PEP carboxylase from Commelina communis leaf epidermis was characterized. Activity was optimal at concentrations of 2 to 3 mM bicarbonate and 5 mM PEP at pH 8·3. The temperature optimum was between 35 and 40°C and the activation energy was 9·1 kcal mol-1. At saturating substrate concentrations and at pH 8·3, malate inhibited PEP carboxylase activity; slight inhibition occurred at 2·0 mM (the lowest concentration tested) while 13·5 mM caused 50% inhibition. At saturating substrate concentrations KC1 and mannitol, both up to concentrations of 250 mM, had no significant inhibitory effect upon PEP carboxylase activity although, thereafter, marked inhibition occurred in the KC1 medium (concentrations of mannitol higher than 250 mM were not tested). 10 -4 M ABA had no effect on PEP carboxylase activity from Tulipa gesneriana leaf epidermis.

The effects of a number of factors were tested both on PEP carboxylase activity and also on stomatal opening in epidermal strips of C. communis. After 1 h, maximum apertures and maximum opening rates occurred at 35°C. The activation energy for stomatal opening was 9·3 kcal mol -1. The effects of malate on stomatal opening in C. communis epidermis were complex: At pH 4·1 to 4·3, 10 mM monosodium malate saturated the opening response; widest openings also occurred at pH 4·0 with no opening at pH 2·7 and some opening at pH 5·0, 6·0 and 7·0.

PEP carboxylase from C. communis epidermal tissue behaved, in most respects, very similarly to PEP carboxylase from C4 leaves. Correlations between the effects of the factors on PEP carboxylase activity and stomatal opening were difficult to make. However, temperature effects were remarkably similar. The effects of osmotica on PEP carboxylase activity suggested that inhibition due to high K+, Cl- or other osmoticum in the guard cell cytoplasm would be negligible. It was also concluded that ABA does not control stomatal movements via its effect on PEP carboxylase.