Cell specialization within the parenchymatous bundle sheath of barley

Structural and physiological aspects of the parenchymatous bundle sheath (PBS) were studied in cultivars of Hordeum distichum L. The PBS of intermediate, lateral and midrib veins consisted of a single layer of cells closely appressed to the mestome sheath. These cells were large, vacuolate and approximately cylindrical in shape, extending parallel to the vein. Mean PBS cell volume was 4 × 10 −5 mm 3 compared to 1.23 × 10 −5 mm 3 for mesophyll cells. Transverse sections revealed three cell types within the PBS, cells with small chloroplasts (S‐type), cells with large chloroplasts (L‐type) and structural cells. The majority of cells were S‐type, containing chloroplasts of approximately a third of the volume of mesophyll chloroplasts; they were able to reduce tetranitro blue‐tetrazolium and synthesize starch. Structural cells interrupted the phloem and xylem are of the sheath in lateral veins and the midrib, whilst between one and four PBS cells within the phloem are of each vein type contained chloroplasts similar in volume and starch content to those of the mesophyll. Only these L‐type cells contained noticeable starch grains at the end of an 8‐h dark period, a further 4 h darkness being required for complete mobilization of starch. Starch deposition within S‐type and structural cells was detectable after 4 h illumination but was only appreciable in leaves excised from the plant and illuminated for 9–12 h. The role of S‐type PBS cells in assimilate transport is discussed in relation to these findings.