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High resolution chlorophyll a ¯uorescence imaging was used to compare the photosynthetic ef®ciency of PSII electron transport (estimated by Fq¢/Fm¢) in guard cell chloroplasts and the underlying mesophyll in intact leaves of six different species: Commelina communis, Vicia faba, Amaranthus caudatus, Polypodium vulgare, Nicotiana tabacum, and Tradescantia albifora. While photosynthetic ef®ciency varied between the species, the ef®ciencies of guard cells and mesophyll cells were always closely matched. As measurement light intensity was increased, guard cells from the lower leaf surfaces of C. communis and V. faba showed larger reductions in photosynthetic ef®ciency than those from the upper surfaces. In these two species, guard cell photosynthetic ef®ciency responded similarly to that of the mesophyll when either light intensity or CO2 concentration during either measurement or growth was changed. In all six species, reducing the O2 concentration from 21% to 2% reduced guard cell photosynthetic ef®ciency, even for the C4 species A. caudatus, although the mesophyll of the C4 species did not show any O2 modulation of photosynthetic ef®ciency. This suggests that Rubisco activity is signi®cant in the guard cells of these six species. When C. communis plants were water-stressed, the guard cell photosynthetic ef®ciency declined in parallel with that of the mesophyll. It was concluded that the photosynthetic ef®ciency in guard cells is determined by the same factors that determine it in the mesophyll.
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Palisade cells are plant cells located within the mesophyll in leaves, right below the upper epidermis and cuticle. They are vertically elongated, a different shape from the spongymesophyll cells beneath them in the leaf. Their chloroplasts absorb a major portion of the light energy used by the leaf.
Guard cells are highly specialized cells that form tiny pores called stomata on the leaf surface. The opening and closing of stomata control leaf gas exchange and water transpiration as well as allow plants to quickly respond and adjust to new environmental conditions. Mesophyll cells are specialized for photosynthesis. Despite the phenotypic and obvious functional differences between the two types of cells, the full protein components and their functions have not been explored but are addressed here through a global comparative proteomics analysis of purified guard cells and mesophyll cells. With the use of isobaric tags for relative and absolute quantification (iTRAQ) tagging and two-dimensional liquid chromatography mass spectrometry.

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