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Review
. 2022 Apr 25:13:802274.
doi: 10.3389/fpls.2022.802274. eCollection 2022.

Physiological Essence of Magnesium in Plants and Its Widespread Deficiency in the Farming System of China

Muhammad Ishfaq  1 , Yongqi Wang  1 , Minwen Yan  1 , Zheng Wang  2 , Liangquan Wu  3 , Chunjian Li  1   3 , Xuexian Li  1
Affiliations
Review

Physiological Essence of Magnesium in Plants and Its Widespread Deficiency in the Farming System of China

Muhammad Ishfaq et al. Front Plant Sci. .

Abstract

Magnesium (Mg) is an essential nutrient for a wide array of fundamental physiological and biochemical processes in plants. It largely involves chlorophyll synthesis, production, transportation, and utilization of photoassimilates, enzyme activation, and protein synthesis. As a multifaceted result of the introduction of high-yielding fertilizer-responsive cultivars, intensive cropping without replenishment of Mg, soil acidification, and exchangeable Mg (Ex-Mg) leaching, Mg has become a limiting nutrient for optimum crop production. However, little literature is available to better understand distinct responses of plants to Mg deficiency, the geographical distribution of soil Ex-Mg, and the degree of Mg deficiency. Here, we summarize the current state of knowledge of key plant responses to Mg availability and, as far as possible, highlight spatial Mg distribution and the magnitude of Mg deficiency in different cultivated regions of the world with a special focus on China. In particular, ~55% of arable lands in China are revealed Mg-deficient (< 120 mg kg-1 soil Ex-Mg), and Mg deficiency literally becomes increasingly severe from northern (227-488 mg kg-1) to southern (32-89 mg kg-1) China. Mg deficiency primarily traced back to higher depletion of soil Ex-Mg by fruits, vegetables, sugarcane, tubers, tea, and tobacco cultivated in tropical and subtropical climate zones. Further, each unit decline in soil pH from neutral reduced ~2-fold soil Ex-Mg. This article underscores the physiological importance of Mg, potential risks associated with Mg deficiency, and accordingly, to optimize fertilization strategies for higher crop productivity and better quality.

Keywords: China's agricultural system; Mg transporter family (MRS2/MGT); crop yield and quality; magnesium deficiency; photosynthetic activity; physiological functions.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Scheme depicting distinct responses of plants to Mg availability. Plant response to MGD can be summarized as (1) sub-optimal photosynthetic CO2 assimilation; (2) impaired carbohydrate transportation from the source (fully expanded older leaves) to sinks (root, young growing points, or yield-forming organs); (3) photoassimilate (starch and sucrose) accumulation in source leaves; (4) reformed rounded and larger chloroplasts; (5) imbalanced light capture and utilization; (6) leakage of electrons from photosynthetic apparatus; (7) generation of reactive oxygen species (ROS); (8) oxidative stress; (9) degradation of chlorophyll (a & b) and reduced activities of Mg-depended enzymes, i.e., Rubisco; and (10) ultimately the appearance of interveinal chlorosis and/or necrotic spots on fully expanded (mature) leaves. Impaired phloem loading results in suppression of root growth, uptake of nutrients and water along with yield and quality traits of agricultural commodities.
Figure 2
Figure 2
The visual Mg deficiency symptoms on tomato seedling. Under low Mg supply, interveinal chlorosis usually first appears on older, fully expanded (mature) leaves, and gradually encompasses younger leaves as Mg deficiency becomes more severe. Where adequate and low Mg were applied as 1.0 mM and 0.02 mM Mg, respectively, in the nutrient solution. Other details on plant growth have been described elsewhere (Ishfaq et al., 2021).
Figure 3
Figure 3
Mg uptake, transport, and homeostasis in model plants Arabidopsis and rice. Mg2+ absorption from the soil by plants is affected by a range of intrinsic and extrinsic factors. In soil, Mg2+ is susceptible to leaching due to its larger hydrated and smaller ionic radius. Once the Mg2+ reached the root surface, plants acquire it via CorA/MRS2/MGT, unspecific K-transporters, and MHX; using such transporters/exchanger, Mg2+ enters into root epidermis, cortex, and then to the xylem after passing through the Casparian strip. The symplastic route depicts Mg movement from cell to cell, whereas, apoplastic refers to movement through extracellular spaces. Followed by, it transfers to shoot and leaves through xylem and stores in vacuole via specific transporters/exchanger. MGT/MRS2, Magnesium Transporters; MHX, Mg2+/H+ Exchanger; MRS, Mitochondrial RNA Splicing; HATS, High-Affinity Transport System; LATS, Low-Affinity Transport System; Dual-ATS, Dual-Affinity Transport System.
Figure 4
Figure 4
Geographical distribution of soil Ex-Mg in agricultural systems. (A) Global soil Ex-Mg status for data available countries, (B) Global ranking of soil Ex-Mg for data available countries (value for China is highlighted by the dark green bar), (C) Frequency per cent of soil Ex-Mg in China's farming systems. Gray color in map indicated data not found.
Figure 5
Figure 5
Spatial distribution and magnitude of Mg deficiency in China's farming system. (A) Spatial distribution of soil Ex-Mg in distinct regions of China, (B) Provincial-level soil Ex-Mg distribution in agricultural soils of China. In China, ~55% of arable lands are Mg-deficient (Ex-Mg < 120 mg kg−1), with an obvious spatially declining trend from northern (331 mg kg−1 on average) toward southern (65 mg kg−1 on average) China. Gray color in maps indicated data not found.
Figure 6
Figure 6
Soil Ex-Mg variations with plant type, soil pH, and climate in China's agricultural system. (A) Fruits, vegetables, sugarcane, tubers, tea, and tobacco contribute higher in the depletion of soil Ex-Mg. Mg deficiency becomes increasingly severe (B) in acidic soils, and (C) in tropical and subtropical climate zones of China.
Figure 7
Figure 7
A proposed model to combat rising Mg deficiency in different agroecosystems. Mg availability to plants highly depends on the abundance of indigenous Mg and other soil factors. To close yield gaps in different croplands, Mg recommendations should be optimized primarily according to its net removal and indigenous soil reserves.
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