Leaf vascular architecture in the atypical C₄ NADP – malic enzyme grass Arundinella hirta

Leaf anatomy, particularly vascular organization, of the grass species Arundinella hirta is atypical for a C 4 NADP – malic enzyme species. Most conspicuously, interveinal distances are large, more similar to those of C 3 than C 4 species. When the longitudinal files of the bundle-sheath-like distinctive cells are included with vein counts, interveinal distance falls within the range for the C 4 NADP – malic enzyme biochemical type. A quantitative analysis of the lengths of distinctive cell files shows that ca. 22% lack any contact with transverse veins or other vascular tissue. This reinforces the idea that the functional significance of typical C 4 anatomy is the spatial relationship between bundle sheath tissue and mesophyll. While the general features of blade and sheath vascular architecture of A. hirta are shared with other C 3 and C 4 grass species, the wide vein spacing potentially creates limitations on the rate of loading photosynthate into vein phloem that are not experienced by other C 4 species. Measurements of phloem cross-sectional area per vein and total phloem cross-sectional area give no indication of a constriction in conductive capacity within the leaf blade or sheath. These observations provide an essential background for studies of short- and long-distance translocation of 14 C-labelled photosynthate in this atypical species.