Changes of Sugar Levels in Cucumber Leaves during Ammonium Toxicity

Toxic effects of high concentrations of ammonia were studied in the cucumber plant (Cucumis sativus cv. Suisei No. 2). When the cucumber plant was cultured with 200 mg/l NH₃-N (as NH₄Cl), some characteristic symptoms, probably due to ammonium toxicity, appeared in the leaves after about 1 week, while no such symptoms were observed in the plants cultured with 20 mg/1 NH₃-N. The level of free sugars in 20 mg/l NH₃-N treated plants decreased with time and was lower than that in plants treated with 200 mg/l NH₃-N. Specially distinct differences were found as regards the levels of fructose and glucose. After 9 days' culture the content of glucose in 200 mg/l NH₃-N plants was 17 times higher than that of 20 mg/l NH₃-N plants. From the results of an incorporation of photosynthesized ¹⁴CO₂ for 3 hours into newly synthesized glucose it is evident that this accumulation of glucose can not be the degradative product of a glucose polymer such as starch. The levels of starch were also studied, and it was found that the starch level decreased due to ammonium toxicity. These results suggest that the translocation of glucose after its synthesis is inhibited by ammonium toxicity, at least up to starch synthesis.