Effects of Potassium Levels on Plant Growth, Accumulation and Distribution of Carbon, and Nitrate Metabolism in Apple Dwarf Rootstock Seedlings

Abstract

Nitrogen (N) is one of the most required mineral elements for plant growth, and potassium (K) plays a vital role in nitrogen metabolism, both elements being widely applied as fertilizers in agricultural production. However, the exact relationship between K and nitrogen use efficiency (NUE) remains unclear. Apple dwarf rootstock seedlings (M9T337) were used to study the impacts of different K levels on plant growth, nitrogen metabolism, and carbon (C) assimilation in water culture experiments for 2 years. The results showed that both deficiency and excess K inhibited the growth and root development of M9T337 seedlings. When the K supply concentration was 0 mM and 12 mM, the biomass of each organ, root-shoot ratio, root activity and NO₃ ^- ion flow rate decreased significantly, net photosynthetic rate (P _n) and photochemical efficiency (F _v/F _m) being lower. Meanwhile, seedlings treated with 6 mM K⁺ had higher N and C metabolizing enzyme activities and higher nitrate transporter gene expression levels (NRT1.1; NRT2.1). ¹³C and ¹⁵N labeling results showed that deficiency and excess K could not only reduce ¹⁵N absorption and ¹³C assimilation accumulation of M9T337 seedlings, but also reduced the ¹⁵N distribution ratio in leaves and ¹³C distribution ratio in roots. These results suggest that appropriate K supply (6 mM) was optimal as it enhanced photoassimilate transport from leaves to roots and increased NUE by influencing photosynthesis, C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport.

Keywords: 13C; 15N; M9T337 seedlings; NUE; biomass; enzyme activity; nitrate transporter gene expression.

FIGURE 1

Root morphology (A), root length (B), root surface (C), and root activity (D) of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 2

K concentrations (mg g^–1 DW) in the leaves, stems, and roots of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 3

Rubisco (A), SPS (B), and SS (C) activities in the leaves of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 4

¹³C accumulation (A) and ¹³C distribution ratio (B) of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 5

NO₃^– ion flow rate in root of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. (A) Net NO₃^– fluxes in the root of apple seedlings for 10 min. (B) Mean rate of NO₃^– fluxes during the entire 10 min. Data are means ± SD (N = 6). min = minutes. Different letters indicate statistically significant differences (P < 0.05).

FIGURE 6

NR activities in the leaves (A) and roots (B); GS activities in the leaves (C), and roots (D) of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 7

NRT1.1 gene expressions (A) and NRT2.1 gene expressions (B) in the roots of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).

FIGURE 8

¹⁵N accumulation (A), ¹⁵N distribution ratio (B) and ¹⁵NUE (C) of M9T337 seedlings treated with K0 (0 mM K⁺), K3 (3 mM K⁺), K6 (6 mM K⁺), K9 (9 mM K⁺), or K12 (12 mM K⁺) in 2018 and 2019. Each treatment had three biological replicates and the assays were repeated three times. Vertical bars indicate ± SD (N = 3). Different letters indicate statistically significant differences (P < 0.05).