Research Progress on the Mechanism of Crop Saline-alkali Tolerance and Mitigation Measures

Ting XU Yantao LIU

Abstract The wide distribution of saline-alkali land in China is a restrictive factor for the sustainable development of agriculture. Saline-alkaline soil inhibits the growth and development of crops, reducing its yield and quality. In this article, we summarized the germination status, physiological characteristics, response mechanisms and mitigation measures of different crops under saline-alkali stress in recent years, aiming to provide important reference for the study of saline-alkali tolerance mechanism in crops, cultivation of crop varieties tolerant to salts and alkalis and improvement of the utilization rate of saline-alkali land, and put forward suggestions for future development trend of saline-alkali land crops and mitigation measures.

Key words Saline-alkali stress; Crop; Response mechanism; Mitigation measures; Research progress

Received: June 5, 2021  Accepted: August 8, 2021

Supported by Tiemenguan Science and Technology Project of the Second Division of Xinjiang Production and Construction Corps (2019NYGG13); XPCC Peoples Practical Livelihood Matter Project of "Improving the Level of Agricultural Science and Technology".

Ting XU (1998-), female, P. R. China, master, devoted to research about physiological response of peanut under saline-alkali stress and its mitigation measures.

*Corresponding author. E-mail: ziheng@126.com.

As the area of saline-alkali land in China increases year by year and the area of arable land continues to shrink, soil salinization restricts the sustainable development of agriculture［1-2］. Xinjiang has a dry climate, low precipitation, large evaporation, continuous accumulation of salt in the soil surface, and lagging drainage facilities in the agricultural irrigation system, resulting in increased salinization and lower crop growth［3-4］. Saline-alkali land contains a lot of salts, which severely damage soil structure, and ions are unbalanced. In such saline-alkali land, crop cells are damaged by salt ions, and photosynthesis, material synthesis and energy metabolism are affected. Alkaline-alkaline soil has almost an impact on crops at various stages from germination to growth and reproduction, harming crops and causing them to form a series of physiological and biochemical reactions［5］. Higher alkalinity of soil hinders the absorption and utilization of crop nutrients, inhibits seed germination, and leads to the destruction of root cell structure and function, resulting in a substantial reduction in crop yield and quality［6-7］. As population grows, food problem is becoming more and more serious. Understanding the reaction mechanism of crops in saline-alkali land, encouraging the breeding of new salt-resistant varieties and enhancing resistance of crops to salinization can effectively improve the utilization rate of saline-alkali land［8］.

For saline-alkali soil management, we can also grow crops tolerant to salts and alkalis and apply amendments in addition to physical methods. Studying the salt tolerance of crops and cultivating new salt-tolerant varieties are of great significance for achieving high yield and high quality of crops and rational utilization of saline-alkali land. At present, there have been studies on the application of a variety of amendments to crop growth and the selection of salt-tolerant crop varieties, and there are many studies on the effects of salt-alkali stress on peanut, sunflower, oat, mung bean, rape, corn, sweet sorghum and other crops. In this article, we comprehensively described the response mechanism of crops to saline-alkali stress and the research progress of the selection of salt-tolerant varieties, aiming to provide an important reference for the research of crops with saline-alkali tolerance.

Effects of Saline-alkali Stress on the Germination of Crop Seeds

Excessive soil salinity leads to inhibition of crop growth and reduced yield［9］. Salt tolerance of crop seeds during the germination period is one of the important basis for characterizing salt tolerance of crops［10］. Li et al. used different saline-alkali types and five saline-alkali concentrations of saline-alkali stress, and found that when the salt concentration was higher than a critical value, the germination of cotton seeds was inhibited, and the germination rate decreased with the increase of salt concentration. Bao et al.［11］ cultivated alfalfa seeds with soil extracts of different pH values and found that high concentrations of soil extracts inhibited the germination of alfalfa seeds. Alfalfa seeds do not germinate when the pH of soil extract was above 9.22, which might be due to the destruction of embryos in high concentrations of salts and alkalis. The germination period of alfalfa seeds is a sensitive period in saline-alkali environments. Alkaline stress reduces the germination rate of seeds which thus cannot absorb enough water, and it is difficult for seed proteolytic enzymes to work, resulting in weakened respiration. Seeds of different rape varieties have different germination under mixed saline-alkali stress, resulting in osmotic effect and ion effect. Ding et al.［12］ showed that low-concentration mixed saline-alkali stress could promote the germination of Fengyou 737 rapeseeds, but inhibit the germination of other varieties of rapeseeds. Zhu et al.［13］ studied the germination of five different varieties of Isatis tinctoria seeds, and found that different varieties of seeds had different saline-alkali tolerance, low saline-alkali concentration could promote the growth of I. tinctoria seeds and stimulate the germination of seeds, and Bozhou Daye was a sensitive variety. Therefore, the research of crop seed germination under mixed saline-alkali stress is of great significance to the selection and cultivation of crops tolerant to salts and alkalis.

Effects of Saline-alkali Stress on Crop Growth and Physiological Characteristics

Studies have shown that higher salt concentration shortens the average length of rape embryos, and low saline-alkali stress promotes embryo growth. High-concentration salt solutions inhibit root growth, resulting in decreased root-to-shoot ratio, decreased contents of chlorophyll and carotenoids, and damaged reaction center［12］. With the increase of the concentration of saline-alkali mixture, the total relative humidity of the leaves of Eruca sativa Mill. decreased. According to different root length changes of E. sativa, it was found that the root length of E. sativa increased under low concentration stress, and the average fresh weight and dry weight of roots in solutions of different concentrations decreased with the increase of salt concentration［14］.  Due to the imbalance of cell osmotic pressure under saline-alkali stress, soluble sugar increases, and plants accelerate water absorption to resist stress. Sunflower seedlings showed different responses to ion poisoning. Under low-salt stress, cell permeability increased and seedling water content increased, resulting in a decrease in cell membrane stability and a relative increase in salt medium and electrical conductivity［15］. With the increase of sunflower defensive enzyme activity, peroxidation reaction weakened, amino acid content decreased, and oxygen free radicals were effectively eliminated. High concentrations of salt stress made it difficult for water to be absorbed by the root system, inhibited its growth, and hindered the absorption and transportation of nutrients by the root system［11］. The increase in the pH value of salt solutions limits the growth of alfalfa seedling roots. Fu［16］ evaluated the adaptability of peanut to salt stress and its salt tolerance. The study showed that the salt tolerance of peanut was strongest in the pod-setting stage, and the salt tolerance was the worst in the seedling stage, which was the salt-sensitive period. Salt stress hindered the formation and material accumulation of peanut seedlings, and its salt tolerance threshold was 0.45%. With the increase of salt stress concentration, chla/chlb showed an upward trend, and the contents of various photosynthetic pigments showed a downward trend. Different salt stress concentrations had different inhibitory effects on seedlings. For "Huayu 25", with the increase of salt stress, the fresh weight of aboveground part and the fresh weight of underground part decreased by 68.3% and 74.7%, respectively; and when the salt stress concentration was 0.15%, the dry weights of the aboveground part and underground part of "Huayu 20" first increased and then decreased［17］. Yu［18］ studied the effects of three different alkaline soils on seedling root morphology, agronomic characteristics and plant dry matter accumulation of two different types of peanut varieties by potting. They concluded that high alkalinity (pH 9.0) severely affected the aboveground production of peanut, and leaf area, dry matter weight of each organ, and root-to-shoot ratio all showed a downward trend. Thus, the alkali tolerance threshold of peanut was pH 8.5.

Response Mechanism of Crops to Saline-alkali Stress

With the increase in the concentration of saline-alkali mixed stress, antioxidant superoxide dismutase (SOD) and catalase (CTA) complement each other and play an important role in reducing plant oxidative damage, preventing salinization, and maintaining normal plant physiological metabolism［19］. In the process of saline-alkali mixed stress, soluble sugar, soluble protein and free proline show lower concentrations, and as osmotic substances, their contents increase rapidly with the concentration of solutions increasing. They play a leading role in plant osmotic adjustment and ensures the physiological activity of plants［20］. Plants cause reactive oxygen species (ROS) damage under salt stress［21］. Due to the accumulation of reactive oxygen species, lipid peroxides in membranes cause cell membrane damage［22-24］. The ability to remove active oxygen is mainly achieved by maintaining the high activity of protective enzymes. Salt-tolerant plants can improve the scavenging ability of active oxygen by reducing active oxygen［18］. Oat can continuously maintain the antioxidant enzyme system and improve its antioxidant capacity. Under the action of salts and alkalis, proline and soluble sugars in oat respond to salt-alkali stress［25］, and it accumulates more proline and soluble protein to maintain the normal physiological state of cells. Nitrogen, phosphorus and calcium can increase the K+ content of oat to a certain extent, reduce the Na+ content, alleviate the influence of salt stress on cell membranes, and promote the growth of oat. Alkaline stress is more harmful than salt stress. When alfalfa is exposed to saline-alkali stress, superoxide dismutase, photosynthetic rate, plant transpiration rate and intracellular carbon dioxide concentration change. In a saline-alkali environment, alfalfa reduces cell water potential through K+ to maintain intracellular water.

Saline-alkali Stress Mitigation Measures

Selection and cultivation of salt-tolerant varieties

The relative main stem height, relative side branch length, relative branch number, relative blighted pod number, relative total pod number, relative full pod rate and other indicators of peanut were measured under mixed saltine-alkali stress, and it was found that the main stem height, side branch length and yield of peanut were the main factors to identify peanut salt and alkali tolerance［4］. Wang et al.［26］ found that the indexity of total branch number has yet to be studied. They screened out the salt-tolerant stable type ‘Fuhua No.1 multi-grain peanut, and the high-yield type ‘Bai 6-330 variety in saline-alkali land, and the pearl-bean type ‘Baisha 1016 with high tolerance to salts and alkalis. Wang［27］ bred saline-alkali-tolerant peanut varieties through chemical mutation and hybrid breeding, and screened out 12 dominant saline-alkali land cultivars including ‘Huayu 31, ‘Huayu 40, ‘Huayu 57 and ‘Huayu 9612 and two high-oleic-acid high-yield varieties ‘15S25 and ‘15S24, all of which can be applied to production［28］, which has important reference value for screening and cultivating salt-tolerant crop varieties (lines) and improving the utilization rate of saline-alkali land.

Administration of Calcium Inhibitors and Exogenous Calcium

The application of exogenous calcium to peanut in saline-alkali land increased the dry matter accumulation of peanuts, and with the continuous increase of exogenous calcium consumption, the dry matter accumulation, maximum growth and pod yield of the shoots increased significantly, shortening leaf senescence cycle, delaying leaf senescence, and increasing photosynthetic rate to 55%［29］. Tian et al.［30］ found through research on related indicators of ‘Huayu 25 that the seeds and pods grew well under the stimulation of exogenous calcium, the seed expansion period was earlier, the protein and linoleic acid contents decreased, and the weights of 100 pods and 100 kernels and yield increased. Spraying exogenous calcium on maize stems and leaves in saline-alkali soil indicated that calcium inhibitor verapamil reduced active oxygen. Because a high concentration of calcium inhibitor verapamil can prevent Ca2+ signals from passing through cells in a short time, and reduce the effect of salinization on plant cell membranes. Under saline-alkali conditions, when the concentration of Ca2+ was high, the catalytic effect of Ca2+ and the calcium inhibitor verapamil was significantly weakened［31］.

Application of biochar, bio-humic acid and bio-organic fertilizer

The application of biochar in saline-alkali soil increases soil biomass, and the application of 10%-20% biochar results in increases of aboveground dry matter, underground dry matter and leaf area of mung bean, and promotes the growth of mung bean. The increase of biochar promotes the increase of chlorophyll content of mung bean leaves, the increase of Mg2+ and K+, Ca2+ contents, and the decrease of Na+ content［32］. Under the conditions of increased biochar and salinity, the average root diameter is not much different. With the increase of biochar, the growth and development of mung bean root system is promoted, and the absorption and transportation efficiency of root nutrients is improved. High-concentration biological humic acid inhibits tobacco root growth, and low-concentration humic acid promotes root growth and increases root number［33］. The most significant factor for increasing cucumber yield in saline-alkali soil is the application of moderate humic acid. With the increase in the amount of biological humic acid applied, soil fertility and cucumber yield increase. After high humic acid treatment, soil pH value decreases, which has a certain inhibitory effect on cucumber growth［33］. Although biological humic acid helps microorganisms to multiply and increase dehydrogenase activity, excessive humic acid content can inhibit the growth of microorganisms and bacteria. In addition, organic fertilizer can promote the formation of soil aggregates and increase the content of organic carbon in the soil, thereby reducing soil bulk density［11］. Organic fertilizer can promote the growth of peanut main stem and the first lateral branch. Application of appropriate amount of organic fertilizer can increase leaf area index, delay peanut senescence, accelerate pod growth, and increase yield per plant［34］.

Application of rhizosphere growth-promoting bacteria

In the process of plant physiological and biochemical changes, rhizosphere growth-promoting bacteria establish a resistance mechanism, synthesize ABA, induce proline gene expression, and reduce the damage caused by salt concentration. Studies have shown that rhizosphere growth-promoting bacteria reduce the concentration of ABA in plant roots under saline-alkali conditions, and the osmotic regulator secreted by them can increase plant resistance to saline-alkali stress, increase soluble sugar content, and reduce plant cell osmotic potential［35］. Inoculation of rhizosphere growth-promoting bacteria can help plants produce micro-aggregates, enhance their drought resistance and stress resistance, and protect crops from salt ions. The microenvironment of rhizosphere growth-promoting bacteria can be changed by extracellular polysaccharides, so that nutrients are adsorbed by extracellular polysaccharides, which accelerates the decomposition of minerals, thereby promoting the absorption of nutrients by plant roots, increasing plants nutrient utilization rate, and the regulation of rhizosphere growth-promoting bacteria to salinity and alkalinity resistance-related genes and to proteins gradually enhances the ability of crops to resist salinity and alkalinity［35］. Liu et al.［36］ performed root irrigation of potted peanut seedlings and found that four composite strains significantly increased the height and fresh weight of peanut, promoted the growth of peanut seedlings, and improved the rhizosphere soil environment. Among them, the compound strain composed of strains with the function of dissolving phosphorus and strains with the function of dissolving nitrogen had the best effect. Liu et al.［37］ studied four rhizosphere growth-promoting bacteria and found that the dry weight, crude protein content, phosphorus content and root length of alfalfa increased after inoculation with rhizosphere growth-promoting bacteria, which improved the ability of alfalfa to remove harmful substances, such as superoxide anions. Han et al.［38］ applied rhizosphere growth-promoting bacteria to rice, and screened two kinds of rhizosphere growth-promoting strains with saline-alkali tolerance. The results showed that the germination rate, germination vigor, dry weight, rice height, root length and enzyme activity were improved, and the two rhizosphere growth-promoting strains effectively alleviated the damage caused by saline-alkali stress in rice.

Agricultural Biotechnology2021

Prospects

Since 2015, China has made great progress in the selection and breeding of salt-tolerant varieties and the genetics and mechanisms of salt-tolerant varieties, and a large number of salt-tolerant genes have been studied［39-44］. It is necessary to focus on collecting salt-tolerant wild plants and wild relatives, exploring salt-tolerant genes［45-46］, so as to screen and cultivate new salt-tolerant varieties, develop and use saline soil, and achieve efficient water use for agriculture. The in-depth study of salt tolerance mechanism is inseparable from the close integration of many disciplines such as soil science, genetics, breeding, physiology and molecular biology. Genetic engineering and biotechnology can be used to identify, isolate and clone salt-tolerant genes and cultivate new salt-tolerant varieties. Saline soil improvement has gradually changed from water and soil conservation measures to biological improvement［47-49］. The research on the relationship between crop salt tolerance and ecological factors provides an important foundation for the exploration of salt tolerance resources.

As the population increases, the area of saline-alkali land continues to expand, and the per capita arable land continues to decrease. It is imperative to solve the problem of land salinization. Amendments can not only reduce the impact of soil salinization on the environment, but also bring certain economic benefits［50］. At present, a large number of researchers have explored the physiological characteristics of crops under saline-alkali stress and screened salt-tolerant varieties, but only with amendments can the crop yield be more effectively increased, such as rhizosphere growth-promoting bacteria, biochar, and calcium inhibitors. The growth and development of a large number of crops such as sunflower, rape, E. sativa, mung bean, tobacco, cucumber, alfalfa, etc., are significantly improved after the application of amendments, and the yields are significantly increased. The saline-alkali tolerance of crops has the characteristics of multi-channel regulation. Strengthening the research on the regulation mechanism of saline-alkali-resistant ion balance, applying genetic engineering technology to crop breeding for saline-alkali resistance, and cultivating new salt-tolerant varieties is a hot research direction in the future, is an important part of the efficient use of saline-alkali land, and is an important guarantee for achieving high yield and high quality of crops.

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