Leaf Ultrastructure of Triodia irritans: a C₄ Grass Possessing an Unusual Arrangement of Photosynthetic Tissues

Abstract

Light, transmission electron and scanning electron microscopy have been used to investigate the ultrastructural arrangement of leaf tissue in Triodia irritans, a C₄ grass adapted to a hot, dry environment. Photosynthetic tissue was found to constitute 25% of the cross-sectional area of the leaf and includes mesophyll and bundle sheath cell types. The mesophyll is restricted to discrete areas and consists of long files of cells interconnected by a complex network of cytoplasm-filled cross bridges. It is in contact with the vascular tissue via extensions from the bundle sheath, which make intimate contact with it by way of plasmodesmata located in frequent pit fields. The mesophyll cytoplasm is largely non-vacuolate, but in other respects is similar to that found in most other species-viz., contains chloroplasts, mitochondria and occasional microbodies. Cells of the bundle sheath extensions exhibit a uniform to slightly centripetal distribution of cytoplasm, consisting mainly of grana-containing chloroplasts, mitochondria possessing elongate cristae, and microbodies. Cells of the bundle sheath proper, which partially encircles the vascular tissue, do not make direct contact with the mesophyll and contain fewer chloroplasts than the extension cells.

The arrangement of photosynthetic and of vascular tissues are discussed in terms of the classical Kranz anatomy that is a feature of C₄ plants, and the possible relationships between leaf structure, photosynthetic activity and the translocation of photosynthate are considered.