III. WHAT IS TRANSLOCATED AND WHY
A. Sugars and Sugar Alcohols
Sugars and sugar alcohols carry 60 to 95% of the translocated carbon and have a total concentration rang-
ing up to 180 mg mL^1 [133]. Sucrose is the primary translocated carbohydrate in most plants [134–137].
In many plants, sucrose represents essentially all of the carbohydrate translocated. Other nonreducing
oligosaccharides, such as raffinose, stachyose [134,138–141], gentianose, umbelliferose [136], and fruc-
tan [138], are translocated in some plants.
Sugar alcohols, along with sucrose, are translocated in a few groups of plants. Specifically, sorbitol
is translocated by several members of the Rosaceae and Oleaceae [134,142], and mannitol is translocated
by celery [143]. Even the giant alga, Macrocystis[144] and Fucus[145] translocated mannitol through
“sieve cells” [146].
Occasionally, there are reports of glucose and fructose being isolated from phloem exudate [133].
However, these may be artifacts that resulted from hydrolysis of sucrose by enzymes released from dam-
aged cells or a portion of the sap being supplied by damaged cells other than sieve tubes [147]. Sucrose
hydrolysis should result in a 1:1 glucose-to-fructose ratio; however, Glad et al. [24] reported a ratio of
about 2:1 from sieve tube exudate of grape. Nevertheless, Swanson and El Shishiny [148] obtained data
from grape labeled with^14 CO 2 and concluded that grape did not translocate hexoses. Wang and Nobel
[137] reported significant amounts of hexoses, especially fructose, in phloem exudate of Agave. However,
the existence of fructan hydrolases in the sap suggests that these hexoses may be an artifact (N Wang, per-
sonal communication).
In general, most if not all of the sugars and sugar alcohols translocated by plants are nonreducing.
Why should that be? Arnold [149] proposed that these are “protected” molecules. Molecules acted upon
by only a few enzymes would be favored for translocation because activity of only these few enzymes
would need to be suppressed within sieve tubes to maintain the protected state.
Why do some plants translocate only sucrose while others translocate a mixture of sucrose, larger
oligosaccharides, and/or sugar alcohols? Handley et al. [150] suggested that plants translocate sugars dif-
ferent from those accumulated in storage tissue, thereby maintaining a concentration gradient for translo-
cated sugar from sieve tubes to sink cells. Examples of such a system are represented for cucurbits [150]
and legumes [1,135]. Even some parasites and symblonts operate similarly, accumulating carbohydrates
not found in appreciable concentrations in their hosts [130,151,152]. Obviously, this does not apply to all
plants, for sugar beets and sugarcane translocate and store sucrose, but they store it within the vacuole that
would keep it away from the transport system. It has been proposed that selection of the carbohydrate that
is translocated is based on phloem loading mechanisms (discussed in the following).
In spite of the variability observed in translocated carbohydrates, all plants appear to translocate
some carbon as sucrose. There is no known study that accounts for this observation. A possible explana-
tion involves the observation that callose synthesis at sleve plates occurs rapidly [153]. This implies that
the required enzymes must be present at all times but are usually inactive. If injury activated the sucrose
synthase that Nolte and Koch [154] located in companion cells, the UDP-glucose substrate of callose syn-
thesis would be provided and an inhibitor of callose synthesis, UDP, would be removed [155]. Neither
larger oligosaccharides nor sugar alcohols could provide appropriate conditions so easily.
B. Other Organic Compounds
Although most of the carbon is translocated through phloem as carbohydrate, other organic compounds
are translocated in significant quantities. Ziegler [156] summarized the literature then available for ni-
trogenous compounds in sieve tube sap. Reported concentrations varied from 0.8 to 137 mol mL^1. Pate
[157] observed sieve tube concentrations of amino acids and amides in the phloem of lupine up to 21 mg
mL^1 and reported that asparagine accounted for over half of that content and glutamine was second most
concentrated. In sieve tube exudate of yucca, glutamine and glutamic acid predominated [133], and in
grape exudate, glutamine predominated [24]. Interestingly, the same amino acids predominate in the
translocation steam of giant algae [144]. However, no single amino acid or small group of amino acids
were dominant in the phloem exudate of Opuntia[158]. Glad et al. [24] also reported that molar quanti-
ties of amino acids approximated those of sugar. It should be noted, however, that on average each
translocated amino acid molecule is much smaller than sucrose; therefore, mass-based quantities strongly
430 HENDRIX