Loading and transport of assimilates in different maize leaf bundles : Digital image analysis of...

The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of (14)CO2. The concentrations of (14)C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of (14)C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of (14)C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained (14)C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more (14)C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were (14)C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport (14)C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.