ing ions [191]. It should be noted that the cells of embryos have essentially no vacuole in which to se-
quester ions. Other studies comparing accumulation of nutrients in phloem sinks (fruit) with accumula-
tion in xylem sinks (leaves) lead to similar conclusions about relative mobility of various ions [192–195].
One must take care in the interpretation of phloem exudate data, for the location of sampling can be crit-
ical. Pearson et al. [196] have shown that Mn and Zn enter wheat grains through phloem but much of the
transfer from xylem to phloem occurs within the inflorescence. However, Grusak [197] reported that iron,
supplied as Fe(III) citrate to leave of pea plants, was loaded into phloem and exported to developing seeds.
It should also be noted that an understanding of relative phloem-xylem mobility of nutrients leads
one to an understanding of which plant parts best supply various materials for human nutrition. In addi-
tion, toxic metal ions are generally more mobile in xylem than in phloem [198]; therefore leafy materials
grown on sites contaminated with such materials are more likely to be toxic to humans and livestock than
are fruit and seeds [195].
Inorganic anions are not as concentrated in the phloem as the cations, for much of the negative charge
is accounted for by organic ions. Van Die and Tammes [133] reported inorganic chloride, phosphate, and
sulfate in sieve tube exudate of yucca. About 75% of the phosphorus was combined into organic ions, and,
undoubtedly, a significant portion of the sulfur was in amino acids. Nitrate is seldom reported to be a com-
ponent of the phloem sap, but it is occasionally reported in low concentrations [146,199,200].
Wolterbeek and Van Die [201], using neutron activation analysis, were able to identify small
amounts of several other inorganics in phloem exudate, including rubidium, copper, bromine, vanadium,
and even gold. These data provide no information regarding relative phloem-xylem mobility of these
materials.
Until recently, there has been conflicting evidence concerning the phloem mobility of boron. How-
ever, Blevins and Lukaszewski [202] have concluded that boron is phloem mobile, probably as a cyclic
diester with mannitol, sorbitol, or other di- and polyols. Furthermore, Brown et al. [203] demonstrated
that wild-type tobacco plants transported very little boron from mature leaves whereas plants transgeni-
cally modified to synthesize sorbitol transported large amounts of boron from leaves. This observation
probably explains earlier conflicts in that plants that normally transport sugar alcohols would exhibit
much higher phloem mobility of boron than plants that do not transport polyols.
IV. MECHANISM OF LONG-DISTANCE TRANSPORT
As early as 1900, one of the predominant hypotheses of assimilate translocation was pressure flow
through sieve tubes [1]. In 1927, Munch [204] proposed an osmotic model for the generation of pressure
in phloem. This came to be known as the Munch pressure-flow hypotheses. Even so, textbooks published
in the early 1930s [205,206] made no mention of a pressure-flow mechanism.
Over the subsequent decades several different mechanisms of long-distance transport through
phloem were proposed [207–210]. Arguments were put forth that the microanatomy of sieve tubes could
not support a flow mechanism because the holes in sieve plates are blocked with “slime plugs” and, even
if they are not blocked, sieve plates would create far too much resistance for flow to occur at observed
rates. During the 1950s and 1960s, a substantial amount of physiological data was interpreted as refuting
a flow mechanism. When plants were supplied with^14 CO 2 , the profile of^14 C in the transport system de-
creased from source to sink in a pattern that would be expected for diffusion, although rates and veloci-
PRODUCTION-RELATED ASSIMILATE TRANSPORT 433
TABLE 2 Chemical Composition of Leaves and Floretsa
Leaves Florets Florets /leaves
Nutrient (mol m^3 )b (mol m^3 )b (K100)c
Potassium 72
6 133
21 100
Sodium 360
18 56
38
Calcium 35
9 25
239
Magnesium 37
5 45
266
Chloride 320
15 51
99
aData for Aster tipolium.
bConcentrations on plant water basis ( standard errors, n3) from Gorham et al. [190].
cFlorets / leaves adjusted such that potassium equals 100, other elements as a percentage of potassium.