Respiratory stress in cucumber roots treated with ammonium or nitrate nitrogen

Summary

Characteristic differences in growth and metabolic responses were observed following treatment of cucumber (Cucumis sativus, cv Suisei No. 2) roots with NH₄ ⁺ or NO₃ ⁻ as the sole source of N in solution culture. The most significant differences were:

• 1. Plant growth was more rapid with NO₃ ⁻ as the nitrogen source. The plants generally appeared healthier, and pH of the nutrient solution was more stable.

• 2. Free ammonia appeared in the tissues of roots grown in NH₄ ⁺ media within 3 hr after introduction of the ammonium ion. A rapid accumulation of amide-N and amino-N in root tissue followed within the next 3 h.

• 3. Characteristic symptoms of ‘rotten root’ appeared on plants growing in ammonium salts, with increasing severity as the volume of solution accessible to each plant diminished. ‘Rotten root’ symptoms were not observed on plants grown in 20 ppm nitrate-N regardless of the solution volume.

• 4. Root oxygen uptake was clearly higher in plants grown with NH₄ ⁺ ion than in those grown in NO₃ ⁻ ion.

• 5. Increased levels of mitochondrial enzyme activity were observed in NH₄ ⁺ grown roots. Malate dehydrogenase, NADH oxidase, succinate dehydrogenase, succinate: cytochrome c oxidoreductase, and NADH: cytochrome c oxidoreductase were all higher in the NH₄ ⁺ grown plants, even when under anoxic conditions. NADH diaphorase, however, was not significantly increased.