The Effect of Abscisic Acid on Amino Nitrogen and Protein Content of Potato Plants in Relation to the Inhibition of Nitrate Reductase Activity1

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Abstract
Treatment of potato plants grown in nutrient solution with 3.8 μM ABA resulted in reduced soluble protein in roots and in leaves at 24 h, but not in stems. This treatment reduced in vivo nitrate reductase activity in all organs for about 48 h with the most pronounced reduction occurring in the roots. Excised root and leaf segments from plants treated with ABA for 24, 48 and 72 h absorbed significantly more 14C leucine, compared to the control but the percent incorporation into protein was not altered in roots. In response to ABA total free amino nitrogen in leaves was lower at 5 and 72 h and in stems at 72 h. Amino nitrogen content of roots was enhanced by ABA at 5, 24 and 72 h due to generally higher levels of aspartate, serine, glutamate, proline and ammonia. There was no consistent relationship between ABA suppression of nitrate reductase activity and ammonia or specific amino acid (except proline) levels in leaves and stems. The increased free amino nitrogen levels in response to the hormone may be the result of impaired NO3− reduction rather than the cause. The results of protein synthesis studies and soluble protein content suggest that ABA inhibition of nitrate reductase is not due to general inhibition of protein synthesis and may involve specific inhibition of nitrate reductase protein synthesis.