Promotion of Assimilation of Ammonium Ions by Simultaneous Application of Nitrate and Ammonium Ions in Radish Plants

摘要

When radish plants were grown in nutrient solutions that contained ammonium ions (NH4+) as the sole source of nitrogen, they grew poorly and accumulated high levels of NH4+in their leaves. However, radish plants cultured in 5 mM NH4+plus 1 mM NO3−(a ratio of 5 : 1 in forms of nitrogen; referred to as 5:lmix-N) grew well and accumulated very low levels of NH4+in their leaves. After radish plants were cultured in solutions that contained NO3−, or NH4+, or 5: lmix-N for a week, they were then supplied with the same nitrogen source labeled with15N for one day. The uptake of15N from labeled NH4+into total nitrogen was the highest in plants supplied with 5:1mix-N. These plants converted far fewer labeled NH4+into free NH4+than did NH4+-fed plants, but converted many more labeled NH4+into the insoluble fraction than did NH4+- or NO3−-fed plants.The presence of a small amount of nitrate was shown to stimulate the assimilation of ammonium ions and the synthesis of pro