PHOTOSYNTHESIS BY GUARD-CELL CHLOROPLASTS OF VICIA-FABA L - EFFECTS OF FACTORS ASSOCIATED WITH STOMATAL MOVEMENT

The properties of photosynthetic O-2 evolution by mesophyll cell chlor oplasts (MCC) and guard cell chloroplasts (GCC) isolated from protopla sts of Vicia faba L. have been studied and effects on O-2 evolution of factors known to regulate stomatal movements have been compared. The O-2 evolution of GCC was CO2-dependent. The saturating light intensity for O-2 evolution was between 150 and 200 mu mol m(-2)s(-1) for MCC a nd was between 400 and 1,000 mu mol m(-2)s(-1) for GCC. Light quality (red vs. blue) had no significant effect on O-2 evolution by either MC C or GCC. The O-2 evolution rate of MCC was strongly dependent on exte rnal K+ concentration, but GCC did not respond significantly to variat ions in external K+ concentration between 0 and 250 mM. The optimal ex ternal pH for O-2 evolution by MCC was approximately 7.5, and either h igher or lower external pH significantly inhibited O-2 evolution. Howe ver, O-2 evolution by GCC was only slightly enhanced when external pH was increased from 6.0 to 8.0. Our observation of differential sensiti vity of MCC and GCC to light intensity and to variations of cytoplasmi c K+ and pH may indicate differential regulation of photosynthesis in MCC and GCC.