Organic Acids as Sources for Drought-induced Proline Synthesis in Field Bean Plants, Vicia faba L.

Abstract

Summary The involvement of organic acids in proline synthesis was studied using different parts from 6-week-old field bean plants after treatment for one day of withholding water and/or light. Water stress induced proline accumulation; N came from nitrate and the C skeleton from organic acids. Loss of nitrate and carboxylates was compensated by synthesis of organic acids of oxidative respiration (citrate, malate, and lactate) to maintain electroneutrality. The concentrations of proline as well as the concentrations of organic acids in parts of water-stressed plants were multiples of corresponding concentrations in plant parts under normal conditions of water supply and light exposure. A functional relation existed between multiples of proline and corresponding multiples of organic acids of the oxidative respiration; the correlation coefficient was 0.8. Increases in concentration of proline and of the above-mentioned acids were due to water loss and increased synthesis; all other drought-induced increases in organic-acid concentrations were a consequence of water loss only. Withholding of light did not prevent proline synthesis in water-stressed plants; ammonia assimilation, which is closely linked to photorespiratory acids (glycerate and glycollate), could not be involved in proline synthesis. It is reasoned that organic acids of oxidative respiration are the direct sources of proline accumulation in plants under conditions of a water deficit. This proline synthesis occurs via glutamate dehydrogenase and is an efficient metabolic mechanism to remove an excess of H + ions.