Influence of Abscisic Acid on Nitrate Accumulation and Nitrate Reductase Activity in Potato Tuber Slices

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Abstract
Abscisic acid (ABA) at concentrations of 1 to 10 μg.ml−1 suppressed development of nitrate reductase activity in fresh tuber slices of Solanum tuberosum L. incubated in KNO3. Suppression of activity was evident after 3 hr and continued for 20 hr before recovery. This recovery may be due to inactivation of the hormone. Nitrate accumulation was enhanced by ABA. At exogenous NO3 levels of 0.1 to 5 mM, the hormone enhanced both NO3 accumulation and nitrate reductase activity. When applied 24 hr after incubation in NO3, ABA promoted a marked decline in enzyme activity in the absence of exogenous NO3, but was less effective in the presence of NO3. Slices incubated in NO3 and ABA also exhibited increased loss of enzyme activity upon removal of NO3. Preincubating slices in the hormone for 24 hr in a NO3- free medium resulted in stimulation of nitrate reductase activity. Addition of NO3 resulted in a marked stimulation of enzyme activity over a period of 8–10 hr. The ABA response is not related to tissue levels of free amino acids and is not affected by different NO3 sources. These results suggest the ABA effect on nitrate reductase activity is influenced by NO3 status of the cells. Where external NO3 levels are low it stimulated NRA while it inhibited activity where NO3 content is high.