Potassium deficiency causes more nitrate nitrogen to be stored in leaves for low-K sensitive sweet potato genotypes

Abstract

In order to explore the effect of potassium (K) deficiency on nitrogen (N) metabolism in sweet potato (Ipomoea batatas L.), a hydroponic experiment was conducted with two genotypes (Xushu 32, low-K-tolerant; Ningzishu 1, low-K-sensitive) under two K treatments (-K, <0.03 mM of K⁺; +K, 5 mM of K⁺) in the greenhouse of Jiangsu Normal University. The results showed that K deficiency decreased root, stem, and leaf biomass by 13%-58% and reduced whole plant biomass by 24%-35%. Compared to +K, the amount of K and K accumulation in sweet potato leaves and roots was significantly decreased by increasing root K⁺ efflux in K-deficiency-treated plants. In addition, leaf K, N, ammonium nitrogen (NH₄ ⁺-N), or nitrate nitrogen (NO₃ ^--N) in leaves and roots significantly reduced under K deficiency, and leaf K content had a significant quadratic relationship with soluble protein, NO₃ ^--N, or NH₄ ⁺-N in leaves and roots. Under K deficiency, higher glutamate synthase (GOGAT) activity did not increase amino acid synthesis in roots; however, the range of variation in leaves was larger than that in roots with increased amino acid in roots, indicating that the transformation of amino acids into proteins in roots and the amino acid export from roots to leaves were not inhibited. K deficiency decreased the activity of nitrate reductase (NR) and nitrite reductase (NiR), even if the transcription level of NR and NiR increased, decreased, or remained unchanged. The NO₃ ^-/NH₄ ⁺ ratio in leaves and roots under K deficiency decreased, except in Ningzishu 1 leaves. These results indicated that for Ningzishu 1, more NO₃ ^- was stored under K deficiency in leaves, and the NR and NiR determined the response to K deficiency in leaves. Therefore, the resistance of NR and NiR activities to K deficiency may be a dominant factor that ameliorates the growth between Xushu 32 and Ningzishu 1 with different low-K sensitivities.

Keywords: N metabolism; leaf K; nitrate; potassium deficiency; sweet potato (Ipomoea batatas).

Figure 1

Difference in the sensitivity to K deficiency between the two sweet potato genotypes (low-K-tolerant “Xushu 32” and low-K-sensitive “Ningzishu 1”). (A) Phenotypic difference of sweet potato plants during K deficiency (−K) and K sufficient (+K) treatments for 10 days. (B, C) leaf and root K contents. (D, E) Leaf and root N contents. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K.

Figure 2

Relationship between leaf nitrogen (N) content, root N content, and leaf potassium (K) content. The solid and dotted lines represent Xushu 32 and Ningzishu 1, respectively; * mean significant at the p < 0.05 probability level.

Figure 3

Effects of K deficiency on steady-state net K⁺ efflux in adventitious roots of two sweet potato genotypes (Xushu 32 and Ningzishu 1). Steady-state net K⁺ flux measured from root apex and mature regions after 10 days of K treatment. Apex region, 500 μm from the root tip; mature region, 15 mm from the root tip. Columns labeled with different letters in the same region indicate significant differences at 0.05 level.

Figure 4

Changes of amino acid and soluble protein content in leaves and roots for two K treatments for Xushu 32 and Ningzishu 1. (A, B) leaf and root amino acid contents. (C, D) Leaf and root soluble protein contents. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K.

Figure 5

Changes of NO₃ ⁻–N and NH₄ ⁺–N content in leaves and roots for two K treatments for Xushu 32 and Ningzishu 1. (A, B) leaf and root NO₃ ⁻–N contents. (C, D) Leaf and root NH₄ ⁺–N contents. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K.

Figure 6

Relationship of protein (A, B), NO₃ ⁻–N content (C, D), and NH₄ ⁺–N content (E, F) in leaves and roots with leaf K content. * mean significant at the p < 0.05 probability level. The solid and dotted lines represent Xushu 32 and Ningzishu 1, respectively.

Figure 7

Changes of NR and NiR activities in leaves and roots for two K treatments for Xushu 32 and Ningzishu 1. (A, B) leaf and root NR activities. (C, D) Leaf and root NiR activities. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K. NR, nitrate reductase; NiR, nitrite reductase.

Figure 8

Changes of GS and GOGAT activities in leaves and roots for two K treatments for Xushu 32 and Ningzishu 1. (A, B) leaf and root GS activities. (C, D) Leaf and root GOGAT activities. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K. GS, glutamine synthetase; GOGAT, glutamate synthase.

Figure 9

Effects of K deficiency on the transcription of N metabolism-related genes in leaves and roots of two sweet potato cultivars. Sweet potato seedlings were subjected to 15 days of K deficiency stress; total RNA was isolated from leaves and roots for real-time PCR analysis. The stages labeled with an asterisk (*) indicate significant differences (p < 0.05) between −K and +K.

Figure 10

Scheme summarizing the effects of K deficiency on primary metabolism in leaves and roots of sweet potato. Biochemical and transport pathways are indicated with solid and dashed arrows, respectively. Increases in metabolite concentrations and enzyme activities under K deficiency are shown in red, decreases are shown in blue, and items that do not change significantly under K deficiency are marked with gray. Putative direct inhibition under K deficiency is indicated with the red bar. Dashed lines indicate the exchange of metabolites between leaves and roots. X32 and N1 represent Xushu 32 and Ningzishu 1, respectively.