[Effects of nitrate nitrogen supply on the growth, photosynthetic characteristics and 15N 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-115NO3- -N, respectively) on the growth, photosynthetic characteristics and 15N absorption, utilization and distribution, Malus hupehensis seedlings were grown in cultural liquid Hoagland by using the 15N-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 15NO3- -N application rates, the leaf net photosynthetic rate (Pn)significantly increased but tended to decease when the 15NO3--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 15N in different organs was significantly different among those treatments. The relatively balanced distribution ratio of 15N appeared in N2 treatment, which the 15N utilization rate also reached relatively higher level. The total N content and 15N absorption of seedlings increased at low 15NO3--N concentration, reached the highest value in N2 treatment with 103.77 and 21.57 mg, and then deceased at high 15NO3--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 15NO3--N stress, and the assimilation of nitrate nitrogen and root growth were restrained under too much higher 15NO3--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.以霍格兰营养液为培养基质,采用15N同位素示踪技术,研究不同浓度15NO3--N (0、2.5、5、10和20 mmol·L-1,分别以N0、N1、N2、N3和N4表示)对平邑甜茶幼苗生长、光合作用、15N吸收、利用及分配的影响.结果表明:与其他处理相比,N2处理幼苗叶绿素含量、叶面积及各器官干质量最大.叶片净光合速率(Pn)随15NO3--N浓度的增加显著增大,但15NO3--N浓度超过N2处理后Pn略有下降.处理20 d时,N2处理幼苗根系活力最大,根系长度、根系总表面积和根尖数也显著高于其他处理.各处理间15N分配率差异显著,N2处理幼苗各器官间15N分配率最均衡,15N利用率也较高;随15NO3--N浓度增加,各处理幼苗全氮量和15N吸收量呈先升高后降低的趋势,且在N2处理时最大,分别为103.77和21.57 mg.处理12 d后,叶片硝酸还原酶(NR)活性以N2处理最高,N4处理最低,至第16天时,N4处理较N2处理降低了84.9%.因此,15NO3--N供应过低抑制幼苗光合作用及氮素吸收,15NO3--N供应过高则抑制幼苗体内硝态氮同化及根系生长,均不利于苹果幼苗生长及氮素营养吸收利用,适量供氮有利于苹果幼苗的生长、光合作用的提高,以及氮素的吸收、利用和分配.