INTRODUCTION:

The Asian cultivated Rice (Oryza sativa L.), being the most important cereal crop of the World ¹, provides the carbohydrate needs for half of the total world population ², serving more than three billion people ³. It is the second most important crop in the world after wheat, in terms of total area under cultivation and to Corn, in terms of production. Rice is cultivated on 167 million hectares of land worldwide and almost 90% farmland in Asia. In India, rice has been cultivated since time immemorial, mostly from the beginning of the civilization and becomes an important cereal crop in terms of area under cultivation.

The trend of rice cultivation is being accelerated during last fifty years to increase the productivity to ensure the food security of the country having almost 60 % area with proper irrigation method. On average 116.48 million tonnes of rice was produced on 44.16 million hectares of land with average yield of 2638 kg per hectare, as evidenced in 2018-19. In terms of total area under production, India ranks first in the World ⁴.

Among the varied topography, soil and climatic conditions prevailing in Indian landmass, Sundarban, a wet coastal stretch is unique of its kind for sustaining world’s largest mangrove forest. This magnificent estuarine ecosystem with deltaic settings of various creeks and canals fed with the freshwater flow of the river Ganges, Brahmaputra and Meghna covers an area of about 10,263 sq. km, shared between India (38%) and Bangladesh (62%). Islands, the sole landmass of this ecosystem with a number of 102, plays the key role in sustaining the mangrove community in 48, as littoral forest and human settlement in 54, with agricultural practices. The transformation of mangrove dense island to agricultural field was started during the late 16^th century when Collector General Clod Russell framed the plan for lending mangrove dominated island to collect timber through forest clearing mainly for revenue generation and making the lands suitable for cultivation. Many people performed such tedious works with much hurdles and started rice cultivation. The farmlands which were often inundated by the brackish water from the adjoining rivers and creeks, most of the rice varieties were not being grown. Through trials and errors, farmers were able to select some rice varieties having salt tolerant capacity which were cultivated in those lands. Thus, many salt tolerant farmers’ varieties with varied degree of salinity tolerance were developed in Sundarban area of which some are being cultivated in few pockets.

Climate change has caused changes in the natural weather cycle resulting climatic calamities like cyclone, flood which has become more common in the recent past. In such scenario, salinity has become one of the most common stress factors for the rice productivity of the World. It is estimated that more than 6% of the world’s farmlands are affected by the salinity. In India, 6.73mha has been identified as saline stressed agricultural field. West Bengal sharing Indian part of Sundarban has 0.44 m ha land affected by salinity, standing at 4^th position, as a state, after Gujarat, Uttar Pradesh and Maharashtra ⁵.

In the present climate change scenario, the global mean sea level has risen 0.2m between 1901 and 2018 and it is expected to rise 0.2m-0.29m by 2050 and 0.63m to 1.01m by 2100 ⁶. So going by the trend, it is assumed that most of the agricultural fields of the Sundarban region will be under sea water by the end of this century. Besides sea level rise, super cyclone happens with frequent occurrence in the last decade. On average 2-4 cyclones are hitting the region every year, which has adverse effects on life and livelihoods of 4.4 million people living in the islands. As rice cultivation is the major livelihood of these island inhabitants, salt tolerant rice varieties of different salinity regime are to be known for food security in such climatic vulnerable regions. Besides, the salinity factor causes the physiological stress in the rice plant which affects their morphology vis-a-vis growth, development and productivity. Thus, it is very much essential to understand the stress response of some traditional rice varieties (the farmer varieties of Sundarban area) with different salinity gradients showing morphological including morphometric variations. Moreover, how the salt stress is being withstood by different traditional rice varieties in the salinity gradients needs to be understood for selection of salt tolerant traditional rice varieties of different salinity regime. With this perspective, the present study was performed which would be helpful in reintroduction of the selected rice varieties in adverse conditions, especilly in the period of saline water intrusion into the agricultural fields due to sea water rise or cyclones, as a result of climate change.

MATERIALS AND METHODS:

Selection of study site: The experimental fields with three different salinity levels were selected in the village of Jamespur, under Gosaba Block of South 24 Parganas district of Indian state, West Bengal. One field (low salinity) was taken in the middle of the usual agricultural field (E88.862˚, N22.097˚), where local farmers cultivate their crops. This served as the control site. Another field (medium salinity) (E88.865˚, N22.096˚) was selected just beside one canal directly linked with the river Datta, with saline water flow. (Fig. 1) The highest salinity field was taken just beside the said river (E88.864˚, N22.093˚). This village is the last inhabited island in the southern part of West Bengal. The agricultural fields are often found to be at lower elevation than the high tide line for which saline water reaching during high tide. The villagers often experience flooding of its land during the cyclone forming in the Bay of Bengal region.

Rice varieties and Experimental design:

During selection of the varieties, local People’s Biodiversity Register (PBR) was consulted. The PBR is a legal document, prepared by the local people, that contains details of bioresources available in that particular area. Community interaction were held with the local farmers to identify the traditional rice varieties that are still being cultivated in the area. From the list of traditional rice varieties, 3 varieties viz. Malabati, Patnai and Gheus were selected for the present study which have been reported by local people as the capacity to grow in somewhat saline condition. The field trial was done for three years, during the cultivation season of 2020-2022 (June-November).

Analysis of soil physical and chemical characters:

In each site the particle size (clay, silt and sand) was analyzed following Robinson Pipette Method ⁷. The soil and water salinity were measured by Hanna Salinity Meter according to the manufacturer’s instructions. The soil organic carbon was quantified by Black method ⁸.

Plot preparation and management practice:

The experiments were carried out in the above mentioned three sites to investigate the effect of saline stress on the growth and morphological characters of the selected varieties. For each variety, a plot of 10ft×10ft was prepared in each of the stress zone. In total, 30ft×10ft plot for each of the varieties was taken for the experiment.

Estimation of Photosynthetic pigments:

Extraction of chlorophyll was done according to Arnon⁹. The total chlorophyll was calculated with the following formula:

Total chlorophyll (mg/gm FW)

= 20.2 (OD645) + 8.02 (OD663)

Estimation of Antioxidant enzymes:

200 mg leaf samples from the plant material grown on three varied saline stress level sets were collected and ground to a fine powder using liquid nitrogen. The powder sample was extracted at 4°C in a chilled mortar and pestle with 0.1 M phosphate buffer (pH 7.0). The homogeneous mixture was then centrifuged for 20 minutes at 12,000g. The supernatant was kept after centrifugation and utilised as a source of enzymes.

Ascorbate Peroxidase Assay (APX):

The Nakano and Asada method ¹⁰ was used to measure the activity of ascorbate peroxidase (APX) in rice leaves. The quantity of oxidised ascorbate was determined to be mol min^-1g^-1 protein using the extinction coefficient (€= 2.8mM).

Catalase Assay (CAT):

The Cakmak and Horst¹¹ technique was used to determine the catalase (CAT) test spectrophotometrically. Using the molar extinction coefficient, € = 39,400 mM^-1cm^-1, the CAT activity was represented as nmol min^-1 g^-1 of protein.

Superoxide Dismutase Assay (SOD):

The capacity of superoxide dismutase (SOD) to prevent the photochemical reduction of nitro blue tetrazolium (NBT) was used to measure its activity ¹². Enzyme activity was reported as unit mg^-1 protein g^-1 following incubation measurements at 560 nm.

Estimation of Total Protein Content:

The sample's total protein content was calculated using the Lowry’s¹³ technique. The quantity of protein was determined using the standard curve of BSA solution, and the estimated protein was represented as mg protein g^-1 of the fresh weight.

Documentation of the morphological characters:

The morphological characters with alternate options i.e., character states of the selected traditional rice varieties were documented according to the Distinctiveness, Uniformity and Stability characters specified in the guideline by the Ministry of Agriculture and Farmer’s Welfare, Government of India.

Statistical analysis:

Data collected in ten sets are reported as ±SD. The corelation coefficient among salinity stress and morphological characters, Analysis of Variance (ANOVA) and Post Hoc analysis were carried out in R statistical package. In Post Hoc analysis, Duncan’s Multiple Range Test (DMRT) at 5% level was done for comparing the means. The graphs and other figures were presented with the help of GraphPad Prism 9.

RESULTS:

In the present study, soil is the major stress factor for rice varieties. Thus, study plots have distinct salinity variations: usual agriculture plot, designated as low salinity with salinity 0.89±0.02 mS/cm; canal site linked to river, as medium salinity with 4.40±0.19 mS/cm; river site as high with 7.26±0.22 mS/cm. (Table 1)

The soil particle size is mostly similar in low and medium salinity sites but high salinity site has higher value of silt (40.11%) and lower value of sand (31.77%). High salinity site also has lower amount of soil organic carbon in comparison to other sites (Table. 1).

The soil in both the experimental plots were fine loamy soils with following details:

Table 2: Variation of Morpho-metric characters for three consecutive years of three selected traditional rice varieties under varied salinity stress. (LS- Low Salinity 0.89 mS/cm, MS- Medium Salinity 4.4 mS/cm, HS- High Salinity 7.26mS/cm)

Character		*Patnai*		*Malabati*		*Gheus*
		Value	Sd	Value	Sd	Value	Sd
Leaf length (cm)	LS	23.73	1.16	29.76	1.12	31.10	2.84
	MS	20.50	0.41	26.14	0.80	25.40	1.84
	HS	14.06	0.55	22.21	0.63	20.43	1.68
Leaf width (cm)	LS	1.15	0.04	1.23	0.08	0.87	0.06
	MS	0.94	0.03	0.97	0.12	0.74	0.03
	HS	0.64	0.01	0.88	0.02	0.69	0.01
50% flowering day (days)	LS	111.33	0.94	122.67	1.70	121.00	1.41
	MS	113.67	1.25	128.67	3.30	126.33	1.25
	HS	115.67	0.94	130.33	4.19	128.67	1.70
Panicle length (cm)	LS	12.68	0.13	13.50	0.41	15.50	0.71
	MS	11.08	0.72	11.67	0.47	12.77	0.21
	HS	6.40	0.19	7.46	0.14	6.38	0.08
Number of panicle per plant	LS	12.22	1.66	14.50	0.41	8.78	0.57
	MS	8.18	0.88	9.55	0.42	6.61	0.55
	HS	5.86	0.59	8.40	0.08	4.14	0.04
Maturity time (days)	LS	158.00	1.63	166.33	1.25	173.33	1.25
	MS	154.33	3.30	163.67	1.70	169.00	1.41
	HS	155.33	2.49	162.00	2.16	164.33	0.94
Weight of 1000 grains (gm)	LS	25.42	0.06	26.28	0.28	28.47	0.37
	MS	23.98	0.66	23.75	0.39	26.93	0.21
	HS	18.88	0.64	22.50	0.19	21.35	0.19
Grain length (mm)	LS	10.87	0.05	8.12	0.03	7.37	0.05
	MS	10.57	0.05	7.45	0.05	6.78	0.06
	HS	10.38	0.08	7.31	0.08	6.37	0.05
Grain width (mm)	LS	2.79	0.03	3.40	0.04	3.30	0.04
	MS	2.52	0.06	3.10	0.05	2.99	0.02
	HS	2.34	0.03	2.86	0.03	2.83	0.01
Decorticated grain length (mm)	LS	8.05	0.06	5.95	0.03	5.60	0.07
	MS	7.85	0.03	5.78	0.02	5.30	0.03
	HS	7.70	0.02	5.63	0.11	5.12	0.02
Decorticated grain width (mm)	LS	2.46	0.04	2.91	0.03	2.86	0.05
	MS	2.31	0.02	2.78	0.02	2.39	0.02
	HS	2.06	0.04	2.10	0.03	2.14	0.01

Table. 3: ANOVA between salinity and morphological characters for three rice varieties.

Variety	Dependent Variable	Factor	F value	P value	R sq. value	Remark (significant/ non- significant) At less than 5% level
Patnai	Leaf length	Salinity	80.19	<0.0001	0.9639	Significant
	Leaf width	Salinity	159.4	<0.0001	0.9815	Significant
	50% flowering day	Salinity	8.467	0.0179	0.7384	Significant
	Panicle length	Salinity	112.6	<0.0001	0.9741	Significant
	Panicle per plant	Salinity	16.01	0.0039	0.8422	Significant
	Maturity time	Salinity	1.09	0.3947	0.2665	Non-Significant
	Weight of 1000 grains	Salinity	217.1	<0.0001	0.9864	Significant
	Grain length	Salinity	30.62	0.0007	0.9108	Significant
	Grain width	Salinity	54.32	0.0001	0.9477	Significant
	Decorticated grain length	Salinity	34.54	0.0005	0.9201	Significant
	Decorticated grain width	Salinity	69.18	<0.0001	0.9584	Significant
Malabati	Leaf length	Salinity	37.15	0.0004	0.9253	Significant
	Leaf width	Salinity	8.898	0.016	0.7478	Significant
	50% flowering day	Salinity	3.113	0.1182	0.5093	Non-Significant
	Panicle length	Salinity	140.5	<0.0001	0.9791	Significant
	Panicle per plant	Salinity	182.3	<0.0001	0.9838	Significant
	Maturity time	Salinity	3.146	0.1163	0.5119	Non-Significant
	Weight of 1000 grains	Salinity	83.43	<0.0001	0.9653	Significant
	Grain length	Salinity	112.6	<0.0001	0.974	Significant
	Grain width	Salinity	86.39	<0.0001	0.9664	Significant
	Decorticated grain length	Salinity	10.58	0.0108	0.779	Significant
	Decorticated grain width	Salinity	466.2	<0.0001	0.9936	Significant
Gheus	Leaf length	Salinity	11.96	0.0081	0.7995	Significant
	Leaf width	Salinity	10.56	0.0108	0.7788	Significant
	50% flowering day	Salinity	14.38	0.0051	0.8274	Significant
	Panicle length	Salinity	239	<0.0001	0.9876	Significant
	Panicle per plant	Salinity	51.81	0.0002	0.9453	Significant
	Maturity time	Salinity	27.35	0.001	0.9012	Significant
	Weight of 1000 grains	Salinity	394.3	<0.0001	0.9924	Significant
	Grain length	Salinity	166	<0.0001	0.9822	Significant
	Grain width	Salinity	175.9	<0.0001	0.9832	Significant
	Decorticated grain length	Salinity	55.24	0.0001	0.9485	Significant
	Decorticated grain width	Salinity	286.4	<0.0001	0.9896	Significant

Maturity time:

Early maturity in high salinity fields were recorded for all the varieties. Overall, it was less evident in case of Patnai (1.6%) and Malabati (2.6%) but distinct in case of Gheus (5.1%). Statistically, the difference of mean among all three salinity levels were non-significant for Patnai and Malabati but it was found to be significant in case of Gheus (Fig. 2).

Weight of 1000 grains:

Significant differences were observed in weight of 1000 seeds for all three varieties with varied salinity stress. Gheus variety had the highest 1000 grain weight of 28.9 gm whereas in case of Malabati it is 26.55 gm and in Patnai it is 25.5 gm. In case of Patnai, a reduction of 25.74% was observed in high salinity compared to low salinity. In Gheus, 24.9% reduction was observed whereas in case of Malabati, it was 14.39%. Varied salinity had significant effect on 1000 grain weight in Patnai, Malabati and Gheus (Fig. 2).

Length of whole grain:

Longest grain was recorded in Patnai (10.9mm) followed by Malabati (8.16mm) and Gheus (7.42mm) in low salinity plot. The reduction in grain length was most evident in Gheus with 13.6% in high salinity plot compared to low salinity plot. The least affected variety was Patnai with only 4.4% reduction and Malabati with 10% reduction. In Patnai and Gheus variety, the mean difference of whole grain length affected by varied degree of salinity was found to be significant, but in Malabati, significant difference was documented in low and medium salinity plots but non-significant in medium and high salinity plots (Fig. 2).

Width of whole grain:

Among the three rice varieties, Malabati variety had the boldest grain reaching upto 3.44mm, followed by Gheus (3.34mm) and 2.81mm in Patnai. Patnai had highest reduction of width of 16.2% followed by Malabati of 15.9% and Gheus of 14.3%. Difference of mean among different level of salinity were found to be statistically significant in all of the three varieties (Fig. 2).

Decorticated grain length:

The study shows that the decorticated leaf length was moderately affected with the increasing salinity. The highest influence of high salinity over low was observed in case of Gheus (8.4%), followed by Malabati (5.4%) and Patnai (4.4%). Difference of mean values for different degree of salinity on the character were recorded as significant for Patnai and Gheus varieties. In Malabati, mean difference were non-significant between medium and high salinity range but significant in low and medium salinity (Fig. 2).

Decorticated grain width:

Data reveals, Malabati had the highest decorticated grain width of 2.95mm, and 2.91mm and 2.47mm for Gheus and Patnai respectively. Massive reduction of grain width was observed in case of Malabati (27.7%), Gheus (25.3%) and Patnai (16%). The difference of mean in all three varieties in response to varied salinity stress were computed to be significant at p<0.05 (Fig. 2).

Reduction of morphometric characters were most prominent in Patnai followed by Gheus and it was least affected by salinity in Malabati. Though the flowering days was delayed but the maturity days were found to be quicker in varieties grown under high salinity level. In all of the three varieties, seed sizes were found to be of smaller, as well as weight. Deviation of the grain parameters were more distinct in case of Gheus followed by Patnai and Malabati. The morphological characters e.g., colours and other distinct ones were unchanged with the varied salinity levels.

To understand the consequences of different salinity stress on the antioxidant enzymes, CAT, APX and SOD were analysed for their ROS scavenging activities. It was observed that the CAT activity increased with increasing salinity in all the varieties. Highest increase was observed in Patnai variety and less evident in Malabati and Gheus (Fig. 3).

The other ROS scavenger, ascorbate peroxidase, increased abruptly in Patnai with the increase in salinity stress, whereas, in Malabati, it decreased in medium salinity and increased with upscaling of same stress. In Gheus variety, the APX activity was increased with salinity and then dropping moderately at high stress (Fig. 3).

The SOD concentration was found to be high in all three varieties with increasing salinity. With the increasing salinity amplitude, highest activity of SOD was recorded in Malabati variety. Patnai coming at second and Gheus at last, for the same assay (Fig. 3).

DISCUSSIONS:

Rice is the most important cereal consumed by large number of people around the world. Unlike wheat, barley, sorghum and oats, rice is susceptible to salinity, thus a large area of Indian Sundarban experience the harsh effect of salinity intrusion into the agriculture fields. Still this region has been nurturing the diversity of indigenous rice varieties since the transformation of mangrove forest into agricultural lands. It is a common practice in agriculture field, especially in Sundarban area, that the selection of salt tolerant rice variety is based upon the inhibition of root and shoot growth under varied salinity. In this study, three traditional rice varieties were cultivated in farmer’s fields under different salinity stress conditions. The varieties showed difference in growth parameters like leaf length and width, length of panicle, maturity time and other grain characters, as well as photosynthetic pigment and antioxidant enzyme variation. When plants are grown in saline conditions reduction in overall growth is a common response due to the inhibitory effect of salinity on the amount of photosynthesis and stomatal conductance resulting excess sodium ion intake that hinders critical physiological parameter and biochemical processes ¹⁴. When faced with high salinity level even the tolerant plant species also show inhibition of growth ¹⁵.Rice is considered as a salt sensitive species. Though all of the three varieties studied were salt tolerant ones, still Malabati variety showed to be the most tolerant, followed by Patnai and Gheus. Malabati variety showed better initial vegetative growth expressed by less reduction of leaf length and width under increasing salinity stress. During the field experiment, 50% flowering time was recorded to be delayed by 4.33 days in Patnai and upto 7.66 days in Malabati and Gheus. This may be due to both Malabati and Gheus varieties have long maturation time (160-165 days for Malabati and 170-174 days for Gheus) and Patnai matures comparatively earlier at 155-158 days. In the reproductive stage, number of panicles per plant as well as length of panicle were reduced owing to insufficient vegetative growth and reduced effective leaf area of the flag leaves. Malabati had better yield in terms of number of panicles per plant than that of Gheus and Patnai. Early leaf senescence due to the salinity stress were observed in all the varieties, as reported by various other authors ^16,17.

In Patnai, reduction of total chlorophyll was observed but in other two varieties, the pigment increased marginally under medium stress, due to their ability to tolerate salinity. Generally, the decrease of chlorophyll in plant cell under high salinity condition affects plant growth and development causing reduction of photosynthetic efficiency resulting to less productivity ¹⁸. However, in the present study, Malabati and Gheus showed increase in chlorophyll content under medium salinity stress due to more salt tolerant capacity. Whereas, Patnai showed the reverse due to its sensitivity to higher salinity stress.

Na⁺ toxicity causes production of reactive oxygen species or ROS. Throughout the evolutionary process, plants have developed ways to protect the cellular structures from oxidative damages by means of ROS scavenging via different antioxidant enzymes. Under hard salinity stress increased level of Catalase (CAT) and Ascorbate Peroxidase (APX) helps the plant to achieve better stress tolerance¹⁹.Hydrogen peroxide gets converted to H₂O and O₂ with the help of catalase. Ascorbate peroxidase regulates the concentration of intercellular H₂O₂ to a non-toxic level, as it has higher affinity towards H₂O₂ than Catalase. Increase of specific plant antioxidant levels have been positively corelated with the decreased oxidative damage and improvement in salinity tolerance of plants ^20,21.

In the present study, based upon the performance of three rice varieties in three different amplitude of salinity Malabati seemed to be most tolerant variety followed by Gheus and Patnai. Being most dynamic ecosystem, Sundarban and its adjoining areas are most valuable especially in present climate change scenario. The recent trend of frequent climatic catastrophe like storms, floods, cyclones adversely affect the landscapes, lives and livelihoods. The saline water intrusion into agricultural fields causes its non-fertile nature leading to zero productivity. The findings of the present study would be helpful for the farmers of the Sundarban and other coastal areas through the adaptation of such tolerant rice varieties especially during post-climatic catastrophe.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

ACKNOWLEDGEMENTS:

The authors would like to convey their sincere thanks to the Chairman and other officials of the West Bengal Biodiversity Board for their support. The authors are indebted to the farmers for extending whole hearted support to conduct the field work.

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Received on 24.12.2023 Modified on 18.03.2024

Research J. Pharm. and Tech 2024; 17(9):4582-4590.

DOI: 10.52711/0974-360X.2024.00707

Plot	Clay %	Silt %	Sand %	Salinity (mS/cm)	pH	Organic matter %
Low salinity plot	28.48±0.12	38.19±0.42	33.32±0.53	0.89±0.02	6.51±0.05	3.84±0.06
Medium salinity plot	27.70±0.36	38.70±0.36	33.60±0.53	4.40±0.19	6.17±0.06	2.41±0.16
High salinity plot	28.12±0.43	40.11±0.17	31.77±0.29	7.26±0.22	6.02±0.07	1.84±0.12