Functional Analysis of Corn Husk Photosynthesis
2011
Pengelly, Jasper J.L. | Kwasny, Scott | Bala, Soumi | Evans, John R. | Voznesenskaya, Elena V. | Koteyeva, Nuria K. | Edwards, Gerald E. | Furbank, Robert T. | Von Caemmerer, S (Susanna)
The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C₃ photosynthetic pathway, in contrast to its leaves, which use the C₄ photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO₂ compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO₂ assimilation rate in the husk was less than that in the leaves and did not saturate at high CO₂, indicating CO₂ diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO₂ compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C₄-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C₄ photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.
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