species [30]. Feeding experiments with cadaverine and spermidine have exhibited reduced phloem sap
levels of putrescine, which demonstrates a competitive effect between various polyamines [35]. Among
the purported roles of polyamines one function may be in stabilization of biomembranes, which may be
important for the regulation of growth in plants.
- Mineral Nutrients
Many of the same mineral ions found in xylem saps are also found in phloem saps (Table 2) [30,31,36,37],
indicating that these nutrients can be removed from the xylem and loaded into the phloem transport sys-
tem. Many sink tissues, being only poorly supplied with these nutrients by the xylem because of low tran-
spiration rates, must depend on phloem transport for much of their mineral requirements [30,37].
In general, the relative mobility of mineral ions in the phloem can be determined by the site at which
deficiency symptoms first appear. Some ions (e.g., boron and calcium) are only poorly loaded into the
phloem [30,31,36,37]. In these cases, deficiency symptoms appear predominantly in sink tissues such as
fruits and young leaves, which must depend on transpiration and xylem movement for a supply of these
minerals [37]. In contrast, in the cases of minerals that are highly phloem mobile (e.g., magnesium, potas-
sium), deficiency symptoms appear first in the mature leaves [37]. This indicates a remobilization of min-
erals from the mature leaves and delivery of these elements to the sink leaves via phloem transport.
Mineral ions may be translocated as free elemental ionic forms (e.g., K, Cl) but frequently may
exist in other chemical forms (e.g., phosphate, sulfate, ammonium). Notably, although free nitrate is a
common constituent of xylem saps, it is never found in phloem saps (Table 2) [30,31,36]. Mineral ele-
ments may also be combined into organic complexes (e.g., ferric chelates, zinc peptides, phosphate es-
ters, sulfur-containing amino acids) for transport in the phloem [30,36]. For example, S-methylmethion-
ine (SMM) in phloem has been reported to provide more than half of the sulfur needed for grain protein
synthesis in wheat [38]. The enzymes involved in synthesis of SMM have shown strong amino acid se-
quence homology with those of Arabidopsisand maize, and one can speculate that a transgenic approach
to increase the copy number of SMM genes might reduce the fertilizer levels required on croplands.
456 MIRANDA ET AL.TABLE 2 Typical Range of Amino Acid Composition of
Phloem Sap
Amino acid Phloem sap concentration (mM)
Aspartate 2–20
Glutamate 7–25
Asparagine 2–275
Glutamine 10–25
Serine 5–15
Glycine Trace–6
Homoserine 0–trace
Citrulline 0–20
Histidine 0–trace
Arginine Trace–5
Threonine 1–10
Alanine 1–8
Proline 5–15
Tyrosine 0.5–2.0
Valine 0–9
Methionine 0–trace
Cysteine 0–1
Isoleucine 2–6
Leucine 0–6
Phenylalanine 3–5
Tryptophan 0–trace
Ornithine 0–trace
Lysine 1–3
Source: Data from Refs. 31–33.