PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 453
Figure 4 Diagrammatical representations of the classes of minor vein companion cells found in source
leaves: (A) Type 1 companion cell (intermediary cell), (B) type 2a companion cell, and (C) type 2b companion
cell (transfer cell).
In constrast, phloem transport occurs through living cells, the sieve elements [1]. The rates of trans-
port are much lower than one sees in the xylem (on the order of 1 cm min^1 ) and can occur in either an
upward or downward direction [21]. Rate and direction of transport are dictated by differences in solute
concentrations between sites of solute synthesis (sources) and solute consumption (sinks) within the plant
body. It is these solute concentration differences that provide the driving force for phloem transport [22].
In contrast to xylem saps, phloem saps contain very high solute levels (Table 1) and particularly high lev-
els of sugars, amino acids, and potassium.
A. The Munch Pressure Flow Mechanism
The pressure flow mechanism first postulated by Munch [22] provides the best explanation for the driv-
ing force for phloem transport presently available based on our knowledge of rates of transport and