DISTRIBUTION AND FREQUENCY OF PLASMODESMATA IN RELATION TO PHOTOASSIMILATE PATHWAYS AND PHLOEM LOADING IN THE BARLEY LEAF

Large, intermediate, and small bundles and contiguous tissues of the l eaf blade of Hordeum vulgare L. 'Morex' were examined with the transmi ssion electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the variou s cell combinations. Plasmodesmata are abundant at the mesophyll/paren chymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bun dles, plasmodesmata are also abundant between vascular parenchyma cell s, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cell s commonly separate the thick-walled sieve tubes from the sieve tube-c ompanion cell complexes. Plasmodesmatal frequencies between all remain ing cell combinations of the vascular tissues are very low, even betwe en the thin-wailed sieve tubes and their associated companion cells. B oth the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastica lly from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and larg e bundles indicate that they (and their associated companion cells, wh en present) are also isolated symplastically from the rest of the leaf . Collectively, these data indicate that both phloem loading and unloa ding in the barley leaf involve apoplastic mechanisms.