Phosphoenolpyruvate carboxylase from spinach leaf tissue: inhibition by sulfite ion

Respiration and gas exchange in the light were studied manometrically with tissue slices from stem material of Opuntia basilaris Engelm. and Bigel. Dark respiration rates were greater in young stems than in mature stems. The timing of the experiment in the day/night cycle influences the magnitude and pattern of respiration and gas exchange in the light. Net dark respiration has a temperature optimum between 35 and 40 C, and is maintained at 60% of the control rate in tissue equilibrated with experimental osmotic potentials of −25 bars. Net gas exchange in the light is regulated by the titratable acidity of the tissue and by the tissue temperature. Increased rates of net CO₂ evolution and net O₂ consumption occur in the light with high levels of titratable acidity and high temperatures. An efflux of CO₂ and influx of O₂ occur following light/dark transitions. These patterns are reversed following dark/light transitions. Similar results were demonstrated at 15, 25, and 35 C, and are interpreted as a mechanism of adaptation to desert environments.