Growth, leaf gas exchange and physiological parameters of two Glycyrrhiza glabra L. populations subjected to salt stress condition

Abstract

Salinity is the major environmental stress affecting growth and physiological processes in plants. Glycyrrhiza glabra (licorice) is a perennial medicinal plant having many secondary metabolites. The effects of 0, 100, 200, 400, 600, and 800 mM NaCl on growth parameters, leaf relative water content, chlorophyll index, gas exchange parameters, membrane stability index, antioxidant activity, phenolic compounds, flavonoid, and soluble carbohydrate contents of licorice populations (Fars and Khorasan) cultivated with different rhizomes diameters were evaluated. The soil and rhizomes elements were also studied. The results indicated that the salinity significantly decreased the growth parameters, relative water content, and membrane stability index, compared to the controls. The thicker rhizomes can grow more than thinner ones in saline soil. Chlorophyll index and leaf gas exchange were declined under salt stress. Besides, the soluble carbohydrate content was decreased. Salinity caused oxidative stress in licorice rhizomes, so that by decreasing the membrane stability index, antioxidant capacity, phenolic compounds, and flavonoid contents of rhizomes have increased in line with NaCl treatments. Na⁺, Cl⁻, and Ca²⁺ levels increased, but K⁺ concentration was reduced. The PCA-biplot analysis revealed that Fars and Khorasan populations had different responses to salinity. The length of rhizomes, flavonoids, phenolic compounds, and soluble carbohydrates of two licorice populations and salinity stress had the largest amount of difference of traits. Thus, the elevation of osmolytes, the nutrient balance, and the antioxidant capacity lead to protect of licorice under osmotic, ionic, and oxidative stress, which is caused by the salinity stress. Accordingly, G. glabra can withstand salinity up to 600 mM NaCl and be introduced as a halophyte plant.

Introduction

Salt stress is the most critical and harmful factor that affects reducing plant growth, development, and yield (Hadi and Karimi, 2012; Saidimoradi et al., 2019). Soil salinity is caused by excessive use of fertilizers, irrigation of plants with low-quality water, high evaporation of soil, as well as applied soils with little drainage (Hadi and Karimi, 2012).

Three major effects of salinity stress are ion toxicity, osmotic, and oxidative stress (Hniličková et al., 2017). Salt stress reduces plant growth and development, which leads to altering in water and mineral uptake, reduction of net photosynthesis and membrane stability, cell division inhibition, damage on photosynthetic pigments, as well as an increase in respiration rate (Saidimoradi et al., 2019).

The short or long-term impacts of salt stress on the photosynthesis process are complex. Plant biomass is an indication of net photosynthesis (Tavakkoli et al., 2011). Prior researches demonstrated the impacts of salt stress on photosynthetic pigments, photosystems, CO₂ assimilation, stomatal conductance, electron transport system, and water use efficiency that finally lead to a reduction of the photosynthetic process (Mahouachi, 2018). Many studies reported that the chlorophyll content and the efficiency of photosynthesis are reduced in Miscanthus giganteus (Płażek et al., 2014) and Pistacia vera (Hajiboland et al., 2014) under the high NaCl content in the rhizosphere.

The osmotic stress caused by salinity leads to reduce osmotic potential, turgor damage, and prevention of water absorption by plants (Mahouachi, 2018). Relative water content (RWC) is a determining factor of the physiological process and survival of plants (Sarabi et al., 2017). Accumulation of compatible solutes is the usual response of plant adaptation to changes in the external osmotic potential and protects the membrane against various stress (Hajiboland et al., 2014; Saidimoradi et al., 2019).

The major ionic stress is related to high salinity and Na⁺ toxicity. Some plant species are also vulnerable to Cl⁻, a key anion in saline soils (Hadi and Karimi, 2012). The high concentration of Na⁺ and Cl⁻ in saline soils leads to competition between sodium and other inorganic nutrients, ion imbalances, and nutrient deficiencies, such as K⁺, Ca²⁺, and Mn²⁺ (Hadi and Karimi, 2012; Hajiboland et al., 2014). Plants can withstand salinity by restricting the absorption of toxic ions and compartmentation of the ions in cell vacuoles (Płażek et al., 2014). Under the high salt levels in the rhizosphere, further selective uptake of K⁺ rather than Na⁺ by plant cells, Na⁺ compartmentation, and distribution in shoots represent to be the important mechanisms for keeping sufficient K⁺ levels (Mahouachi, 2018).

Cell membrane stability has been widely considered as an index of cellular damages under the high salt concentration because the salinity leads to produce reactive oxygen species (ROS) in plants (Sarabi et al., 2017; Saidimoradi et al., 2019). Both non-enzymatic and enzymatic antioxidant mechanisms have a considerable function in the detoxifying ROS generated from the electron transport chains of mitochondria and chloroplasts (Behdad et al., 2020a).

Phenolic compounds and flavonoids are secondary metabolites that have antioxidant activities due to the presence of free OH groups in their structure (Behdad et al., 2020b). Glabridin, liquiritin, liquiritigenin contents, and antioxidant capacity were different in the licorice rhizomes of different populations from Iran (Esmaeili et al., 2019). Plant responses to soil salinity depend on plant growth stage, plant populations, and the duration and severity of salinity to which the plants are exposed (Behdad et al., 2020a). The production of the plant secondary metabolites is strongly related to environmental factors (Esmaeili et al., 2019; Behdad et al., 2020b).

Glycyrrhiza glabra L. (Licorice) is the oldest and famous medicinal plant belonging to the Fabaceae family (Shirazi et al., 2019). The rhizomes of Glycyrrhiza species are one of the valuable and most important sources of biologically active compounds, including glycyrrhizin, phenolic compounds, flavonoids, different sugars, saponins, sterols, starches, amino acids, tannins, and essential oils (Behdad et al., 2020b). G. glabra grows as a wild plant from North to South of Iran. The major producer and exporter of licorice rhizomes on a global scale are Iran, especially Shiraz, the region located in the southwest of Iran (Esmaeili et al., 2019). Because of the high economic and medical importance of licorice, the demand and export of licorice rhizomes have increased, although the availability of wild licorice has declined (Esmaeili et al., 2019; Behdad et al., 2020b). On the other hand, there are 6.8 million hectares of salt-affected agricultural lands in Iran (Moameni, 2011). Thus, it's necessary to study the cultivation of licorice rhizomes in saline soils.

To the best of our knowledge, limited information is available on the impacts of salt stress on licorice cultivated from seeds (Amanifar et al., 2019; Shirazi et al., 2019). According to the wide distribution of G. glabra in Iran and its tolerance to environmental stresses, evaluation of the response of licorice populations to salinity stress for the cultivation extension of this important medicinal plant in regions with saline soil of Iran is essential. Moreover, the major proliferation of licorice is through its rhizomes (Behdad et al., 2020a). The objective of this research was to survey the impacts of different treatments of salt stress (0, 100, 200, 400, 600, and 800 mM NaCl) on growth parameters, leaf relative water content (LRWC), chlorophyll index (SPAD), gas exchange parameters, membrane stability index (MSI), antioxidant activity, phenolic compounds, flavonoid, and soluble carbohydrate contents, as well as, element contents of Fars and Khorasan rhizomes grown with two different diameters.