Evidence for the functional organization of chloroplasts in adaxial guard cells of Vicia faba leaves by single cell analysis

Abstract

The functional organization of chloroplasts in adaxial guard cells (GCs) of Vicia faba leaves was investigated by the saturation pulse method of chlorophyll fluorescence induction using single GC pairs. Quantitative imaging analysis of chlorophyll fluorescence performed on adaxial GCs upon dark–light transition revealed a large chlorophyll fluorescence transient. Adaxial GCs displayed PS II quantum yields and relative electron transport rates (ETR) comparable to the abaxial GCs. The slow chlorophyll fluorescence transients in the two cells provided further evidence for the activation of the Calvin cycle in GC chloroplasts on both sides of the leaf. The ETR upon increasing light intensity was, however, differentially expressed in the two cell types. When the rates were normalized to their maximum ETR activity, half-saturation was achieved at 76 and 45 μmol m −2 s −1 in adaxial and abaxial GCs, respectively. These results provide unequivocal evidence that the chloroplasts of adaxial GCs have a highly expressive functional organization and that the photosynthetic apparatus of adaxial GCs is well adapted to elevated light intensities.