LESSENED MALATE INHIBITION OF GUARD-CELL PHOSPHOENOLPYRUVATE CARBOXYLASE VELOCITY DURING STOMATAL OPENING

Leaflets of Vicia faba with closed stomata or with opening stomata wer e freeze-dried. Excised guard-cell pairs were assayed individually und er suboptimal conditions (pH 7.1 and subsaturating substrate) for phos phoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) using quantitative his tochemical procedures. L-Malate, 400 mu M, significantly inhibited gua rd-cell PEPC activity of closed stomata but not that of opening stomat a. We postulate that the lessened sensitivity of guard-cell PEPC activ ity to malate inhibition is an important regulatory feature of stomata l opening, which is associated with malate accumulation.