Zinc in suitable level improves absorption and assimilation efficiency of carbon and nitrogen of Malus hupehensis seedlings

Objectives Nitrogen nutrition is one of restrictions for efficient fruit production. The photosynthetic characteristics and the absorption, utilization of C and N of Malus hupehensis seedlings were studied under different zinc supply levels, aiming to find an effective way for improving nitrogen nutrition.

Methods ¹⁵N and ¹³C tracer technique was used in a sand culture experiment in which Malus hupehensis seedlings were watered with a nutrient solution containing 0.01 g of Ca (¹⁵NO₃)₂. The Zn²⁺ treatment levels in the nutrient solution included 0, 2.0, 4.0, 8.0, 16.0 μmol/L. The nutrition solution was replaced every other three days, and after the seedlings grew for 25 days, then 0.2 g/seedling of Ba¹³CO₃ (¹³C abundance was 98%) was added in each pot of the nutrient solution, and the samples were taken at the 24, 48 and 96 hours afterward to determine the ¹³C abundance of each organ. After 30 days^, treatment, the seedlings were harvested, and dry mass, root morphology, photosynthetic characteristics, key enzyme activity, nitrogen content and ¹⁵N abundance in each organ were measured.

Results Among the five Zn treatments, Zn²⁺ of 4.0 μmol/L treatment had the highest biomass and root activity, and the highest root morphological indexes (root length, root surface area and root tip number). The net photosynthetic rate (P_n), stomatal conductance (g_s), maximum photochemical efficiency of PSⅡ (F_v/F_m), Rubisco, NR and SPS enzyme activities in all the five treatments were increased first and then decreased with the increase of Zn supply level, all of the maximum values were appeared under 4.0 μmol/L treatment, with the values of 15.21 μmol/(m²·s), 184.12 μmol/(m²·s), 0.839, 8.4 μmol/(g·min), 103.25 μg/(g·h), 96.6 mg/(g·h) respectively, and the lowest values were all appeared in CK treatment, which were 0.65, 0.71, 0.92, 0.51, 0.60 and 0.52 times of those in 4.0 μmol/L treatment. The Zn content of all organs increased with the increase of the Zn supply level, and reached the maximum value under treatment of Zn²⁺ 16.0 μmol/L. With the increase of Zn supply, ¹³C accumulation, total nitrogen, ¹⁵N uptake and ¹⁵N utilization efficiency of the whole plant increased first and then decreased, all the highest values of these indexes were obtained in treatment of Zn²⁺ 4.0 μmol/L, with the values of 1.95 mg, 59.5 mg, 0.57 mg and 15.8%, respectively. The distribution rates of ¹³C and ¹⁵N were also affected by Zn levels. Among them, both the distribution rate of ¹³C in roots and that of ¹⁵N in leaves was the highest in treatment of Zn²⁺ 4.0 μmol/L, which were 1.53 and 1.18 times of those in CK treatment, respectively.

Conclusions Both insufficient and excessive Zn supply will inhibit the absorption and utilization of N by seedlings. On one hand, the suitable Zn supply can improve the photosynthesis of leaves, increase the activities of carbon metabolism related enzymes, strengthen the carbon fixation of leaves, improve the competitiveness of root system for photosynthetic products, strengthen the transportation of photosynthetic substances from leaves to roots, promote the growth and development of root system, improve the root morphology and root activity, and then enhance the nitrogen content of seedlings absorption. On the other hand, it promotes the transport of nitrogen from root to leaf, and increases the nitrate reductase activity of leaves, and enhances the assimilation and utilization ability of nitrogen by seedlings, which further promotes the absorption of nitrogen by seedlings. Therefore, suitable zinc supply will coordinately promote the absorption and assimilation of carbon and nitrogen of seedlings, and benefit the growth of seedlings.