Nitrogen accumulation and nitrate reductase activity in MM. 106 apple trees as affected by nitrate supply

The effect of nitrate supply in the nutrient solution on N accumulation was investigated in various organs of MM.106 apple trees (Malus domestica Borkh.). Increasing nitrate supply increased the concentration and amount of total N in leaves and stems, but there was a saturation point of total N accumulation in response to the nitrate supply. The increased accumulation of total N in leaves and stems with increasing nitrate supply were due to increases in reduced N, since nitrate N remained relatively constant. Reduced N contents of trunk bark and trunk wood were significantly decreased when low nitrate was supplied, whereas that of roots was changed insignificantly. Most organs contained only a small portion of their total N as nitrate. The activities of nitrate reductase (NR) were measured using an in vivo assay either with or without nitrate for estimating reduced N accumulation. The in vivo —N03^- NR assay may be a better way to estimate the accumulation of reduced N than the in vivo +N03^- NR assay since the in vivo +N03^- nitrate reductase activity (NRA) was not related to the changes in the reduced N concentration with the differential nitrate supplies. Increasing nitrate supply significant increased in vivo —N03^- NRA of all organs. The rate of increase of the in vivo —N03^- leaf NRA was greater than those of stems and roots with increasing nitrate supply. Consequently, the proportion of leaf NRA to the total activity increased as the nitrate supply increased. The roots exhibited higher NRA than leaves and stems. The stem also contributed considerably to the total NRA. It can therefore be concluded that the reduction of nitrate can occur in various organs of MM. 106 apple trees as long as nitrate is available.