phloem structure. Phloem loading in source tissues leads to the very high solute concentrations charac-
teristic of the phloem. The high solute levels create a water potential gradient within the sieve element,
and water moves into the sieve element from the adjacent xylem tissues. Because the sieve element is a
living cell and has a functional plasma membrane, this influx of water creates a very high hydrostatic pres-
sure within the sieve element. At the sink end, the solutes are removed from the sieve element for use by
the sink cells, and the hydrostatic pressure is reduced. This combination of solute loading at the source
end and solute unloading at the sink end of the phloem system creates a strong hydrostatic pressure gra-
dient. Because the sieve elements are linked end to end by open sieve plates, water containing the dis-
solved solutes passes through the pores of the sieve plates in response to the pressure gradient and solutes
are moved by this bulk flow from source to sink.
B. Solutes Translocated in the Phloem
A mature, fully expanded leaf not only is the primary site of photosynthesis but also has the highest rate
of transpiration. As a result, a significant percentage of the dissolved mineral nutrients present in the
xylem sap will end up in leaves, not in the agronomically important plant parts. The phloem of the minor
veins of leaves is therefore very important, not only for the transport of photosynthate produced in the
leaves but also for the redistribution of mineral elements delivered by the xylem. In addition, phloem
transport plays a major role in the transduction of developmental and environmental stimuli via the trans-
port of growth regulators and systemic signal molecules.
- Carbohydrates
Carbohydrates translocated in the phloem are all low-molecular-weight nonreducing sugars (Figure 5A)
or sugar alcohols (Figure 5B). The dissacharide sucrose (Figure 5A) is ubiqutous in the phloem of crop
plants. However, many important crop species transport sugars in addition to sucrose [23]. Plant species
that possess type 1 companion cells (Figure 4A) all translocate the raffinose family oligosaccharides such
as raffinose and stachyose [6–8,13–15,24], which are galactoside derivatives of sucrose (Figure 5A).
Some members of the Rosaceae, including tree crops such as apples, cherries, plums, and apricots, also
translocate significant quantities of the sugar alcohol sorbitol [23,25,26]. Members of the Apiaceae, such
as celery, transport the sugar alcohol mannitol in addition to sucrose [25,27–29]. Still other plant species,
such as olive [12] and euonymus [23], translocate both raffinose family sugars and a sugar alcohol (man-
nitol and dulcitol, respectively).
- Nitrogen-Containing Compounds
Most protein amino acids are found in phloem saps (Table 2) [30–33]. The predominant amino acids tend
to be those having a high ratio of nitrogen to carbon (Figure 5C), particularly the amides asparagine and
454 MIRANDA ET AL.TABLE 1 Typical Ranges for Components of Xylem and Phloem
Saps in Higher Plants
Concentrations (g mL^1 )
Substance Xylem Phloem
Sugars Absent 140,000–210,000
Amino acids 200–1000 900–10,000
P 70–80 300–550
K 200–800 2800–4400
Ca 150–200 80–150
Mg 30–200 100–400
Mn 0.2–6.0 0.9–3.4
Zn 1.5–7.0 8–23
Cu 0.1–2.5 1.0–5.0
B 3.0–6.0 9–11
NO 3 1500–2000 Absent
NH 4 7–60 45–846
Source: Data from Refs. 31 and 36.