Continuous and constant supply of potassium in suitable level stimulates the growth and absorption and utilization of nitrogen in M9T337 dwarf rootstocks seedlings

Objectives Potassium plays an important role in nitrogen metabolism. The growth, nitrogen absorption and utilization of apple were studied under different K supply conditions, which would help further understanding the influencing mechanism of potassium nutrition on nitrogen metabolism and providing support for reasonable K fertilization in apple production.

Methods Sand culture method was used, and M9T337 dwarf rootstocks seedlings were used as materials and the culture lasted 60 days. At the base of Hoagland solution, K was supplied in five ways: deficient (0.5 mmol/L, K1) in 1–60 days, deficient (0.5 mmol/L) in 1–30 days and excess (12 mmol/L) in 31–60 days (K2), appropriate (6 mmol/L) in 1–60 days (K3), excess (12 mmol/L) in 1–30 days and deficient (0.5 mmol/L) in 31–60 days (K4), excess (12 mmol/L) in 1–60 days (K5). The nutrition solution was replaced every other three days, and 0.01 g of Ca(¹⁵NO₃)₂ was added into each pot with the replacement of solution and 0.2 g was added in total. At the 31st day and 60th day of treatment, seedling samples were collected for the measurement of growth, root traits, N content, root NO₃^– flux and seedling nitrate reductase (NR) activity.

Result At the 60th day after treatment, K3 obtained the highest plant dry matter, the largest total root length and surface area, and significantly higher root activity. From 31 days to 60 days, the greatest increase of above-ground dry weight was in K2 treatment (174.8%), the largest increase of root dry weight was in K4 treatment (176.3%) and the greatest increase in total biomass, root length and root surface area were in K3 (12.99 g/plant, 1059 cm/plant and 1113 cm²/plant, respectively). In both the measurements at 31 days and 60 days, the highest root absorption rates of NO₃^– were all in K3 treatment 29.63 and 36.19 pmol/(cm²·s), respectively. In treatments K1 and K4, the NO₃^– ion flow in root was inward from day 1 to day 30 and became outward from day 31 to day 60. During the whole treatment period, K3 treatment had the highest NR activity; during 41–60 days, the NR activity increased significantly in K2 treatment but decreased significantly in K4. The results of ¹⁵N tracer showed that the N absorption ability of seedlings under K3 treatment was the highest, in which the seedling total N accumulation, ¹⁵N utilization rate and ¹⁵N allocation rate in leaf were all significantly higher than those in other treatments. At the 60th day after treatment, the total ¹⁵N absorption was in order of K3>K5>K2>K4>K1.

Conclusions Deficient, excess or varied K supplement all impede root growth and nitrogen movement to shoot in apple seedlings, and are not conducive to nitrogen uptake and utilization. Constant and appropriate K supply will improve root activity and nitrate reductase activity, maintain a high NO₃^– uptake rate of root system, and promote the upward transport of nitrogen. So the constant and suitable K supply should be considered through fertilization for the healthy and efficient growth of apple rootstock seedlings in practical K management.