Effects of nitrogen supply levels and stability on growth and ¹⁵N absorption and utilization of M9T337 dwarf rootstocks seedlings

Objectives Studying the effects of the stability and levels of nitrogen supply on the growth and nitrogen absorption characteristics of apple seedlings will help further understanding the physiological mechanism of apple seedlings, which will provide theoretical basis for nitrogen fertilization in apple orchards.

Methods Hydroponic culture method was used with M9T337 dwarf rootstocks seedlings as tested materials. NO₃^–-N concentrations of deficiency, appropriate and excess (NO₃^–-N 0.5, 5 and 25 mmol/L in turn) were setup in the cultural solution. The seedlings were cultured in the three solutions for 10 days, then the treatments were transferred as the following five ones: NO₃^–-N concentration varied from deficit to excess treatment (N1), from excess to deficit treatment (N2), keeping appropriate (N3), keeping deficiency (N4) and keeping excess (N5). The 10 days seedlings were continually cultured for another 10 days in above treatments. The plant dry matter, root morphology and root NO₃^– uptake flux, contents of NO₃^– and nitrate reductase enzyme activity in the roots and leaves, and the plant total N content and ¹⁵N absorption were measured.

Results The highest plant dry matter was obtained in the N3 treatment and the least in the N4 treatment. The biggest increase of shoot dry matter was found in the N1 treatment at the 20th day after the treatment. The most significant effect on root length, root surface area and quantity of tips of seedlings was in the N3 treatment, followed by the N4 treatment, and least in the N5 treatment. The biggest increase appeared in the N2 treatment, with the root length and root surface increment of 31.5% and 34.9% respectively between the two sampling intervals. One day after the change in NO₃^–-N concentration, the highest NO₃^– uptake flux was found in the N1 treatment, which was 46.37 pmol/(cm²·s), and no significant difference with that in N3 treatment. On the 10th day after NO₃^–-N concentration change, the root NO₃^– uptake flux in the N3 treatment was significantly higher than in the others. A clear trend of NO₃^– outflow was found in the N5 treatment, and the root NO₃^– uptake flux in the N1 treatment was decreased by 62.0%, in the first day of NO₃^–-N concentration change. The Ndff values in different organs, the total N content and ¹⁵N absorption were the biggest in the N3 treatment, the smallest in the N4 treatment, and the biggest increases appeared in the N1 treatment. At the 11th day after the treatment, the contents of NO₃^– in the roots and leaves reached the highest in the N5 treatment, and there was no significant difference between the N5 and N3 treatments. While the NO₃^– content in leaves under the N3 treatment was significantly decreased by13.4% compared with the N5 treatment in 20th day. The leaf nitrate reductase activity in the N5 treatment was significantly lower than that in the N3 treatment at the 12th day after the treatment, and it displayed an order of N3 > N1 > N5 > N2 > N4 at the 20th day after the treatment.

Conclusions Low NO₃^– supply will lead to insufficient absorption of NO₃^– in plants, excess supply will restrain the assimilation of NO₃^–-N and root growth. Appropriate and steady supply of nitrogen could maintain high root NO₃^– uptake flux and Ndff value, thus enhancing the leaf nitrate reductase activity gradually, and then increasing the absorption and utilization of nitrogen by seedlings, which is the preferred form to the growth of apple seedlings.