Sporobolus virginicus leaf salt glands: morphology and ultrastructure

The leaves of the halophytic, C-4 grass, Sporobolus virginicus, possess bicellular salt glands that are more abundant on the adaxial surface. Each gland is composed of a voluminous, flask-shaped basal cell, embedded in the leaf and a smaller, dome-shaped cap cell that protrudes beyond the epidermis. An ornamented, porose cuticle overlies the protruding gland. The dense cytoplasm of the basal cell is partitioned into channels by a system of paired membranes which originate from invaginations of the plasma membrane, adjacent to the common wall between basal and cap cells. Closely associated with the partitioning membranes are numerous mitochondria, microtubules and endoplasmic reticulum. The basal cell is intimately connected to adjacent cap, epidermal and mesophyll cells by numerous plasmodesmata. The dense cytoplasm of the cap cell lacks partitioning membranes, but contains numerous small vacuoles and a concentration of organelles in close proximity to the outer surface. The basal cell appears to be suitably located and designed to access and direct ions from surrounding mesophyll and epidermal cells into the channels formed by the partitioning membranes. Subsequent ion movement is probably symplastic via the cytoplasm and plasmodesmata to the cap cell. The abundant mitochondria, which are closely associated with the partitioning membranes, are probably important in ion transport through the cytoplasm of the basal cell. The ions appear to be compartmentalised and transported across the cap cell in small vacuoles and accumulate in the cuticular cavity prior to elimination via cuticular pores or through rupture of the cuticle.