Current knowledge about Na2SO4 effects on plants: what is different in comparison to NaCl?

In some areas of the world, high levels of sodium sulfate (Na₂SO₄) are found in the soil together with sodium chloride (NaCl). However, most studies on salinity are performed utilizing only NaCl as a salinizing agent. Generally, plant species have different tolerance/susceptibility responses when grown in the presence of these salts. Some studies showed that Na₂SO₄ seems to be more inhibitory than NaCl for the growth of species such as barley, wheat, sugar cane, beet, tomato, wild potato, and others. However, studies focusing on how Na₂SO₄ can affect the biochemical and physiological processes of plants are very scarce. This review provides an overview on the effects of Na₂SO₄ on different crops and plants species with a special emphasis on the tolerance/non-tolerance mechanisms of the halophyte Prosopis strombulifera under elevated NaCl and Na₂SO₄. A better understanding of the tolerance mechanisms in this particular species will help to identify cultivars of crop species that are more tolerant to Na₂SO₄. This knowledge could be used to extent cultivation of certain crop plants on Na₂SO₄ containing soils.

Na2SO4 and NaCl salts differentially modulate the antioxidant systems in the highly stress tolerant halophyte Prosopis strombulifera

Prosopis strombulifera (Lam.) Benth. is a halophytic shrub abundant in high-salinity areas in central Argentina, with high tolerance against NaCl but strong growth inhibition by Na₂SO₄. In the present study, the modulation of the antioxidant systems (enzymatic and non-enzymatic components) was analyzed under different salt treatments (NaCl, Na₂SO₄ and the iso-osmotic mixture) in hydroponic cultivation. Na₂SO₄-treated plants showed strong indications of oxidative stress (H₂O₂ and O₂-• increase). Modifications in antioxidant enzymes activities were observed mainly under Na₂SO₄ treatment, where CAT seems to play an important role in early detoxification of H₂O₂ in roots, whereas SOD and APX have a predominant role in leaves. As part of the non-enzymatic system, 21 compounds were identified in leaves, being polyphenols the most abundant. Control plants contained the major variety of detected phytochemicals (14). Na₂SO₄-treated plants contained 10 compounds and NaCl-treated plants nine compounds, but with a different profile. NaCl-treated plants showed the highest antioxidant capacity. Our findings confirm that different types of salt treatments provoke a differential modulation of the antioxidant systems. Polyphenols and other ROS-detoxifying compounds, in a joint action with the enzymatic antioxidant system, are proposed to have a fundamental role in the cellular protection of P. strombulifera plants under severe oxidative stress. Our findings also highlight the potential of this halophyte as a valuable source of bioactive compounds with high antioxidant activity and health benefits.

New insights into the salt tolerance of the extreme halophytic species Lycium humile (Lycieae, Solanaceae)

Knowledge about Solanaceae species naturally adapted to salinity is scarce, despite the fact that a considerable number of Solanaceae has been reported growing in saline environments. Lycium humile Phil. inhabits extreme saline soils in the Altiplano-Puna region (Central Andes, South America) and represents a promising experimental model to study salt tolerance in Solanaceae plants. Seeds, leaves and roots were collected from a saline environment (Salar del Diablo, Argentina). Seeds were scarified and 30 days after germination salt treatments were applied by adding NaCl salt pulses (up to 750 or 1000 mM). Different growth parameters were evaluated, and leaf spectral reflectance, endogenous phytohormone levels, antioxidant capacity, proline and elemental content, and morpho-anatomical characteristics in L. humile under salinity were analyzed both in controlled and natural conditions. The multiple salt tolerance mechanisms found in this species are mainly the accumulation of the phytohormone abscisic acid, the increase of the antioxidant capacity and proline content, together with the development of a large leaf water-storage parenchyma that allows Na⁺ accumulation and an efficient osmotic adjustment. Lycium humile is probably one of the most salt-tolerant Solanaceae species in the world, and, in controlled conditions, can effectively grow at high NaCl concentrations (at least, up to 750 mM NaCl) but also, in the absence of salts in the medium. Therefore, we propose that natural distribution of L. humile is more related to water availability, as a limiting factor of growth in Altiplano-Puna saline habitats, than to high salt concentrations in the soils.

Biochemical and molecular approach of oxidative damage triggered by water stress and rewatering in sunflower seedlings of two inbred lines with different ability to tolerate water stress

Water stress accelerates the generation of reactive oxygen species, which trigger a cascade of antioxidative defence mechanisms comprising enzymatic and nonenzymatic antioxidants. The aim of this study was to investigate the oxidative damage and the antioxidative defence systems in seedlings of the water stress-tolerant (B71) and the sensitive (B59) inbred lines of sunflower (Helianthus annuus L.) in response to water stress and rewatering. In addition, we characterised the transcriptomic profile associated with enzymatic antioxidative defence. An elevated electrolyte leakage in B59 indicated increased plasmatic membrane permeability, which correlated with greater sensitivity to water stress. In response to water stress, both lines showed an increase in malondialdehyde and H2O2 content but these increases were more noticeable in the sensitive line. In both lines, an increase in enzymatic activity (e.g. peroxidase and ascorbate peroxidase) was not sufficient to overcome the H2O2 accumulation triggered by water stress. Upon water stress, the overall expression level of genes associated with the enzymatic antioxidant system increased in B71 and decreased in B59, which showed downregulated levels of most genes in the shoots. The general profile of phenolic compounds was clearly different between organs and between inbred lines. The B59 line activated nonenzymatic antioxidant responses to counteract the oxidative stress caused by water stress. The tolerance of B71 to water stress could be associated with compensatory antioxidant mechanisms based on the expression of genes encoding enzyme components of the ascorbate-glutathione and redoxin cycles, which contributed to explaining, at least partly, the response of this line.

Growth responses and ion accumulation in the halophytic legume Prosopis strombulifera are determined by Na2SO4 and NaCl

Halophytes are potential gene sources for genetic manipulation of economically important crop species. This study addresses the physiological responses of a widespread halophyte, Prosopis strombulifera (Lam.) Benth to salinity. We hypothesised that increasing concentrations of the two major salts present in soils of central Argentina (Na2SO4, NaCl, or their iso-osmotic mixture) would produce distinct physiological responses. We used hydroponically grown P. strombulifera to test this hypothesis, analysing growth parameters, water relations, photosynthetic pigments, cations and anions. These plants showed a halophytic response to NaCl, but strong general inhibition of growth in response to iso-osmotic solutions containing Na2SO4. The explanation for the adaptive success of P. strombulifera in high NaCl conditions seems to be related to a delicate balance between Na(+) accumulation (and its use for osmotic adjustment) and efficient compartmentalisation in vacuoles, the ability of the whole plant to ensure sufficient K(+) supply by maintaining high K(+)/Na(+) discrimination, and maintenance of normal Ca(2+) levels in leaves. The three salt treatments had different effects on the accumulation of ions. Findings in bi-saline-treated plants were of particular interest, where most of the physiological parameters studied showed partial alleviation of SO4(2-)-induced toxicity by Cl(-). Thus, discussions on physiological responses to salinity could be further expanded in a way that more closely mimics natural salt environments.