Changes of Starch Synthetase Activity of Cucumber Leaves during Ammonium Toxicity

The changes of granule bound starch synthetase activity in cucumber leaves (Cucumis sativus L. cv. Suisei No. 2) were investigated during ammonium toxicity. Generally speaking the quantity of starch granules of injured plants were less than that of normal plants. ADPG is a more effective glucose donor to starch synthesis than UDPG. It was found that the starch synthetase activity of injured plants was decreased compared to the normal plants. This variation of enzyme activity was higher when UDPG was used as glucose donor. The addition of K⁺ and NH₄⁺ generally inhibited the enzyme activity when UDPG was used as glucose donor, but stimulated it when ADPG was used. This stimulation was found to be more effective in enzymes prepared from injured plants than from normal plants. The level of potassium bound to starch granules was not changed markedly between normal and injured plants.