Chlorophyll a Fluorescence Transients in Mesophyll and Guard Cells

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Abstract
Chlorophyll fluorescence transients from mesophyll and guard cell chloroplasts of variegated leaves from Chlorophytum comosum were compared using high resolution fluorescence spectroscopy. Like their mesophyll counterparts, guard cell chloroplasts showed the OPS fluorescence transient indicating the operation of the linear electron transport and the possible generation of NADPH in these organelles. They also showed a slow fluorescence yield decrease, equivalent to the MT transition in mesophyll, suggesting the formation of the high energy state and photophosphorylation. Unlike the mesophyll chloroplasts, the fluorescence from guard cell chloroplasts lacked the increment of the SM transition, indicating that the two types of chloroplasts have some metabolic differences. The presence of CO2 (supplied as bicarbonate, pH 6.7) specifically inhibited the MT-equivalent transition while its absence accelerated it. These observations constitute the first specific evidence of a guard cell chloroplast response to CO2. Control of photosynthetic ATP levels in the guard cell cytoplasm by CO2 may provide a mechanism regulating the availability of high energy equivalents at the guard cell plasmalemma, thus affecting stomatal opening.