075N.HTML???????? x//U Kranz Anatomy

 

Grass leaf with Kranz anatomy: Sugar cane

Single bundle sheath

Sugar cane (Saccharum officinarum), a C4 plant. In this grass Kranz palisade cells are apparently shorter than in amaranth and orach, and the bundle sheath cells (wreath) are smaller and more numerous. Bundle sheaths consist of one layer of parenchyma cells.

There is a two-stage system of carbon fixation in C4 plants. The first stage is a temporary fixation of CO2 in the cytoplasm of mesophyll cells (without the direct involvement of chloroplasts) to form oxaloacetic acid which moves into the mesophyll chloroplasts and is converted into malic or aspartic acid, both of which are 4-carbon molecules (hence the "C4" acronym). These acids move through plasmodesmata to the bundle sheath cells where they are broken down to yield CO2 again.

The second stage is carbon dioxide dissociation in the relatively oxygen-impoverished environment of the bundle sheath. The inner chloroplasts permanently fix the CO2 into traditional C3 intermediates without stimulating photorespiration. Only the mesophyll plastids generate O2, whereas carbon fixation is spatially separated into a different tissue.

Identify: xylem vessels, phloem, bundle sheath, Kranz palisade mesophyll, water-storage cells, stomata and trichomes.

 
   
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