High-resolution images of the chlorophyll fluorescence parameter F q Ј/F m Ј from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. F q Ј/F m Ј estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic

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... synthetic electron transport. Over a range of light intensities, values of F q Ј/F m Ј were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of F q Ј/F m Ј of guard and mesophyll cells to changes of CO 2 and O 2 concentration were very similar. There were similar reductions of F q Ј/F m Ј of guard and mesophyll cells over a wide range of CO 2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, F q Ј/F m Ј of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO 2 was low, but no change when ambient CO 2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO 2 concentration. fax 44 -1206 -873416. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/