Photosynthesis-Related Functions of Vasculature-Associated Chlorenchymatous Cells

Abstract

In most biochemical, molecular, and genetic studies, a leaf is regarded as a uniformly responding unit, however leaves are not homogeneous in structure and function. Leaf venation is in continuity with the vascular system within leaf petiols and stems. Leaf veins are typically encircled by bundle sheath (BS) cells containing chloroplasts and photosynthetic cells adjacent to the vasculature are also found in petiols and stems. In C3 plants, BS cells have been shown to be preadapted for the role in C4 photosynthesis and this may explain the polyphyletic evolution of C4 photosynthesis. The photosynthetically active radiation (400–700 nm) reaching the chloroplast-containing cells adjacent to the vasculature in leaves, petiols, and stems is of lower intensity and enriched with longer wavelengths (~500–700 nm) when compared with that absorbed by mesophyll cells. The CO2 diffusion from the air to the vasculature-adjacent chlorenchymatous cells is also expected to be slow in comparison to mesophyll cells. However, the vasculature can be supplied with malate which releases CO2 after decarboxylation and with respiratory CO2 from heterotrophic tissues transported in the xylem. It could be expected that high CO2 concentration at the green cells around the vasculature supports carboxylation and photosynthesis. However, CO2-rich environment in stems impedes the photochemical activity of the photosynthetic vascular cells possibly through acidification of protoplasm and impairment of the pH-dependent excess energy quenching followed by reduction in the efficiency of heat dissipation. Light-dependent reduction in CO2 release, as shown in experiments on stems can predominantly be attributed to corticular refixation. All these can affect chloroplast ultrastructure, the composition of photosynthetic electron transport chain components, and the photosynthetic enzymatic machinery in these cells.