This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999
The distance between sites of synthesis of assimilates and the site of phloem
loading can be large, and specialized leaf cell layers such as the paraveinal
mesophyll (PVM) might act to enhance the efficiency of transport. A number of
techniques were used to analyse PVM of legume leaves with respect to a
hypothesized function in transfer of assimilates between tissues. Of 39 legume
species examined, PVM was found in 22. Leaves of all PVM-containing species
had multiple palisade parenchyma layers, while non-PVM species generally had
only one distinct palisade layer. Morphometric analysis identified a
significant correlation between PVM presence and greater numbers of palisade
cells per unit leaf surface area. Comparison of photosynthetic rates of four
PVM and four non-PVM species showed the PVM species had higher rates on a leaf
area basis than all but one of the non-PVM species. Microautoradiography of
14CO2 pulse–chase studies in
soybean demonstrated PVM is an intermediary tissue in transfer of assimilates
to vascular bundles. In addition, PVM cells but not mesophyll cells, were
enriched in a sucrose binding protein previously found to be associated with
sucrose-transporting tissues. The structural, positional and transport data
support the hypothesis that the PVM acts as a transport pathway between the
vascular system and photoassimilatory cells of the leaf, and has probably
evolved to overcome diffusion limitations imposed by multiple palisade layers.