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

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Abstract
Leaf anatomy, particularly vascular organization, of the grass species Arundinella hirta is atypical for a C4 NADP – malic enzyme species. Most conspicuously, interveinal distances are large, more similar to those of C3 than C4 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 C4 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 C4 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 C3 and C4 grass species, the wide vein spacing potentially creates limitations on the rate of loading photosynthate into vein phloem that are not experienced by other C4 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 14C-labelled photosynthate in this atypical species.