Minor Veins of the Potato (Solanum tuberosum L.) Leaf: Ultrastructure and Plasmodesmatal Frequency

Abstract

The minor veins and surrounding chlorophyllous tissue of mature leaves of potato (Solanum tuberosum L.) were examined with the electron microscope to determine the ultrastructural characteristics of the various cell types and the distribution and frequency of plasmodesmata between them. The abaxial phloem of the sixth- and seventh-order veins is characterized by a regular arrangement of cells such that each sieve-tube member is in contact with at least two companion cells. The abaxial phloem of the fifth-order veins has a variable cell arrangement, but each sieve-tube member is also in contact with at least two companion cells. The adaxial phloem of the fifth- and sixth-order veins also has a variable cell arrangement, but each sieve-tube member commonly is in contact with only one companion cell. The companion cells and phloem parenchyma cells of the abaxial phloem are quite distinct from one another cytologically; in contrast, these two cell types in the adaxial phloem are often difficult to distinguish from one another. Whereas the abaxial phloem of fifth- and sixth-order veins consists entirely of living cells, the adaxial phloem of the same vein orders commonly contains dead sieve-tube members and companion cells. Plasmodesmata occur along all possible routes from the mesophyll to the sieve-tube members of the minor veins, although the sieve tubes of the adaxial phloem may be more isolated symplastically than those of the abaxial phloem. Statistical analyses of the plasmodesmatal frequencies show that a preferential pathway for photosynthate transport, based on frequency alone, does not exist in the mesophyll, but that significant differences occur between the plasmodesmatal frequencies of cell combinations in the veins, perhaps indicative of an apoplastic step of vein loading.