The Suppression of Starch Synthesis and the Accumulation of Uridine Diphosphoglucose in Cucumber Leaves due to Ammonium Toxicity.

This work is a study of the metabolic disorder due to ammonium toxicity in cucumber (Cucumis sativus L. cv. suisei No. 2) leaves. The cucumber was cultured with 20 and 200 mg/1 NH₃-N for 5 days. In the first half of this experiment, the plant leaves were photosynthesized for 3 hours to study the distribution of ¹⁴C in starch and other fractions. The incorporation of photosynthesized ¹⁴C into not only starch but also other higher polymers was suppressed by ammonium toxicity. On the other hand, the rate of ¹⁴C incorporation in 80 % ethanol soluble fraction was higher in the treatment of 200 mg/1 NH₃-N. In the latter half of experiment, phosphate esters in the leaves were analyzed by column chromatography. The opposing contents of uridine diphosphoglucose (UDPG) and the uridine compound containing sugar or sugar derivative (UDPX) were determined by different ammonia levels. Toxicity of ammonia resulted in an increase of UDPG and a decrease of UDPX. The level of ATP was not changed so much. The content of glucose-6-phosphate (G-6-P) in injured plants was lower than that of normal plants, while glucose-1- phosphate (G-l-P) and fructose-6-phosphate (F-6-P) were higher in injured plants. These results further suggested the disorder of carbohydrate metabolism due lo ammonium toxicity which was reported previously. [ABSTRACT FROM AUTHOR]