Lessened malate inhibition of guard-cell phosphoenolpyruvate carboxylase velocity during stomatal opening.

Leaflets of Vicia faba with closed stomata or with opening stomata were freeze-dried. Excised guard-cell pairs were assayed individually under suboptimal conditions (pH 7.1 and subsaturating substrate) for phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) using quantitative histochemical procedures. L-Malate, 400 microM, significantly inhibited guard-cell PEPC activity of closed stomata but not that of opening stomata. We postulate that the lessened sensitivity of guard-cell PEPC activity to malate inhibition is an important regulatory feature of stomatal opening, which is associated with malate accumulation.