between the two cell types. Because of the high degree of symplastic continuity between the companion
cell and sieve element, these are often referred to as sieve element–companion cell (SE-CC) complexes
[2–4].
B. Minor Vein Structure
The venation of source leaves is designed such that individual photosynthetic mesophyll cells are never
more than a few cells away from a minor vein (Figure 2). This arrangement drastically reduces the dis-
tance that assimilates must travel from the sites of photosynthesis to the phloem transport system [5]. It
is in the SE-CC complexes of the leaf minor veins that loading of the phloem transport system with pho-
tosynthetic products is initiated. Not surprisingly, the companion cells within the minor veins are very
much larger than the sieve elements with which they are associated (Figure 3), which more than likely re-
flects the added metabolic activity imparted by the phloem loading process.
Minor vein companion cells form a key interface between the photosynthetic tissues of the leaf and
the conduits of the phloem system. Based on ultrastructural differences, Gamalei [6], van Bel and Gar-
nalei [7], and van Bel [8] have categorized three classes of minor vein companion cells within source
leaves (Figure 4).
Type 1 companion cells (Figure 4A), referred to as “intermediary cells” [9–11], are characterized by
large numbers of plasmodesmata, which link the cytoplasm of these cells to that of the adjacent photo-
synthetic cells. These cells are specialized for the symplastic transfer of assimilates from the photosyn-
thetic cells to the sieve elements. Companion cells of this type are common in many species of horticul-
tural importance, including tree species such as olive [12], most woody ornamental vines and shrubs
[6–8], culinary herbs and ornamentals of the mint family such as coleus [13], and the cucurbit vine crops
[14,15]. Crop species that have this companion cell type tend to be of tropical or subtropical origin [16].
Type 2a companion cells (Figure 4B) lack the extensive plasmodesmatal connections to the photo-
synthetic tissues that are typical of type 1 cells. Assimilates produced in leaves with this type of com-
panion cell, therefore, do not have an elaborate symplastic pathway through which to travel into the SE-
CC complex and must be released from the cytoplasm of the photosynthetic cells [6]. Assimilates are
PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 451
Figure 2 Paradermal section of a lilac (Syringa vulgarisL.) source leaf, showing the arrangement of the mi-
nor venation within the photosynthetic tissues. (Paraffin section slide courtesy of D. A. DeMason.)