Salinity is one of the main abiotic factors; it is caused by different factors, including planting agricultural crops near coastal areas. It has a direct impact on the quality of agricultural soils and significantly affects the agricultural potential of crops ^[1][2][1,2]. Approximately 6% of the world’s arable land is affected by salts, representing more than 800 million hectares, with monetary losses amounting to 12 billion dollars in agricultural production ^[3][4][5][3,4,5]. Agricultural land is classified as saline when the electrical conductivity (EC) is 4 dS m⁻¹ (approximately 40 mM NaCl) or more [6].

There are two types of salinization: natural (primary salinization) and anthropogenic (secondary salinization). The first is closely related to the water table with marine origins and the effects of sea intrusions in coastal areas, the primary minerals that form the rocks, the deposition of salts transported by the wind, seepage, upward capillary flow due to evapotranspiration ^[7][8][9][7,8,9]. On the other hand, secondary irrigation is due to poor management and use of poorly adapted soils, with drainage problems and unsuitable for irrigation, incorrect use of irrigation depths and their irregular distribution due to poor irrigation management, as well as excessive and intensive use of amendments or fertilizers and the use of industrial wastes or the use of wastewater for agricultural irrigation ^[10][11][12][10,11,12].

In addition, the intensive exploitation of groundwater resources, with special emphasis on coastal aquifers, induces saline intrusion through the developed artificial canals, the densely meshed system of rivers and/or natural reaches, resulting in the loss of water quality used for irrigation ^[13][14][13,14].

Under conditions of salt stress, plants absorb a large amount of salts, which are transferred from the soil solution to the outer cells of the root system, to the xylem vessels located in the radicle, and in turn, is transported from the roots to the shoots; then to the transpiratory flow through the leaves, which finally inhibits the absorption of nutrients by the plant ^[15][16][17][15,16,17]. Other effects caused by the presence of these salts on agricultural surfaces are a reduction in plant expansion, root vigor, inhibition and the retardation of growth and development, accelerated wilting, inhibition of the photosynthetic process, loss of turgor, cellular pH instability, accumulation of reactive oxygen species (ROS), membrane damage, ionic toxicity, osmotic imbalance, water imbalance, among others ^[18][19][20][18,19,20].

The main cause of salinization is sodium chloride (NaCl), which is abundant in most agricultural soils and is highly soluble; it limits the productivity and quality of areas devoted to agricultural cultivation internationally. Excessive concentrations of these salts and the deficit of water resources are factors that cause the conversion of fertile fields into marginal ones. The Food and Agriculture Organization of the United Nations (FAO) estimates that the impact of salinity on agricultural land amounts to more than 33% ^[21][22][21,22].

Various physiological, biochemical and molecular processes, water relations, transpiration, photosynthesis, cellular homeostasis, hormonal and enzymatic activities and gene expression patterns in plants are negatively affected by sodium salt stress. An accumulation of sodium salt causes an increase in soil pH and alkalinity, which in turn leads to osmotic stress and nutrient deficiency in plants due to its interference with the uptake of nutrients such as phosphorus, manganese, zinc, iron and copper ^[23][24][23,24].

Besides the osmotic and ionic stress induced by NaCl salt stress, this in turn causes other secondary stresses, for example, nutritional imbalances and oxidative stress cause the creation of reactive oxygen species (ROS) in plant radicles, such as hydrogen peroxide (H₂O₂), superoxide (O₂) and hydroxide (OH) [25]. In addition, several biological processes are modified by the influence of high salt concentrations, such as germination, seed vigor, vegetative growth, flowering and fruit development [26].

Rice (Oryza sativa L.) is currently the main source of food for millions of people as the second most cultivated cereal in the world. Unfortunately, the poor management of soil resources, an increase in the presence of pathogens and the accumulation of phytotoxic substances affect the productivity of this crop, which still does not meet the existing demand ^[27][28][27,28].

Sodium salt stress is one of the factors that cause the greatest damage to crop growth, development and yield. Especially in the rice crop, this factor represents the main limiting factor in its productivity; the vegetative, reproductive and grain-filling stages are the most prone to this stress. Among the main symptoms caused by NaCl on O. sativa are the white tips of affected leaves, a decrease, retardation and irregular growth of seedlings, a reduction in tillering and, in severe cases, the death of this crop. This stress causes a significant reduction in the number of stems per plant, the number of spikelets per panicle, fertility, length and the number of panicles ^[29][30][29,30].

The complex mechanism of salt tolerance in soils involves responses at both the cellular and molecular levels. Therefore, it is necessary to urgently develop and investigate different methods and strategies for the elimination of the toxic effect of this stress [31].

Cuba has an agricultural area of 8709.3 million hectares and nearly 1 million of these have salinization problems, which represents 9.1% of the country’s surface area [32].

2. Morphological Effects of Sodium Salinity

The main factors that trigger the decrease in growth under salt stress are adverse changes in morphological structures, which undergo physiological changes due to salinity.

2.1. Effect of Sodium Salinity on Plant Height and Root Length

Plant height is a fundamental morphological parameter that under any abiotic and/or biotic stress condition undergoes modifications, which indicate changes in growth and development in the crop. The stomatal closure caused by salinity stress leads to an increase in temperature and a reduction in leaf elongation ^[33][34][34,35]. When the rice crop is exposed to sodium salinity concentrations, cell elongation and cell division are affected, which induces a significant reduction in the growth and productivity of roots and leaves ^[35][36][36,37]. Such stress induces a high uptake and accumulation of sodium (Na⁺) in the root zones of rice and, in turn, a low uptake, translocation and antagonistic accumulation of potassium (K⁺), which reduces the ability of the plant to perform osmotic adjustment and maintenance of turgor by suppressing plant growth or inhibiting metabolic activities. Direct competition between K⁺ and Na⁺ in the plasmalemma, impairment of the K⁺ transport process in xylem tissues due to Na⁺ and/or root leakage of K⁺ induced Na⁺, are some of the main causes of decreased tissue K⁺ concentrations ^[37][38]. Apoplastic leakage is the most important pathway in the cultivation of O. sativa because, being an aquatic species, it has limited control over water (H₂O) loss from cell to cell. Large gaps develop in the cortical parenchyma of the root zone of this cereal to ensure oxygen transfer H₂O from cell to cell. In the cortical parenchyma of the root zone of this cereal, large lacunae develop that ensure the transfer of oxygen. The entry of Na⁺ through the roots and the movement of Na⁺ to the leaves causes competition in the crop for K⁺ uptake and thus induces K⁺ deficiency in the plant, which activates the transport system for ions that have high affinity for K⁺ and low affinity for Na^{+ [38]}[39].

2.2. Effect of Sodium Salinity on Seedling Growth

Plant cells under the influence of sodium salt stress induce a reduction in shoot development and elongation due to dehydration and shrinkage. These modifications result in the development of symptoms in the form of visual lesions, especially in salt stress-sensitive rice genotypes. With general crop growth over a period of weeks and months, lesions and reduced lateral shoot development become clearly visible in those plants under the influence of sodium stress compared to those under non-saline conditions ^[39][40]. The seedling stage in the growth cycle of the crop in question is the most sensitive to the effects of salinity; there are several reports in which it is expressed that in this stage there is a significant reduction in the growth of shoots and roots. A prolonged saline exposure of rice cultivars induces a more rapid senescence of the leaves; the visible symptoms of this process begin 3–4 days after the crop is exposed to the effect of salts as yellowing and necrotic lesions on the tips of the oldest leaves ^[40][41]. The most critical stage in seedling establishment is seed germination, as it determines successful crop production. At these stages, it is particularly important to understand plant responses, in order to elucidate the mechanisms of plant resistance or sensitivity to salinity and its super-survival. There are reports of salt-sensitive cultivars of O. sativa, in which concentrations of 100 mMol NaCl or higher affect the germination of this cereal ^[41][42].

2.3. Effect of Sodium Salinity on Rice Leaf Growth and Mortality

The anatomical characteristics of leaves, such as the thickness of the leaf and mesophyll tissue, undergo significant changes when exposed to different concentrations of salt. The histological characteristics of the bundle, such as length, width, the thickness of phloem tissue and the diameter of the metaxylem vessel also undergo modifications due to the influence of this stress ^[42][43][43,44]. Salt stress changes in plant structural components, including leaf structure, are closely related to the physiological and biochemical activities of the leaves. Decreased photosynthetic rate, ultrastructural and metabolic damage and sequential leaf death are closely correlated with salt accumulation in expanding leaves. In rice plants subjected to sodium salt stress, leaf cells can also be damaged by transpiration and thus lead to growth inhibition in the crop ^[44][45][45,46]. There are reports that a greater increase in salt stress in the early seedling stage increases leaf mortality in rice. Leaf mortality at this stage varies from 0 to 100% when exposed to sodium salts for more than one week, causing in a short time, a reduction in the growth and development of the crop ^[46][47]. NaCl stress may be the reason for low leaf number by inhibiting leaf primordium formation. In the tillering stages of some rice varieties, leaf area indices and leaf area are also inhibited due to the effects of sodium salinity ^[47][48]. Several studies report on growth damage and leaf mortality due to the inhibitory effect of salinity, where a reduction in the growth and longevity of seedling leaves is appreciable after exposure to concentrations of 50 mMol NaCl ^[48][49].

2.4. Effect of Sodium Salinity on Vegetative and Reproductive Phases

In the morphophysiological parameters of the rice crop, the main effects of salinity stress are the inhibition of seed germination, a delay and reduction in root and shoot growth, a reduction in the number of stems per plant, in the number of grains per panicle and pollen viability, delays in seed establishment and the appearance of sterile spikelets, a reduction in total dry matter accumulation, poor leaf area development and direct effects on the establishment of the crop surface ^[49][50]. The most sensitive stages to NaCl stress in rice correspond to the early seedling growth and reproductive stages. Severe stress (NaCl > 100 mM) causes plants to die before reaching maturity. While in less severe conditions (NaCl < 50 mM), the delay in panicle initiation and flowering is appreciable, which causes a reduction in pollen viability and thus poor seed set. The presence of different Na⁺ concentrations in the panicle negatively affects O. sativa yield parameters such as tillering, number of spikelets, sterility and grain weight ^{[50][51][52][53]}[51,52,53,54]. An increase in salt stress of 5 to 7.5 dS m⁻¹ decreases the growth and fresh weight of rice seedlings. Several physiological parameters such as photosynthesis and plant growth are affected within a few weeks, depending on the salinity concentrations to which they are exposed. The main causes are changes in the osmotic and ionic state of the cell, a considerable increase in plant growth regulators and organic osmolytes such as abscisic acid (ABA), a loss of membrane permeability, a decrease in the partial pressure of intercellular carbon dioxide (CO₂), a reduction in the turgor and stomatal conductance of the protective cells, direct effect on the efficiency of the photosynthetic process, and a reduction in the photosynthetic process ^{[54][55][56][57][58]}[55,56,57,58,59]. If the tolerance mechanisms are not high enough to exclude salt from the transpiration flow, those leaves with longer transpiration time accumulate high levels of salt to toxic levels, triggering the death of the plant. In rice cultivation, the growth of new leaves is supported by the export of carbon dioxide from mature leaves; a correct balance between the rate of death of mature leaves and the production of young leaves is what sustains the life of the plant ^[46][59][47,60]. The root disposition of O. sativa is damaged by the presence of Cl⁻ and Na⁺ salts. The damage of these salts in this cereal is easily recognizable since the damage generated by Cl⁻ is recognizable by the wide-cut edge of the leaf that indicates burning; Na⁺ causes mottling and curling of the leaf ^[60][61].