**1. Introduction**

### **1.1 Background**

Rice (*Oryza sativa*) is the staple food of an estimated 3.5 billion people from East and South Asia, the Middle East, the West Indies, and Latin America, accounting for 50–80% of their daily calorie intake [1]. It is also the primary source of income and

employment for more than 200 million households in developing countries [2]. Rice is the main staple providing over 45% of the total caloric and 40% of the protein requirements for the Sri Lankan population [3, 4]. In the country, rice cultivation occupies 16% of the total land area and 34% of the total cultivated area whereby 11.6% of the total local population and 32% of the total labor force are directly engaged in the rice sector [5]. The rice sector contributes 7% to the national agricultural GDP [6], and therefore is associated with food and nutrition security and is critical for generating livelihood income for the Sri Lankan population.

Salinization of arable soils is an increasing challenge in global agriculture. Worldwide about 20% of total cultivated and about 33% of irrigated agricultural lands are afflicted due to soil salinity [7, 8]. Soil salinization is aggravated by the adverse effects of climate change causing abandonment globally of 0.3–1.5 million hectare year<sup>1</sup> [9]. Consequently, an estimated 50% of the arable land would be salinized by the year 2050 [10]. The cost of global crop loss owing to salinization is estimated to be USD 27.3 billion [11]. Rice, with a threshold of 3 dSm<sup>1</sup> for most cultivated varieties, is very sensitive to salinity [12], and rice cultivation is extremely vulnerable to soil salinization. In moderately saline areas, the yield loss of rice is 10–15%, whereas in highly saline areas, the yield was reduced by 30–45% [13]. Soil salinity has become a major limiting factor for local rice production, especially in the irrigated farming systems in the north-central and eastern plains, and in the rain-fed systems in the west coast and the Jaffna peninsula [14]. The net income of rice production is reduced up to 22% and 43% in moderate and highly saline areas, respectively in Sri Lanka [15], and therefore salinity has a significant impact on the economic sustainability of local rice production systems.

### **1.2 The concept of soil salinity**

Saline soils have an excess accumulation of Na<sup>+</sup> , K<sup>+</sup> , Ca2+, Mg2+, HCO3 , Cl, NO3 , SO4 2 , and CO3 <sup>2</sup> or mixtures of these ions [16], thereby affecting the normal functions of plant growth. Based on the physiochemical properties: electrical conductivity (EC), exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR), and pH, saline soils can be categorized as saline, sodic (alkali), and salinesodic soils. Optimal soil conditions for crop growth are EC <4 dSm<sup>1</sup> , pH = 6.5–7.0, SAR <13, and ESP < 13 [17]. Saline soils are characterized by EC of saturated extract >4 dSm<sup>1</sup> , pH <8.5, SAR <13%, and ESP <15% of the exchangeable cations. However, saline soils often are in normal physical conditions with good structure and permeability, and therefore with proper management measures can be used for crop cultivation. In contrast, sodic soils are low in total salts but are characterized by high exchangeable Na<sup>+</sup> . Sodic soils record high ESP >15%, pH > 8.5, and SAR >13%, but EC is often <4 dSm<sup>1</sup> [18, 19]. High levels of sodium and low total salts result in dispersed soil particles, and poor physical properties and sodic soils are sticky when wet, but hard, cloddy, crusty, and nearly impermeable to water when dry. Salinesodic soils contain large amounts of total soluble salts but greater than 15% of exchangeable Na<sup>+</sup> . The pH is <8.5, SAR >15%, and the EC is >4 dSm<sup>1</sup> . The physical properties of saline-sodic soils are good as long as an excess of soluble salts is present.

Soil salinity can arise due to natural processes causing primary salinity or due to manmade secondary salinity. In field conditions, low salinity *viz.* EC 2–4 dSm<sup>1</sup> can arise from natural salinity and/or irrigation salinity. Species with low to moderate salt tolerance can be grown successfully in low saline soils. Moderate to high salt*The Scale and Complexity of Salinity Impacts on Sri Lankan Rice Farming Systems:… DOI: http://dx.doi.org/10.5772/intechopen.112651*

tolerant plant species can be grown when the EC is between 4 and 8 dSm<sup>1</sup> [20] usually present in water logged irrigated conditions. Under high salinity with an EC value of >9 dSm<sup>1</sup> only halophytes can be grown [21], and therefore the choice of crops is a useful measure of salinity management. Rice expresses low tolerance and based on current guidelines rice yields decrease by 12% for every unit of (dSm<sup>1</sup> ) salinity increase above 3.0 dSm<sup>1</sup> [22, 23]. Detailed analysis of the salinity problem including associated intricate mechanisms, therefore, can help in developing effective management practices including the selection of tolerant rice varieties.

#### **1.3 Major factors underlying salinity tolerance**

Salinity tolerance in plants involves a number of traits that act in isolation (independent) or in combination (less independent) [24, 25]. The adverse effects of salinity on plant growth are generally associated with the osmotic potential of the soil solution and the high level of toxicity of sodium (and chloride for some species) that causes multiple disturbances in crop metabolism, growth, and development at the molecular, biochemical and physiological levels [26]. Plant response to salinity is expressed in two major phases: the initial, rapid osmotic phase that inhibits growth and, a later, slower ionic phase that accelerates tissue and organ senescence [27]. Salinity-induced osmotic effects reduce plant biomass and yields; however, selected ions, such as Na<sup>+</sup> , Cl, Ca+2, and Ba+ , cause additional injury and crop damage [28]. Accordingly, three distinct salinity response mechanisms were described in tolerant germplasm, including osmotic stress tolerance, Na<sup>+</sup> exclusion from photosynthetic and other sensitive tissues, and tissue tolerance against accumulated Na<sup>+</sup> and possibly also accumulated Cl [27].

Rice is relatively tolerant to salinity at germination and late vegetative growth, compared to the early seedling stage (3-leaf stage) and reproductive stage (pollination and fertilization). Therefore, apparently, plants at different developmental stages may express one or more of the different tolerance mechanisms. However, there is a poor correlation between the tolerance mechanisms expressed at the two most salt-sensitive stages, the early seedling and reproductive stages [27]. Multiple tolerance phenotypes, associated mechanisms, and genes were identified from different salinity-tolerant germplasm at different development stages [3, 29]. Therefore, a thorough understanding of the molecular mechanisms associated with tolerance traits, and advanced technological innovations to incorporate the traits into elite varieties can accelerate breeding programs targeting saline tolerance.

This chapter aims to review the literature available on the nature and the scale of soil salinization problems in local rice farming environments. The analysis, thereby, would provide actionable insights and potential targets to reduce the genetic vulnerability of elite germplasm and to improve soil salinity management strategies in local rice farming and production environments.
