Photosynthetic Properties of Guard Cell Protoplasts from Vicia faba L.

Guard cell protoplasts were isolated enzymatically from the epidermis of Vicia faba L. and their photosynthetic activities were investigated. Time courses of light-induced changes in the chlorophyll a fluorescence intensity of these protoplasts showed essentially the same induction kinetics as found for mesophyll protoplasts of Vicia. The transient change in the fluorescence intensity was affected by DCMU, an inhibitor of photosystem II; by phenylmercuric acetate, an inhibitor of ferredoxin and ferredoxin NADP reductase; and by methyl viologen, an acceptor of photosystem I. Low temperature (77 K) emission spectra of the protoplasts had peaks at 684 and 735 nm and a shoulder near 695 nm. A high O2 uptake (175 μmol mg−1 Chl hr−1) was observed in guard cell protoplasts kept in darkness, which was inhibited by 2 mM KCN or NaN3 by about 60%. On illumination, this O2 uptake was partially or completely suppressed, but its suppression was removed by DCMU, which indicates that oxygen was evolved (150 μmol mg−1 Chl hr−1) photosynthetically. We concluded that both photosystems I and II function in guard cell chloroplasts and that these protoplasts have high respiratory activity.