**6. Discussion**

Secondary salinization is increasingly becoming a productivity constrain in local rice farming systems in the "mega-cultivation environments" in north central and eastern inland fields and rain-fed systems in the west, southwest and eastern coastal line. The spatial and temporal dynamics in climatic, edaphic, and hydrological regimes, and associated variations in the farming practices create complex and diverse saline stress environments in local rice farming systems. Furthermore, salinity stress coincides with the susceptible 20-day period, the 3-leaf stage and PI of the rice crop life whereby early season salinity coincides with the 3-leaf seedling stage and late season salinity coincides with the PI stage. Secondary salinization of cultivation environments, therefore, is the result of the interaction among multiple factors, including sea intrusion, sub-optimal quality of the irrigation water, injudicious use of irrigation water, suboptimal farming practices, and unavailability of local elite germplasm, and hence is a problem that operates at system level; therefore, system-level interventions are necessary to mitigate and/or to manage the impacts.

Analyzing the variability in physical and chemical field characteristics and crop growth stage enabled zoning of the saline-afflicted production environments into three major categories: (1) late-season salinity in irrigated mega-cultivation environments during the minor cultivation season, where the soil EC values can be higher in selected locations but on average ≥ 7 dSm<sup>1</sup> , (2) late season salinity in rain-fed farming systems in the west, southwest, and eastern coastal line during the minor cultivation season, where the soil EC values can be higher in selected locations but on average ≥ 20 dSm<sup>1</sup> , and (3) early season salinity in rain-fed farming systems in west, southwest. and eastern coastal line during the minor cultivation season, where the soil EC values can be higher in selected locations but on average ≥ 12 dSm<sup>1</sup> . Although, detailed studies were not available, early season salinity during the major season is reported in the intermediate and dry zone rice fields as a result of residual salts from the previous season that was not washed off due to inadequate rainfall from northwest monsoon (personal observation).

Mega-cultivation environments that contribute nearly 70% of the average annual rice production [108] is an important economic enterprise and critical for the local economy and food availability. However, the intensive irrigated farming system is destabilized as a result of secondary salinization. Irrigation practices in the absence of

#### *The Scale and Complexity of Salinity Impacts on Sri Lankan Rice Farming Systems:… DOI: http://dx.doi.org/10.5772/intechopen.112651*

proper drainage management trigger accumulation of salts in the root zone negatively affecting crop productivity. Poor drainage, undulating topography, reuse of irrigation water, and mono-cropping for long periods are recognized as the main causes of salinity developments in inland areas [109]. The seasonal soil salinization and desalinization cycles are driven by the deposition of salt by irrigation and evaporation and the removal of salts from the root zone by washing off due to rain and by gradual leaching. However, imbalances in this seasonal cycle can create residual effects leading to the accumulation of salts in the system, increasing residual salinity levels, and gradual salinization of the soils. As late-season salinity is largely an irrigation-limited issue, optimizing farm irrigation management strategies and robust water governance can reduce salinity impacts significantly and enhance productivity, while minimizing long-term land degradation in rice farming systems. Standards for irrigation and irrigation water quality based on rainfall, edaphic, and crop parameters [110–113] can be useful for efficient use of irrigation water and also sustainable land use. Actionable guidelines for irrigation and management of water quality are, therefore, an immediate need to prevent or minimize rapid soil salinization in mega-cultivation environments.

Furthermore, late season crop in mega-rice cultivation environments is subjected to simultaneous occurrence of multiple abiotic stresses, including water stress, high temperature, and salinity. Sterility of panicles is very common in saline-afflicted rice environments possibly as a consequence of a decreased pollen viability or receptivity of the stigmatic surface or both [114–117]. Salinity stress at the reproductive stage, especially after the booting stage, cause a reduction of grain yield due to the reduced number of filled grains, total number of grains and total grain weight per panicle, 1000-grain weight, and total grain weight per plant [118–120]. Evidence found that the reduction in grain number and grain weight in salinized panicles was not only due to reduced pollen viability but also due to higher accumulation of photosynthates (sugars) in panicle branches and panicle stem coupled with reduced activity of starch synthetase in developing grains. Accumulation of Na+ in floral parts impedes starch synthase activity that transfers glucose to starch primer in developing grains resulting in the failure of the seed set [121]. However, early grain initiation, loss of pollen fertility, failure of pollination, spikelet death or zygotic abortion, changes in carbohydrate availability, and changes in the kernel sink potential are also the consequences of water deficit [122, 123]. The optimum temperature range for normal growth of rice is 22 to 32°C [124] and the threshold temperature of rice during anthesis is 33.7°C [125]. Under local open field conditions, pollen sterility was recorded at 31 0.8°C [126]. Temperature more than 35°C under high relative humidity (near 85–90%) has an effect on the evaporative cooling of spikelet and makes complete spikelet sterility, and thus subsequent yield losses [127]. Therefore, designing mitigation measures would require a holistic approach whereby management practices and technology interventions need to be integrated effectively to avoid, overcome, or prevent the parallel occurrence of multiple abiotic stresses during the late season.

Early minor cultivation season, and occasionally in selected fields during major season, salinity can affect the seedling crops as a result of residual salinity from the previous season that is not washed off owing to insufficient rainfall or due to continuous seawater intrusion causing inundation of coastal low-lying rice lands. Early season salinity devastates seedling fields causing total loss of the crops. Transplanting aged seedlings is commonly practiced in coastal rice fields to avoid early-season salinity whereby farmers transplant 18 days old seedlings instead of 12 days and practice gap filling in case of mortality. Old seedlings with an established root and

shoot system can tolerate salinity stress [128]. In addition, at manageable levels, salinity-afflicted coastal rice lands can produce rice by the cultivation of tolerant cultivars such as At354, At401, and Bw400 and by practicing recommended good agricultural practices (GAP) [113]. These include proper management of saltwater drainage canals, proper management of soil, maintenance of satisfactory fertility levels, pH and structure of soils by maximization of soil surface cover, application of organic manure, using crop rotation, minimum tillage, proper leveling, and adding gypsum (CaSO4), which can cause leaching out of excess sodium [55].

Despite being one of the most susceptible cereal crops, rice cultivation was traditionally practiced in coastal floodplains as an effective way of land utilization in South and East Asia [129, 130], including Sri Lanka. In experimental plots, salinity of soil was controlled below 3% by growing rice. Soil desalination rate was increased from 65 to 74% by combining physical remediation measures with planting of rice [131]. The reason for this counterintuitive practice is that rice thrives well in standing water, which helps in leaching salts from the root zone to lower layers. Persistence of multiple abiotic stresses, including waterlogging in the wet season and soil and water salinity in both wet and dry seasons make managing these rain-fed production systems challenging. Therefore, salt-tolerant varieties, while increasing yields, enable effective use of saline lands. Integrated approaches based on a detailed understanding of the land potential can help in effective land diversification and crop diversification for developing sustainable land use strategies, and cultivation of tolerant rice varieties can be part of the solution.

Zoning of saline-afflicted production environments enables identifying genetic vulnerabilities, and thereby the unique targeted phenotypes (ideotypes) and locally adapted varieties. Ideotype breeding can create multi-trait genotypes to increase genetic yield potential by modifying selected individual traits to improve crop performance in saline production environments. Since salinity levels in local farming systems are transitional with significant yearly and seasonal fluctuations, maximizing resource use and profits in farms would require a clear understanding of the tradeoffs between performance of varieties under benign conditions and under stress. The vigor/ stress response tradeoff of the varieties, therefore, is an important selection criterion for varietal selections. Knowing the vigor/stress response tradeoff and the traits associated with the tradeoff enables designing potential ideotypes. Accordingly, three major ideotypes can be described. Two different ideotypes can target late-season salinity in irrigated mega-cultivation environments *viz*. (1) high yielding, 3 to 3.5 months varieties that are tolerant at >7 dSm-1, for intensive irrigated farming systems affected due to late season salinity (PI stage) and (2) high yielding, 2.5 to 3 months varieties for intensive irrigated farming that can avoid late season salinity. (3) the third, ideotypes targeting the rain-fed system in the west, with southwest, and the eastern coastal line will have to express tolerance throughout the crop life including seedling and PI stages. Varieties with average yield under salinity up to 12 to 20 dSm<sup>1</sup> would be a practical selection criterion for breeding varieties for the semisubsistence rain-fed cultivation systems in these sites.

Climate change and rising temperatures have a strong positive correlation with increasing soil salinity [132]. Sri Lanka being a tropical island is highly vulnerable to climate change whereby the average temperature is increasing at a rate of 0.01–0.03°C per year [133–135] along with significant fluctuations in rainfall resulting in frequent droughts and floods [34, 136–138]. Higher temperature intensifies the excessive deposition of salt on the surface due to evaporation and increased capillary action. Once reached the surface leaching salts below the rooting zone is extremely difficult,

*The Scale and Complexity of Salinity Impacts on Sri Lankan Rice Farming Systems:… DOI: http://dx.doi.org/10.5772/intechopen.112651*

especially under water-limited conditions in the dry zone. Moreover, as an indirect effect of increased temperature sea level rises resulting in increased salinity encroachment in coastal and deltaic areas that have previously been favorable for rice production [138]. The sea level rise in Sri Lanka is 0.3 m by 2010 with a predicted increase of sea level up to 1 m by 2070 [139]. Natural land degradation in coastal areas through sea level rise and increased coastal salinity has changed land use patterns [140]. In the western coast, in addition to rising sea level a combined impact of multiple factors, including groundwater extraction, river damming for hydropower, and riverbed mining has caused sinking of the shorelines and drawing seawater inland.
