INTRODUCTION:

Environmental stresses are the main constraints for sustainable plant productivity and crop yield, for food and nutritional security to the human population in developing countries, particularly in India where arable land per capita is shrinking while human and livestock population is steadily increasing. Consistent increasing trend of human and livestock population worldwide and particularly in Asian subcontinent primarily in India, China and neighbouring countries, an immediate priority and mandate for agriculture is to achieve maximum production of food and feed in environmentally sustainable and cost-effective manner with decreasing arable land area. This scenario has reflected the importance and direct correlation of environment, economy and development with plant productivity and efficient utilization of natural resources.

Amongst several factors that influence plant productivity and crop yields, perhaps the consistent losses due to abiotic components particularly salt and water stress under climate change situation are of paramount importance^1,2. Plants are considered to be under stress when they are subjected to less than ideal growing conditions. Stress in terms of energy cost can be defined as the reduction of energy acquired by plants or redistribution of energy from growth into stress defense³. In environment, plants have to face two types of stresses –biotic stress and abiotic stress. The biotic stress includes damage caused by living organisms such as bacteria, fungi, virus, weeds, pathogens etc. whereas, abiotic stresses are physical stresses which include high and low temperature, drought, UV stress, salinity etc. Among the abiotic stresses, salinity is one of the major threats which is faced by the field crops^4,5,6. In both soil and water, salt stress can severely affect the plant growth and productivity particularly in arid and semi-arid regions. Soil salinity is responsible for limiting global crop production by affecting large areas of the world’s cultivated land⁷.Reduction in the major crop yield due to salinity is more than 50% ⁴. Area thought to be salt affected is 21.5 million hectares in Asia alone, with India having 8.6 million hectares of such area⁸. High-salt conditions of the soil, high atmospheric temperature as well as freezing temperatures contribute to water deficit or stress. Thus, it results in efflux of cellular water, leading to plasmolysis, inhibition of photosynthesis and eventually cell death. In a plant, the water deficit may accelerate the increase in the concentration of toxic ions and the loss of protective hydration ‘shell’ around the protein molecules. It has also been proposed that plants subjected to water deficits are more vulnerable to their parasites^9,10,11.

After wheat, rice is the 2^nd most important crop in the world and covers about 9% of the Earth’s cultivable land. Rice offers food for more than 50% of world’s population (~3 billion people), the percentage use of rice for direct human consumption in the world is 85%, and the percentage of world’s rice crop traded in the world market is 6-7%. Thus, rice is one of the world’s most important staple crops^12,13. According to the most recent official data, with a production volume of over 210 million metric tons in 2017, China was the world’s leading paddy rice producer, followed by India. In India, rice cultivation is predominant since ages. India ranks first in area and second in production of rice after China, which is the largest producer of rice¹². Asia accounts for 90% of the world’s rice production. World’s largest exporters of rice are Thailand, China, Vietnam and United States. 1.5% of the world’s rice crop is produced by United States with Arkansa, California and Louisiania producing 80% of the US rice crop. Other major rice exporting nations included Indonesia, Bangladesh, Thailand, Vietnam and Mynmar (FAO STAT 2017). Rice plants are now being in phytoremediation of metals from soils and rice plant parts are used in solid state fermentation etc.^14,15,16

Rice (Oryza sativa) belongs to the family Poaceae and genus oryza with 22 cultivated wild species. Oryza sativa and oryza glaberrima L. are the cultivated rice species. Oryza sativa is grown worldwide while oryza glaberrima has been cultivated for about last 3500 years in West Africa. Rice has a basic chromosome number of n = 12. The species can be diploid as well as triploid. Both Oryza sativa and oryza glaberrima L. are diploid species (2n= 24). Considerable amount of recommended zinc and niacin is provided by rice¹⁷. The digestibility of rice protein is very high (88%) and thus is considered as biologically richest protein. Energy provided by rice to global human per capita is 21%. In Asia, calories from rice are particularly important, especially among poor people where it accounts for 50-80% of daily caloric intake. Two methods are used for rice cultivation. First being transplanting and second being direct seeding. The later method is employed and preferred by farmers nowadays¹². Although rice is grown with different production systems under different conditions, however the most common method which is used worldwide is submerged in water. Rice is the only cereal crop that grows in standing water for such long time. 57% of the rice is grown on irrigated land, 25% on rainfed lowland, 10% on the uplands, 6% in deepwater and 2% in tidal wetlands¹⁸. The flooded rice paddy provides home to fishes, amphibians, mollusks and crustaceans; thus, it is a field of biodiversity. The diet of poor and malnourished people that farm rice living in low and middle-income countries can be incorporated with protein by using this aquatic biodiversity¹⁹. As rice is grown in different environmental conditions using different production methods, different rice varieties have different characteristics which make one variety of a particular area more popular than another variety. According the size of the grain, rice can be short, medium or long grain size. Rice can also be waxy (sticky) or non-waxy. The colour of the rice grain also varies, which can be brown, red, white, purple and black ²⁰.

Plant stress is the measure of environmental effects on the health of plants. Out of 7 ×10⁹ha arable land of the world about 10% consists of brackish soils. Salinity and other parameters affects the germination and plant growth and hence reduces the crop yield^{21- 24}. Salt stress causes ionic toxicity, osmotic stress, nutritional imbalance, oxidative stress etc. in plants^25-34. Reduction in seedling growth and leaf expansion causing reduced photosynthesis are some the physiological effects of salt stress. Yield components are severely affected by salt stress. Panicle length, spikelet number per panicle and grain yield were greatly affected when treated with salt ³⁰. Emergence of panicle is also delayed along with flowering. Decreased seed set also resulted through reduced pollen viability when salt stress was induced³¹. At low concentrations, salt suppresses plant growth and at higher concentration can cause death³². When there is high exchangeable sodium in the soil, that condition is known as sodicity, which results in deterioration of the physical properties of the soil³³. Concentration of salt in soil is measured by electrical conductivity (EC), with SI unit dSm⁻ˡ. Saline soil as defined by USDA salinity laboratory staff is the one having ECe ≥ 4 dSm⁻ˡ which is equivalent to approximately 40mM NaCl. ECe is the electrical conductivity of the “saturated paste extract” (solution made by mixing soil sample with sufficient water to form a saturated paste)³⁴. The rice crop comes under the sensitive division from 0dSm⁻ˡ to 8dSm⁻ˡ according to the classification of crop tolerance to salinity. Salinity threshold level is defined as the maximum allowable salt concentration without reduction in the yield and the percent of yield reduction per unit increase in salinity beyond the threshold is called “slope”. 3.0dSm⁻ˡ is the salinity threshold level of rice with 12% yield reduction per unit increase in ECe (dSm⁻ˡ) above this level³⁵.

According to a report, rice at seedling stage is sensitive to salinity and at vegetative stage, it becomes tolerant and becomes susceptible in terms of grain yield at reproductive phase³⁶. In order to fulfill the need of the increasing population which continues to grow each year the rice production in turn needs to be increased. Salinity is an abiotic stress which challenges this. Estimations were done and found that by the year 2025 rice production needs to be increased by 21% in order to feed the teaming millions³⁴. In coastal rice fields of India such as Tamil Nadu state, rice productivity is reduced to a great extend due to soil salinity. To counteract high salt stress, plants have developed different mechanisms such as mineral ions homeostasis and accumulation of compatible solutes such as proline. Moreover, the response of plants to salt stress may depend on genotype, salt type and its concentration³⁷.

Therefore, to ensure future crop production, it is important to screen the genotypes of important crops such as rice which are salt stress tolerant. Also, for successful crop production in saline environment, it is important to identify the sensitivity and tolerance level of different varieties at early seedling stage. Therefore, the objective of this study is to screen the most salt tolerant variety among the five Indica varieties.

MATERIAL AND METHODS:

Seed material:

Five different rice variety seeds used in this study were collected from Kerala Agricultural University. The rice varieties include Karuna, Neeraja, Thekkancheera, Rasmi and Aryan.

Seed sterilization:

Morphologically healthy and uniform seeds of each variety were washed with tap water and rinsed with distilled water. The seeds were then surface sterilized with NaClO for 15 min, followed by several washings with double distilled water.

Salt stress:

Different NaCl concentrations used in this experiment were 50mM, 100mM, 150mM and 200mM. Glass petri plates containing circular whatman filter paper and double layers of tissue paper was used. For each variety 5 petri plates were used: one as control and the other 4 for different salt concentrations. Each petri plate contained 50 seeds. The petriplates were incubated in laboratory conditions and the temperature set at 27^oC ± 2. When the moisture content of the tissue paper declined, the control seeds of each of the 4 varieties were treated with distilled water while, equal volume of salt solutions was added to the seeds kept in test petri plates. The tissue paper was changed after every two days to avoid fungal growth. The experiment was conducted for 20 days. Seeds with protruded radical and plumule through seed coat were considered germinated and germination percentage was calculated by the following formula:

GP = SNG / SNO × 100 %

Where, GP = Germination Percentage

SNG = Number of seeds germinated

SNO = Number of seeds tested

After two weeks of seed germination, the plumule and the radical length was measured. Then plumule and radical length of the seeds without NaCl stress (control) was compared to that subjected to different NaCl concentrations.

RESULTS AND DISCUSSION:

Effect of salinity on seed germination:

Seed germination percentage was decreased with increasing NaCl concentration. At higher concentration of NaCl, inhibition of germination was strongest as compared to control and lower concentrations. Also, at different salt concentrations, various varieties respond differently. Germination percentage at 50Mm showed less variation whereas the salinity effect was prominent above 50mM NaCl concentration i.e, as the NaCl concentration increased, the germination percentage decreased (Table 1). At 200mM salt concentration, seed germination was completely inhibited in Thekkancheera and Aryan with 0% germination percentage, while Karuna, Neeraja and Rasmi showed little germination. Thus, these three varieties are the salt tolerant varieties as they showed some germination at 200mM. Among these three varieties, Rasmi showed highest germination percentage (20%) as compared to Karuna (16%) and Neeraja (10%).

Table 1. Germination of rice seeds under various salt stress conditions.

Variety	Number of seeds inoculated	Seed germination percentage at different NaCl concentration
		Control	%age	50mM	%age	100mM	%age	150mM	%age	200mM	%age
Karuna	50	42	84	36	72	31	62	25	50	8	16
Neeraja	50	42	84	34	68	28	56	22	44	5	10
Thekkancheera	50	40	80	30	60	26	52	22	44	0	0
Rasmi	50	44	88	39	78	32	64	28	56	10	20
Aryan	50	38	76	26	52	22	44	16	32	0	0

Fig.1: Effect of salt stress on seed germination, shoot and root length.

(A)

(B)

Fig.2: Effect of salt stress on (A) shoot length and (B) root length at different NaCl concentrations.

Effect of salinity on radical and plumule length:

Both the radical and plumule showed a decrease in length with the increase in NaCl concentration. Among the five selected varieties, it was observed that Rasmi was least affected. The average plumule length of Rasmi in control was 9.2cm which decreased to 0.8cm at 200mM NaCl concentration. Similarly, the average radical length of Rasmi in control was 6.5cm which decreased to 0.2cm at 200mM NaCl concentration. While, the most affected variety was Thekkancheera which showed an average plumule length of 6.3cm and an average radical length of 5.6cm in the control but, failed to germinate at 200mM NaCl concentration (Fig 1 and Fig. 2).

CONCLUSION:

From this study, it was concluded that increasing salt concentration affects the seed germination and early seedling growth and even causes death at very high concentration. Among the five varieties of rice which were taken for study, it was observed that Rasmi is the most tolerant varieties as compared to other varieties as it showed more germination percentage and was least affected during its early seedling stages at higher salt concentration. Thus, in future, after further physiological and molecular evidences, this variety can be strategically grown as an alternative to other cereal crops, under salt-stressed environments.

ACKNOWLEDGEMENT:

The authors are grateful to the authorities of Lovely Professional University (LPU), Punjab, India for infrastructural support.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 13.06.2018 Modified on 19.06.2018

Research J. Pharm. and Tech 2018; 11(9): 3866-3870.

DOI: 10.5958/0974-360X.2018.00708.4