The C4 grass Arundinella hirta is characterized by unusual leaf blade anatomy: photosynthetic carbon reduction takes place both within the chlorenchymatous bundle sheath cells of the longitudinal veins and within longitudinal strands of "distinctive cells" that form part of the leaf mesophyll and are often completely isolated from vascular tissue. Although they are equivalent physiologically, these two cell types have different ontogenetic origins: bundle sheath cells are delimited from procambium early in leaf development, whereas distinctive cells differentiate from ground meristem at a later developmental stage. Although the two cell types share numerous cytological features (large chloroplasts with reduced grana, thick cell walls with a suberin lamella), we also found significant differences in cell lengths, length to width ratios, cell cross-sectional areas, organelle numbers per cell cross section, phenol content of the cell walls, and numbers of pit fields in the longitudinal cell walls. The size and shape of bundle sheath cells are likely a direct consequence of procambial origin. The thicker walls of bundle sheath cells (in major veins) and their greater lignification may reflect the inductive effect of cell differentiation in the proximity of sclerenchyma and vascular tissues. Differences between major and minor vein bundle sheath cells may reflect differences in the timing of initiation of procambial strands. Our analysis of cell wall characteristics has also shown the presence of numerous primary pit fields in the transverse walls between adjacent distinctive cells in a file; plasmodesmata in these pit fields form a pathway for longitudinal symplastic transport not previously known to exist.
Photosynthetic Carbon Metabolism in Isolated Maize Bundle Sheath Strands