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Rice (Oryza sativa) is a crop found to be the grass and is included in the cereals like wheat, oat and barley. It is found to be the native of tropical and subtropical of Africa and Southeastern Asia. It can be defined as the plant which requires more hot and wet climate with 2 to 6 feet height and having needle like pointed leaves bearing grain producing flowers. It is found to be the most important cereal in tropics and temperate areas of the world (Nez-Atienza et al., 2007). Rice is considered to be the largest produced cereal in the world after wheat. Lafitte et al., 2004 stated that the rain fed areas are considered as 45% of the total area which is producing rice worldwide. According to Francois and Mass 1994, rice makes up 43% of the world food production. Half of the world population use rice as its staple food. Rice provides about 30 to 80% of daily calories requirement (Narciso and hossain, 2002). The countries that are producing rice at greatest scales are found in Western and Eastern Asia (china and India). Outside the Asia, Brazil is one of the major rice producing country followed by United State.
Like all other crops, many problems are faced by the farmers to grow rice in the field. These problems are biotic and abiotic stresses that limit the production of the crop. Biotic stresses include the insect pest, pathogen and diseases whereas the abiotic involve the nutritional deficiencies, drought, salinity and heat stress. For the insect pests and diseases, many efforts are carried out and large scale chemicals are being used to overcome these problems, but in case of abiotic stresses, it seems to be difficult to overcome their effects. Salinity is found to be the utmost importance in abiotic stresses because it affects the agriculture production adversly all over the world (Borsani et al., 2003). Rice is found to be the salt sensitive crop (Mass and Hoffman, 1977) and in tropics there is no alternative of the rice crop to grow because it is only crop that will withstand in flooding (Shaheen and Nowotony, 2005). In south and Southeast Asia, 54 million hectares area is found to be effected from salinity (Akbar and Ponnamperuma, 1982) whereas in all over the Asia, 21.5 million hectares area is salt effected (Sahi et al., 2006) which is found to be the major rice production region. Rice crop is observed to be very sensitive to salinity especially in early growth stages and it was observed that the salinity affects the kernel and aromatic characteristics of rice heavily. It also disturbs the antioxidants machanisms and osmoprotectants balance of the plant (Singh et al., 2007). Salinity affects the osmotic and ionic balance of plant with soil (Greenway and Munns, 1980). Rice growth in salinity is dependent on the developmental stages of the plant as rice is relatively more resistant at germination, tillering and maturation whereas seedling stage, early reproduction stage, pollination and insemination are more sensitive stages (Babu, 1985).
Furthermore it is also concluded that the plant physiological age also affects the salinity tolerance capability of plant and it varies from cultivar to cultivar even in one species (Shaheen and Nowotony, 2005). Rice is a C3 plant species (Farquhar et al., 1982). According to Zeng and Shannon, 1999, decrease growth rate, meristem damage in growing shoots and nutritional disorder are the visible effects of salinity. The salinity in root zone affects both seedlings as well as maturity stages. Seedling density is directly related to the panicle density but yield does not increase because spikelets/plant are not filled completely as in standard density of rice plant (kumar et al., 2007). The scientists, in all over the world have done great work both on the rice production as well as about the stages affected abruptly by the salinity. Review of some of the works is given below that show the effects of salt on different stages of rice crop.
Review of literature about the effect of salinity on rice crop:
Singh et al., 2007 studied the effect of salinity on different parameters of salt sensitive as well as the tolerant varieties and compared their response in two concentrations of salt that were 6 dS m-1 and 12 dS m-1. They grew varieties in hydroponics with salt solution. In susceptible varieties, the found reduction in shoot length, leaf area, dry weight, relative water contents, photosynthetic rate and membrane stability contents as compared to resistant varieties. Catalyze enzymes activity was higher in susceptible cultivars. The overall adverse effect of salinity was found on seedling stages.
Zeng and Shannon, 2000 performed an experiment to check the effect of salinity on grain yield and yield components of rice. They grew plants in the green house in sand, irrigated with nutrient solutions. They took 400, 600 and 720 seeds/ m2 of densities in their experiment with three levels of salt concentrations to compare their effect. Salinity has highly significant effect on grain yield, plant strength, seed weight /plant and per panicle and also on spikelet/ panicle. They also studied the effect of higher densities on the grain production in the saline soil and they found that densities did not effect significantly to overcome the losses. According to their work, it was found that the grain yield was not increased by increasing the densities but the seed weight was reduced in saline soil, no matter what the densities were. So they concluded that even under moderate saline conditions, densities of rice plants do not managed the yield losses.
Ehler and Bernste, 1958 studied that different varieties of rice with respect to salt sensitivity and concluded that all are affected significantly by saline soil and their germination stage is found to be more sensitive to the salt. They suggested that due to poor and inadequate internal drainage of the standing water in the rice field, the salt concentration increases in root zone.
According to Ehler, 1960 the chloride ions of salt also have decrement effect on the production of rice and it was noticed that at the maturity, grain yield was more effective by salt rather than vegetative growth. According to him, salinity has adverse effect on both stages of growth in rice due to higher osmotic pressure. Due to increase in osmotic pressure yield is affected more adversely rather than vegetative growth.
Mehmood et al., 2009 studied effects of salinity on growth, yield and yield components in basmati rice germplasm. They took 4 varieties and 17 breeding lines of basmati rice and estimated the influence of salinity at early growth stages and at maturity in natural environmental conditions. They concluded that there is direct relationship between rice sterility and salinity, but they also found that number of tillers/plant, plant height, shoot dry weight and grain straw ratio was also reduced due to high salt contents. They suggested that it is better to choose plant for tolerance for breeding purposes in early growth stages.
Thach and Pant, 1999 studied the salt tolerance in rice in in-vitro conditions. They took CSR-27 and HBC 19 cultivars that are salt tolerant and sensitive respectively. They performed with 0, 0.5, 1.0, 1.5 and 2.0% NaCl solution for four weeks. Then they compared the seedling and callus growth under saline conditions. According to their study, seedling stages got more abrupt effects as compared to callus. They also suggested that callus can be used in screening process to check the effect of salinity because the proline production is found more in callus under saline condition in susceptible cultivars as compared to the resistant one.
Murtaza et al., 2009 evaluated the rice and wheat cultivars for salinity and found that rice is more sensitive than wheat to EC/SAR ratio. Higher ratio adversely affects the yield of crop. They used three levels of salinity and concluded that in case of rice, higher ratio caused severe loss while lower ratio had not significant difference after comparing the yield and growth of crop with control. But in case of wheat all three levels had significant adverse effect on growth and yield.
Asch et al, 1999 studied the sodium and potassium uptake of rice crop with respect to yield and yield component. They compared two cultivars, resistant and susceptible and observed that susceptible cultivar had more sterility of panicle as compared to resistant one. They also observed that the number of panicles was not affected by salinity but these panicles were partially filled with grain. By their work, they concluded that the sodium uptake can be measured at two factors, one is before flowering through root properties and second factor is from flowering to onward (panicle transpiration).
Fabre et al., 2005 presented that how salinity affects the grain development, grain weight and size distribution. They took 1000 grain weight and considered it constant due to full hull development and studied the salinity effects on it. They used histogram for their analysis as it gave bimodal as well as monomodel patterns for grain weight. They took samples from different field that had different levels of salinity in the Camargue delta area of France. They found that more salinity results more spikelet without grain and with reduced grain weight. So they concluded that salinity affects before flowering at the time of hull development.
Hassan et al., 2001 studied the comparative performance of rice varieties in ameliorated and nonameliorated soils. According to their work, it was concluded that the sterile tillers/ plant were less in ameliorated soil as compare to the non-ameliorated soils. They perform their experiment in the field and found that the soil conditions affect the yield significantly in susceptible cultivars as compared to the resistant cultivars. They also suggested that the resistant cultivars were affected less in comparison to susceptible one, at all the growth stages of life.
Screening of rice for tolerance:
To maintain the yield in rice crop, it is necessary to solve the problems concerning with salinity in rice. Screening for tolerance can be done either in field or in greenhouse with some drawbacks in both circumstances. In field the prevailing conditions cannot control as the wind which is blowing in its direction and cause the seed of one crop to be mixed with other one. So precision is not obtained in the field. On the other hand, in the green houses with the control conditions, the crop will not come in contact with the natural environment and one cannot find the exact level of resistance in the crop. As the situation of resistance is completely different in the field as compare to green houses but screening can be done in both circumstances.
Screening method of rice cultivars at three stages is taken from the work of Puard et al., year in Green House:
salt tolerance screening at germination stage
- Seed of different rice cultivars were taken to check their germination in saline soil.
- Soil with 2g/l NaCl solution was prepared to grow the seed in it (Using the distilled water for irrigation).
- Petri dishes with saline soil were prepared to grow the seeds.
- Environmental conditions were kept constant at 30oC in the green house.
- Observations about the germination of seed were taken after 14 days of sowing.
Salt tolerance screening at sprouting stage
- Seed of different rice varieties were sown in Petri dishes in green house at 30oC for 5 to10 days.
- After 5 to 10 days, the Petri dishes were divided into categories as with distilled water and saline water ( 1.27g/l to 10.2 g/l) to check and compare their sprouting rate and dry matter production.
- The conditions of laboratory or green house were kept constant.
- After 20 days the production of dry matter was checked to estimate the effect of saline conditions.
Salt tolerance screening at 5th leaf stage
- Pots with sands were used for seed germination and were irrigated with distilled water.
- At this stage, screening was done by measuring the photosynthesis and respiration rates of plants with different salt concentrations.
- After 20 days of cultivation, the plants were set in the pots having different concentration of salt solutions in the special rooms having gas variations measure (to measure the CO2 and water vapor production by plants).
- Formulas are used to measure the gas exchange.
- Then the rates of different varieties were compared with each other and with control to measure the effects.
Another screening method was obtained from the work of Rao et al., 2008.
- Seeds were grown in standard normal environmental and salt conditions.
- After 30-35 days, the seedlings were transplanted in the field at 1.5 meter row length with 3 replications of each.
- 2 seedlings per hill were planted at 15-20 cm distance.
- All the requirements of crops were fulfilled before and after the experiment of screening.
- 3 plants/ genotype were tagged in each of 3 replications (9 plant/ genotype).
- For screening the tolerance, following measurements were done.
- Vegetative stage
- Numbers of tillers and panicles/plant
- Reproductive stage
- Fertile floret/panicle
- Spikelet sterility
- Flowering date was considered, when 50% of the plants had flowering.
- Straw and grain yield of the cultivars were counted after harvesting.
- They used 25 genotypes in the screening experiment and they distribute them into 3 categories.
- Tolerant (< 25% grain yield reduction as compare to control)
- Semi-tolerant (30-35% grain yield reduction as compare to control)
- Sensitive (> 25% grain yield reduction as compare to control)
- All the counting of grains and grain yield was done with electronic seed counter and weighed.
One general method is developed for screening of crops by the staff of Servicio de Investigacion Agro Limentaria in Zaragoza, Spain in the natural environmental conditions.
This method has following steps:
- First of all, identify the plot with salinity by visual symptoms.
- Salinity map of the plot is obtained by measuring the apparent electrical conductivity of soil through reading of electromagnetic sensor on regular grid (2m x 2m).
- Crop should be grown in 4 or 5 rows in the direction of maximum salinity gradient (number of rows depends upon row spacing).
- All conventional and agronomic practices should be kept under consideration.
- Regular electrical conductivity readings should be taken after every 15 days or after each irrigation, so that variation in soil salinity can be checked.
- Take at least 3ECa-ECe measurements during all growing seasons (ECe= a+b-ECa).
- Crop is harvested for grain yield or any other target stage for screening by hand because machine does not give accuracy (for this purpose, whole area is divided into small zones (0.5-1 m2) with uniform crop appearance).
- In the end graph is plotted by taking the observation by yield- ECe.
- This method can also be used to estimate the salinity tolerance parameter and thresholds.
This salinity screening method can be used in controlled conditions in the green houses to overcome the problems and drawbacks of open field conditions.
Some salt tolerant varieties:
CSR10, CSR13, CSR23, CSR27, CSR30, CSR36 and Lunishree, Vytilla 1, Vytilla 2, Vytilla 3, Vytilla 4, Panvel 1, Panvel 2, Sumati, Usar dhan 1, 2 & 3 (from India); BRRI dhan 40, BRRI dhan 41 (from Bangladesh); and OM2717, OM2517, OM3242 (from Vietnam). CSSRI30, POKKALI , NONA BOKRA, BR-5331,CSR-27, PANVEL-3, Shaheen basmati, B.P. 95.