[Effects of nitrate nitrogen supply on the growth, photosynthetic characteristics and 15 N absorption, utilization of Malus hupehensis seedlings.]

To explore the effects of different nitrate nitrogen levels (N0 , N1 , N2 , N3 and N4 were equivalent to 0, 2.5, 5, 10, 20 mmol·L-1 15 NO3 - -N, respectively) on the growth, photosynthetic characteristics and 15 N absorption, utilization and distribution, Malus hupehensis seedlings were grown in cultural liquid Hoagland by using the 15 N-labeled tracer method. The results showed that the leaf chlorophyll content, leaf area and dry mass in different organs were the highest in N2 treatment. With the increase of 15 NO3 - -N application rates, the leaf net photosynthetic rate (Pn )significantly increased but tended to decease when the 15 NO3 - -N concentration exceeded N2 treatment. In the 20th day after treatment, the root activity, root length, root surface area and number of tips of seedlings in N2 treatment were significantly higher than those in the other treatments. The distribution ratio of 15 N in different organs was significantly different among those treatments. The relatively balanced distribution ratio of 15 N appeared in N2 treatment, which the 15 N utilization rate also reached relatively higher level. The total N content and 15 N absorption of seedlings increased at low 15 NO3 - -N concentration, reached the highest value in N2 treatment with 103.77 and 21.57 mg, and then deceased at high 15 NO3 - -N concentration. At the 12th day after treatment, the leaf nitrate reductase (NR) activity was the highest in N2 treatment and the lowest in N4 treatment. The leaf nitrate reductase (NR) activity deceased by 84.9% in N4 treatment compared with N2 treatment at the 16th day after treatment. Our findings indicated that the photosynthesis and absorption of nitrate nitrogen were inhibited under low 15 NO3 - -N stress, and the assimilation of nitrate nitrogen and root growth were restrained under too much higher 15 NO3 - -N level, which was not good for the growth, nitrogen absorption and utilization of apple seedlings. The appropriate nitrogen level could promote plant growth, enhance the photosynthesis and also increase the absorption, utilization and distribution of nitrogen.