Foliar phloem infrastructure in support of photosynthesis.

William W Adams,Christopher M Cohu,Barbara Demmig-Adams,Onno Muller

doi:10.3389/fpls.2013.00194

Abstract

Acclimatory adjustments of foliar minor loading veins in response to growth at different temperatures and light intensities are evaluated. These adjustments are related to their role in providing infrastructure for the export of photosynthetic products as a prerequisite for full acclimation of photosynthesis to the respective environmental conditions. Among winter-active apoplastic loaders, higher photosynthesis rates were associated with greater numbers of sieve elements per minor vein as well as an increased apparent total membrane area of cells involved in phloem loading (greater numbers of cells and/or greater cell wall invaginations). Among summer-active apoplastic loaders, higher photosynthesis rates were associated with increased vein density and, possibly, a greater number of sieve elements and companion cells per minor vein. Among symplastic loaders, minor loading vein architecture (number per vein and arrangement of cells) was apparently constrained, but higher photosynthesis rates were associated with higher foliar vein densities and larger intermediary cells (presumably providing a greater volume for enzymes involved in active raffinose sugar synthesis). Winter-active apoplastic loaders thus apparently place emphasis on adjustments of cell membrane area (presumably available for transport proteins active in loading of minor veins), while symplastic loaders apparently place emphasis on increasing the volume of cells in which their active loading step takes place. Presumably to accommodate a greater flux of photosynthate through the foliar veins, winter-active apoplastic loaders also have a higher number of sieve elements per minor loading vein, whereas symplastic loaders and summer-active apoplastic loaders have a higher total number of veins per leaf area. These latter adjustments in the vasculature (during leaf development) may also apply to the xylem (via greater numbers of tracheids per vein and/or greater vein density per leaf area) serving to increase water flux to mesophyll tissues in support of high rates of transpiration typically associated with high rates of photosynthesis.

Highlights

Foliar vasculature, as the network of tissue linking the leaf ’s chloroplast-containing mesophyll cells to the rest of the plant, should be expected to display some concomitant adjustments to those of photosynthesis during leaf development
For apoplastic loaders, which rely on membrane-spanning proteins [adenosine triphosphatases (ATPases) to actively pump protons into the cell wall space and, e.g., sucrose-H+ co-transporters to move sucrose from the apoplast into the phloem], a greater total membrane area of cells engaged in active loading of sugars or sugar alcohols could be achieved via greater cell wall invagination, greater cell size, or greater numbers of cells involved in loading
For the winter annuals A. thaliana and spinach, growth under lower temperature resulted in a greater number of sieve elements, companion cells, and phloem parenchyma cells in the minor loading veins compared to growth at warm temperature, as well as a higher intrinsic capacity of photosynthesis

Summary

Introduction

As the network of tissue linking the leaf ’s chloroplast-containing mesophyll cells to the rest of the plant, should be expected to display some concomitant adjustments to those of photosynthesis during leaf development.

Results

Conclusion