1. Introduction

Rice (Oryza sativa) is an important staple food crop worldwide. As the global population continues to rise, rice production also needs to increase. However, global rice production is threatened by climate change^[1]. Rice is considered to be a salt-susceptible species^[2], and its salt tolerance depends on growth stage, organ type, and genotype^[3][4][5]. Generally, the seedling and reproductive stages are more susceptible to salinity than the vegetative stage, roots are more sensitive than other organs^[3], and japonica rice is more sensitive than indica rice^[4]. Salinity stress suppresses photosynthesis and growth, leading to biomass loss, as well as partial sterility, which ultimately results in reductions in rice yield^[6][7]. Therefore, the breeding of salt-tolerant rice cultivars is considered to be one of the most economic options to assure food security.

2. Salt-Tolerant Rice Identification and Evaluation Methods

The development of an efficient and reliable evaluation system is a prerequisite for breeding salt-tolerant rice varieties. The current rice salt tolerance indicators are divided into two aspects: morphological parameters and physiological parameters^[8][9]. The morphological parameters evaluation method is to conduct salt treatments at different growth stages of rice and then observe and record the salt damage symptoms of plants, leaves, tillers, and spikelet fertility^[10][11]. The standard evaluation score of visual salt injury was proposed by The International Rice Research Institute (IRRI); this method scores the salt tolerance of rice from 1 to 9 based on the tiller number, leaf symptoms, and the growth status of the whole plant under salt stress; the lower score (1) indicates tolerant and higher score (9) denotes sensitive genotypes^[12]. However, this method is greatly affected by human qualitativeness, and there are time differences in the rate of leaf death and plant death of different materials. Therefore, this identification and evaluation system cannot completely and accurately reflect the salt tolerance of rice varieties.

Salinity stress induces metabolite changes, and several physiological mechanisms are perceived to contribute to the overall ability of rice plants to cope with excess salts^[13][14][15]. Studies have shown that the Na+/K+ ratio, proline content, hydrogen peroxide, peroxidase activity, and sugars, etc. are affected under salt stress^[11][16]. Therefore, it can be used to screen salt-tolerant rice varieties by comparing the changes of physiological and biochemical indices in rice with or without salt treatment. However, physiological and biochemical parameter methods lack specific evaluation standards, and the measurement of these indices requires corresponding instruments or kits, which are relatively cumbersome to operate.

Rice salt tolerance is a complex genetic and physiological characteristic, and the extent of its sensitivity varies during different growth and developmental stages^[3][4][5]. The salt tolerance during the whole life of rice is a comprehensive reflection of the salt tolerance in each growth and developmental stage, which is closer to production practice and has more practical significance. However, due to soil heterogeneity, climatic factors and other environmental factors may influence the physiological processes; it is difficult to screen salt-tolerant rice varieties at the field level. Hence, screening under laboratory conditions is considered to be advantageous over field screening. Since the salt types in saline-alkali fields are double salts, the salt tolerance identified in the laboratory does not always correlate with that in the field. Therefore, the most reliable way to evaluate the salt tolerance of rice is to compare the changes of morphological parameters and physiological parameters in various growth and developmental stages under salt treatment and normal condition, both in the laboratory and in the field.

3. Breeding of salt-tolerant rice varieties

Developing elite salt-tolerant rice varieties is considered to be the most economically viable and environmentally friendly method to effectively use saline-alkali land. As a food crop, yield is an important indicator to evaluate the merits of rice varieties. Salt-alkali tolerance in rice is defined as the ability to grow on land with a 0.3% saline-alkali concentration, with a yield of more than 4500 kg per hectare. Through years of hard work, breeders have sought out, collected, evaluated, and developed many salt-tolerant rice resources.

Ceylon (modern-day Sri Lanka) was the first country to carry out the screening and cultivation of salt-tolerant rice varieties, introducing the first salt-tolerant rice variety, Pokkali, in 1939^[17]. Subsequently, India and the Philippines bred a series of salt-tolerant rice varieties, such as Kala Rata 1-24, Nona Bokra, Bhura Rata, SR 26B, Chin.13, and 349 Jhona. Bangladesh bred BRI, BR203-26-2, Sail, and other salt-tolerant rice varieties. Thailand bred the salt-tolerant rice variety FL530. Japan bred the salt-tolerant rice varieties Mantaro rice, Kanto 51, Hama Minoru, Chikushiqing, and Lansheng. The United States bred the salt-tolerant rice variety American Rice. South Korea bred the salt-tolerant rice varieties Dongjinbyeo, Ganchukbyeo, Gyehwabyeo, Ilpumbyeo, Seomjimbyeo, and Nonganbyeo. Russia bred 16 the salt-tolerant rice varieties, including VNIIR8207 and Fontan^{[18][19][20][21][22][23][24][25][26]}. IRRI hosts more than 127,000 rice accessions collected worldwide, providing a rich source of genetic diversity. By evaluating the salt tolerance of these rice varieties, researchers identified approximately 103 varieties that were moderately to highly salt tolerant, including Nona Bokra, Pamodar, Jhona349, IR4595-4-1-13, IR4630-22-2-5-1-3, IR9764-45-2-2, and IR9884-54-3^[27][28].

China began to study the salt tolerance of rice in the 1950s. In the 1980s, China launched a national collaborative research program on the salt-alkali resistance of rice and wheat. During the Seventh Five-Year Plan period (1986-1990), China began evaluating the salt tolerance of rice germplasm. This large-scale national cooperation resulted in some progress. The Liaoning Saline or Alkaline Land Utilization and Research Institute launched a salt-tolerant rice breeding program in the 1970s and cultivated a series of salt-tolerant japonica rice varieties, such as Liaoyan No. 2, Liaoyan 241, and Liaoyan 16. In 1984, the institute developed highly salt-tolerant indica rice varieties 81-210. Since 1989, salt-tolerant varieties, such as Salt-resistant No. 100, Yangeng 29, Yanfeng 47, and Yangeng 228, have been cultivated^[25]. The Jiangsu Institute of Agricultural Sciences in Coastal Areas began identifying and evaluating salt-tolerant rice germplasm resources in the 1980s. From more than 1300 germplasm resources, this group obtained 61 that were salt-tolerant^[26], from which they developed many salt-tolerant materials, such as Yancheng 156, Yandao No. 10, and Yandao No. 12^[26]. In addition, breeding institutes and breeders in China have used existing salt-tolerant germplasms or conventional breeding methods to obtain salt-tolerant rice varieties, such as Changbai No. 6, Changbai No. 7, Changbai No. 9, Changbai No. 13, Jigeng No. 84, and Jinyuan 101^[25] [42].

Sea Rice 86 (SR86) is a new cultivar domesticated from a wild strain of rice that was first found in 1986 in saline-alkaline soil submerged in sea water near the coastal region of the city of Zhanjiang in Southeast China^[29]. SR86 showed a significantly higher ability to cope with high salinity than a highly salt-resistant rice variety, Yanfen 47, measured by both germination and salt inhibition rates^[29]. After more than 20 years of breeding and selection, SR86 retains an extraordinary tolerance to salinity and is considered to be a strategic germplasm resource for the development of new rice varieties. SR86 is being used to investigate the mechanism of salt tolerance and effective breeding strategies.

In recent years, our laboratory has also carried out a breeding program to develop salt-tolerant rice. We collected more than 750 rice accessions, including 500 rice accessions from all over the world and 250 salt-tolerant rice varieties from domestic coastal cities, such as Tianjin, Liaoning, Shandong, and Jiangsu. We investigated the salt tolerance of these varieties in Dongying, Shandong Province (37°31′29”N 118°33′57”E), a typical saline-alkali field in China. Briefly, thirty-day-old seedlings were transplanted to a normal field or saline field (0.35% NaCl, pH 8.2) at a spacing of 20 cm×15 cm, and all agronomic traits were performed at the maturity stage. Through comparing the agronomic traits of the normal field and saline field, we selected a series of varieties with excellent agronomic traits and salt tolerance (Table 1) (unpublished data). Using these varieties, we made more than 300 hybrid combinations, generated more than 100 salt-tolerant genetic populations, and identified more than 1000 high-yielding salt-tolerant recombinant inbred lines.

Table 1. Rice varieties with excellent agronomic traits and salt tolerance results.

“Control” indicates that the variety was grown in a normal field. “Salt” indicates that the variety was grown in a field containing 0.35% NaCl and pH 8.2 throughout its life cycle. “DYST” means Dongying Salt Tolerance.