S demon-
strated that a small amount of protein was translocated through sieve tubes [162]. Even though translo-
cation of protein is not quantitatively important, it appears to be involved in maintaining the functional
integrity of the sieve tubes [163] and may be involved in control of sink development, thereby affecting
partitioning indirectly [163,164]. Furthermore, nucleic acids have been isolated from phloem sap. Some
are viral and represent the mechanism by which viruses become systemic. There is evidence that viral
RNA enters sieve tubes via plasmodesmata by a mechanism normally used by plant mRNA [165]. This
plant mRNA may be important in integrating growth, development, and assimilate partitioning [164,166].
Movement of macromolecules through plasmodesmata into and through phloem has been reviewed [167].
Reduction of nitrate and sulfate in mesophyll cells produces OH[168,169], which is neutralized by
the Hsupplied as organic acids are synthesized. Acid anions are then loaded into the phloem. Although
the concentration of organic anions in sieve tube sap of yucca was only 7 me/mL [133], these anions are
exceedingly important in maintaining the pH of sieve tube sap [168], for, together with amino acids, they
produce salts of weak acids and strong bases with inorganic cations in sieve tubes. This yields a pH near
8.0, which facilitates phloem loading and may be critical in the control of enzymes that might otherwise
degrade translocated saccharides.
Many other organic compounds are undoubtedly translocated through phloem. Data indicate that
auxins [170], cytokinins [171], and gibberellins [172] as well as abscisic acid [173], salicylic acid [174],
and jasmonic acid [175] are phloem mobile. These substances are in such low concentrations within sieve
tubes that their osmotic influence on transport process is minimal; therefore, they move passively with the
flow through this system. However, they undoubtedly influence translocation of assimilates indirectly by
modifying the metabolic activity of phloem sinks [176] and, possibly, even of the membrane processes of
sieve tubes and companion cells [177–180].
C. Inorganics
Most plant scientists consider phloem as the conduit for organic materials, yet are also aware of the con-
cept of phloem mobility of inorganic nutrients. As already noted, the combination of inorganic cations
and organic anions contributes to the control of sieve tube pH.
The concentration of inorganic materials of Yucca[133] and lupine [181] sieve tube sap was about
2 mg/mL. This low concentration is, nevertheless, an extremely important component of the phloem
transport system, for nutrients that enter mature leaves via the transpiration stream could not be reparti-
tioned within plants were they not transported through phloem. Organs such as expanding buds, young
leaves, and developing flowers and fruit that get most of their nutrients through phloem would receive in-
adequate supplies of inorganic nutrients were they not translocated through phloem. Therefore, mobility
of inorganic nutrients through phloem is critical to plant growth and development.
Standard classroom deficiency experiments, often used to indicate mobility of nutrients in phloem,
are based on the assumption that the initial appearance of deficiency symptoms in older leaves indicates
PRODUCTION-RELATED ASSIMILATE TRANSPORT 431