phloem mobility of the nutrient being tested, whereas the initial appearance of symptoms in younger
leaves indicates phloem immobility. Analysis of phloem exudates generally supports that conclusion.
However, this is not always so. In addition, one must not use absolute concentrations of phloem exudate
to determine mobility because requirements for different nutrients vary by orders of magnitude.
A more reasonable approach is to compare phloem to xylem concentrations in plants that were ade-
quately supplied with nutrients. Table 1 [181–185] offers such a comparison for studies in which con-
centrations of nutrients of both xylem and phloem exudate were obtained from the same plant within any
one experiment. The following assumptions were used to construct Table 1:
- Over short periods of time, the same amount of potassium is translocated into a leaf through the
xylem and out through the phloem. - Therefore, mobility of any nutrient is made relative to potassium by computing the ratio of
phloem (P) to xylem (X) concentration (PN/XN) of any nutrient (N), dividing by the same ratio
for potassium (K), and multiplying by 100:
mobility
((
PP
KN
//
XX
KN)
)100 (1)
Equation (1) sets potassium equal to 100, whereupon phloem mobility of any other nutrient becomes
a percentage of potassium. When that was applied to data from several studies (Table 1), phloem mobil-
ities of sodium, calcium, and manganese were low, iron and zinc were intermediate, and magnesium was
variable. A low value should reflect the accumulation of that nutrient in a leaf as it ages, as does calcium
[186,187].
One must wonder why this approach to analysis of phloem mobility of iron disagrees with the stan-
dard nutrient deficiency experiment. It may be that under deficiency conditions Iron is rapidly incorpo-
rated into cellular components as it enters a leaf and, therefore, is unavailable to phloem, whereas with
sufficient supply, some iron is available to be translocated through phloem. Loneragan et al. [188] re-
ported that copper supplied to leaves of copper-deficient plants was retained in the leaves, but when sup-
plied to copper-sufficient plants, it was exported rapidly. That principle may also apply to iron. In addi-
tion, variability in sufficiency of magnesium in various experiments may explain the variable results for
that nutrient (Table 1). Certainly, the availability of organic anions, including amino acids, is critical to
maintaining solubility of iron, copper, and zinc [189].
Data of Gorham et al. [190], reproduced in Table 2, support the foregoing conclusions regarding
phloem mobility of inorganic ions, for ions with high phloem mobility were at higher concentrations in
the phloem sink, while ions of lower mobility were at higher concentrations in xylem sinks. It seems likely
that this pattern has survival significance by protecting the next generation (seeds) from enzyme-inhibit-
432 HENDRIXTABLE 1 Relative Phloem Mobilitya
Lupinus Lupinus Quercus Ricinus Lupinus
Nutrient angustifolab albab rubrac cammusd albae
K 100 100 100 100 100
Na 20 12 7 9 23
Mg 173 18 5 NAf 49
Ca 9 7 2 4 0.4
Fe 69 32 NA NA NA
Mn 15 14 NA NA NA
Zn 78 85 NA NA NA
aKylem /phloem concentrations of several ions divided by the same ratio of concentrations for potassium, thus setting all val-
ues for potassium equal to 100; values for other nutrients as a percentage of phloem mobility of potassium. See Eq. (1) in the
text.
bFrom Ref. 181.
cFrom Ref. 182, compiled by Pate [23].
dFrom Refs. 183 and 184, compiled by Pate [23].
eFrom Ref. 185.
fNA, data not available.