Histochemical Approach to Properties of Vicia faba Guard Cell Phosphoenolpyruvate Carboxylase

Abstract

Properties of phosphoenolpyruvate carboxylase in guard cells dissected from frozen-dried V. faba L. leaflets were studied using quantitative histochemical techniques. Control experiments with palisade cells and whole leaflet extract proved that the single cell approach was valid. Most characteristics of enzyme activity in guard cells were identical to those in the leaflet extract. The activities were highly dependent on temperature, with maximum activity at 25-35.degree. C. Half-maximum activity (with 1 mmol phosphoenolpyruvate [PEP]) was observed at 0.1 mmol Mg2+. Two hundred mmol NaCl inhibited the reaction by 50%. With frozen-dried leaflet extract, the apparent Km(PEP) was 0.15 mmol at pH 7.7; with guard cells, the values were 1.49, 0.5-0.8 and 0.24 mmol in 3 successive experiments. Additional experiments showed that apparent Km(PEP) of guard cell activity from plants within a single growth lot was reproducible and did not change during stomatal opening. Mixed extract experiments proved that soluble compounds were not responsible for the difference observed between leaflet and guard cell activities. The differences in apparent Km(PEP) of guard cell activity could not be unambiguously interpreted. The physiological implications of the properties of this enzyme in guard cells are discussed.