The Effect of Abscisic Acid on Amino Nitrogen and Protein Content of Potato Plants in Relation to the Inhibition of Nitrate Reductase Activity

Treatment of potato plants grown in nutrient solution with 3.8µM ABA resulted in reduced soluble protein in roots andin leaves at 24 h, but not in stems. This treatment reducedin vivo nitrate reductase activity in all organs for about 48h with the most pronounced reduction occurring in the roots.Excised root and leaf segments from plants treated with ABAfor 24, 48 and 72 h absorbed significantly more ¹⁴C leucine,compared to the control but the percent incorporation into proteinwas not altered in roots. In response to ABA total free amino nitrogen in leaves was lowerat 5 and 72 h and in stems at 72 h. Amino nitrogen content ofroots was enhanced by ABA at 5, 24 and 72 h due to generallyhigher levels of aspartate, serine, glutamate, proline and ammonia.There was no consistent relationship between ABA suppressionof nitrate reductase activity and ammonia or specific aminoacid (except proline) levels in leaves and stems. The increasedfree amino nitrogen levels in response to the hormone may bethe result of impaired NO₃– reduction rather than thecause. The results of protein synthesis studies and solubleprotein content suggest that ABA inhibition of nitrate reductaseis not due to general inhibition of protein synthesis and mayinvolve specific inhibition of nitrate reductase protein synthesis. ¹ Contribution No. 684, Department of Plant Science, Universityof Manitoba.