Studies on Nitrogen Metabolism in Forage Sorghum : I. Effect of basal- and topdressing-N under potassium and phosphorus deficiencies on regrowth and nitrogen metabolism in sorgo plant

Abstract

The proper combination of N-level of basal dressing and N-topdressing was effective for increase in the cutting yield but N-topdressing under large-N condition brought about not only reduction in the cutting yield but also an increase in cyanide content harmful to domestic animals. These N-reactions were strongly developed especially under large-N condition lacking in K or P. The proper topdressing quantity of N in the field was considered to be 5-10 kg/10a. Further, the concentration of P in the bleeding sap was also markedly reduced especially when K or P was deficient under large-N condition. It was considered that abnormal metabolism, such as the increase and accumulation of free amino acids in vivo, brought about a marked increase in cyanide or nitrate-N in the leaf blade and stem. The increase of asparagine in the bleeding sap and stem was especially marked as a result of the N-reaction in the sorgo plant accompanied by a rise in the N-level of basal dressing. From this, the purpose in the nutritional diagnosis of the sorgo plant can be achieved by the simple detection of asparagine in the bleeding sap.