Grafting alleviates potassium stress and improves growth in tobacco

Abstract

Background: Potassium is a nutrient element necessary for tobacco growth. Tobacco leaves with high potassium content are elastic and tough, rich in oil. And the same time, potassium can also improve the scent and aromatic value of flue-cured tobacco by regulating the synthesis of aromatic hydrocarbons in leaves.. It is an important quality indicator for flue-cured tobacco. However, the potassium concentration in tobacco leaves in most areas of China is generally lower than the global standard for high quality tobacco. Two tobacco genotypes were grafted to each other under different potassium levels to test whether potassium content and plant growth can be improved by grafting in tobacco.

Results: The growth of tobacco in all treatments was inhibited under potassium starvation, and grafting significantly alleviated this potassium stress in 'Yunyan 87'. The trends in whole plant K⁺ uptake and K⁺ transfer efficiency to the leaves corresponded to the growth results of the different grafts. The nutrient depletion test results showed that the roots of 'Wufeng No.2' had higher K⁺ absorption potential, K⁺ affinity, and K⁺ inward flow rate. K⁺ enrichment circles appeared at the endoderm of the root section in the energy dispersive X-ray figure, indicating that the formation of Casparian strips may be partly responsible for the lower rate of lateral movement of K⁺ in the roots of 'Yunyan 87'. Gene expression analysis suggested that energy redistribution at the whole plant level might constitute one strategy for coping with potassium starvation. The feedback regulation effects between scion 'Wufeng No.2' and rootstock 'Yunyan 87' indicated that the transmission of certain signaling substances had occurred during grafting.

Conclusions: 'Wufeng No.2' tobacco rootstock grafting can increase the K⁺ uptake and transport efficiency of 'Yunyan 87' and enhance plant growth under potassium stress. The physiological mechanism of the improved performance of grafted tobacco is related to higher K⁺ uptake and utilization ability, improved xylem K⁺ loading capacity, and up-regulated expression of genes related to energy supply systems.

Keywords: Genes; Grafting; Potassium; Root; Tobacco; X-ray microanalysis.

Fig. 1

Effect of grafting combination and K⁺ supply level on whole plant K⁺ uptake (I) and transfer efficiency of K⁺ to the leaves (II). The tobacco graft combinations included the ungrafted tobacco W (Wufeng No.2) and Y (Yunyan 87), and grafted tobacco Y/W (Y grafted onto W) and W/Y (W grafted onto Y). Different uppercase letters denote significant differences (P < 0.05) under normal potassium levels (5 mmol L^− 1) and lowercase letters indicate significant differences (P < 0.05) under starvation (0.5 mmol L^− 1)

Fig. 2

Absorption kinetics curve of K⁺ in tobacco roots under different treatments. +K represents normal potassium levels (5 mmol L^− 1) and -K represents potassium starvation (0.5 mmol L^− 1). Ungrafted tobacco W (Wufeng No.2) and Y (Yunyan 87), and grafted tobacco Y/W (Y grafted onto W) and W/Y (W grafted onto Y) during the 20 h nutrient depletion test

Fig. 3

Elemental distribution of K⁺ in various tissues of the tobacco roots under different treatments. Data represent the relative atomic content of K⁺ as a percentage of all test elements (Na⁺, K⁺, Ca²⁺, P, S, Cl⁻, Mg²⁺) in a given region. The tobacco graft combinations included the ungrafted tobacco W (Wufeng No.2) and Y (Yunyan 87), and grafted tobacco Y/W (Y grafted onto W) and W/Y (W grafted onto Y)

Fig. 4

Scanning electron micrographs of a root tissue section under different magnifications. Different lowercase letters represent different magnifications

Fig. 5

Images of the K⁺ distribution in tobacco roots from the X-ray microanalysis. Blue particles represent the charge of K⁺. The tobacco graft combinations included the ungrafted tobacco W (Wufeng No.2) and Y (Yunyan 87), and grafted tobacco Y/W (Y grafted onto W) and W/Y (W grafted onto Y)

Fig. 6

Relative expression levels of genes (a. HAK1, b. NKT1, c. NHA1, d. NVP1) related to potassium transport in tobacco roots under two potassium supply conditions. The tobacco graft combinations included the ungrafted tobacco W (Wufeng No.2) and Y (Yunyan 87), and grafted tobacco Y/W (Y grafted onto W) and W/Y (W grafted onto Y). Different uppercase letters denote significant differences (P < 0.05) under normal potassium levels (5 mmol L^− 1) and lowercase letters indicate significant difference (P < 0.05) under potassium starvation (0.5 mmol L^− 1)