THE INTERMEDIARY CELL - MINOR-VEIN ANATOMY AND RAFFINOSE OLIGOSACCHARIDE SYNTHESIS IN THE SCROPHULARIACEAE

Minor-vein anatomy, sugar content, sugar synthesis, and translocation were studied in mature leaves of nine members of the Scrophulariaceae to determine if there is a correlation between companion-cell type and class of sugar translocated. Three types of companion cell were found : intermediary cells with extensive plasmodesmatal connections to the bundle sheath; transfer cells with wall ingrowths and few plasmodesmat a; and 'ordinary' companion cells with few plasmodesmata and no wall i ngrowths. Alonsoa warscewiczii Regal., Verbascum chaixi Vill., and Mim ulus cardinalis Dougl. ex. Benth. have intermediary cells and ordinary companion cells in the minor veins. These plants synthesize large amo unts of raffinose and stachyose as well as sucrose. Nemesia strumosa B enth., and Rhodochiton atrosanguineum Zucc. have both intermediary cel ls and transfer cells and make proportionately less raffinose oligosac charide than the species above. In N. strumosa, a single sieve element may abut both an intermediary cell and a transfer cell. The minor vei ns of Asarina scandens (Cav.) Penn. have transfer cells and what appea r to be modified intermediary cells that have fewer plasmodesmata than other species, and occasional wall ingrowths. Asarina scandens synthe sizes little raffinose or stachyose. Cymbalaria muralis P. Gaertn et a l. and Linaria maroccana Hook.f. have only transfer cells and Digitali s grandiflora Mill. has only ordinary companion cells; these species m ake a trace of galactinol and raffinose, but no stachyose. Translocati on experiments indicate that there is long-distance movement of raffin ose oligosaccharide in these plants, even when it is synthesized in ve ry small quantities in the leaves. We conclude that intermediary cells are as distinct a cell type as the transfer cell. In contrast to tran sfer cells, which are specialized for uptake of solute from the apopla st, intermediary cells are specialized for symplastic transfer of phot oassimilate from the mesophyll and for synthesis of raffinose oligosac charide. This supports our contention that raffinose oligosaccharide s ynthesis and symplastic phloem loading are mechanistically linked (Tur geon and Gowan 1990, Plant Physiol. 94, 1244-1249). Minor-vein anatomy and sugar synthesis may be useful characters in determining the phylo genetic relationships of plants in this family.