Regulation of cytosol acidity in plants under conditions of drought

In plants under water stress, the activity of photosynthesis declines most. Stimulation of the oxidative respiration and fermentation results in an increase in the amount of related organic acids: citrate, malate and lactate. In spite of some decline in photo‐respiratory activity, dehydration may enhance the concentration of related organic acids, glycerate and glycolate. The resulting amount of H+ should stimulate NAD(P)H reduction of organic acids by dehydrogenases. Accumulation of proline could be the consequence of such reactions. In the oxidation of glycine, regeneration of NAD(P)H does not liberate H+ but NH4+. Assimilation of NH4+ by cytosolic glutamine synthetase (EC 6.3.1.2) results in positively charged glutamine. It is also conceivable that the charge is essential in the final asparagine synthesis by cytosolic asparagine synthetase (EC 6.3.1.1). At low pH the activity of the oxidative respiration declines. In water‐stressed plants, maintenance of oxidative respiration will depend on the availability of sufficient amounts of carbohydrates and on adequate removal of excess H+ by accumulation of proline and asparagine.