Venekamp, J, H, 1989, Regulation of cytosoi acidity in plants under conditions of drought, Plant, 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 photorespiratory 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 hberate H+ but NH4+, Assimilation of NHJ by cytosoiic 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 cytosoiic 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.

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Field bean plants, Vicia faba L., in the field were used to study which conditions determine decline in leaf water potential (ΨW) and coinciding proline accumulation. Cultivars Minica and Optica, having a short period of elongation growth, were used and the cultivars Alfred and Kristall, with a long elongation growth period. The plants were exposed to a “dry” and a “moist” treatment in a randomized block design. Leaf ΨW proline concentration and rate of elongation growth were measured weekly in June and July of both years. The measurements and samplings were conducted from 6 a.m. to 6 p.m. at 4‐hour intervals. Some leaf samples were selected for amino acid analysis. It was concluded that a drought‐induced proline accumulation only occurred when the plants were still in the phase of elongation growth. In this phase, a pre‐dawn leaf ΨW lower than ‐2 bars, combined with high temperatures during the night, were the main determinants for this accumulation. Weather conditions in day time were less decisive for proline accumulation at a certain leaf ΨW decline. The same applies to the diurnal rhythm in physiological patterns, the genetic properties of the plants, the rate of leaf ΨW decline and the amino acid metabolism. Proline accumulation increased when two or more determinants acted together.