Rice is intimately involved in the culture as well as the food ways and economy of many societies

Apr 26 - May 09, 2004

Soil infertility is a major problem in arid and semiarid regions, where rainfall is insufficient to leach salts and excess sodium ions down and out of the root zone. These areas often have high evaporation rates, which can encourage an increase in salt concentration at the soil surface through capillary rise. The presence of a cemented hardpan at varying depths and insufficient precipitation for leaching often adds to the problem. Newly established irrigation projects, with improper planning and management practices may also add salts to soils. Historically, soil salinity contributed to the decline of several ancient civilization. Despite the advanced technologies available today, salinization of millions of hectares of land continues to reduce crop productivity severely worldwide. Of the approximately 13 billion hectares total land on earth, about 1 billion are affected by salinity/sodicity. According to a report, saline/sodic soils cover about 26% of the world's cultivated land. Incidentally, most of the developing and under-developed countries of south and southeast Asia, Africa and South America are the worst affected by this menace. More than 80 million hectares of such soils are in Africa, 50 million hectares in Europe, 357 million hectares in Austral-Asia, nearly 147 mha in Central, North and South America. Similarly, a large bulk of about 320 million hectares of land in South and South East Asia is under the grip of salinity. It shows that no continent on our planet is free from salt-affected soils.

Rice is now one of the most important crops at the global level, as it is used as a staple food in most countries of the world and will continue to be for the foreseeable feature. Rice is intimately involved in the culture as well as the food ways and economy of many societies. Indica type rice feeds more than two billion people, predominantly in developing countries. In the coming 30 years, the world will require 70% more rice than that it requires today. According to conservative estimates, 800 million tons of rice will have to be grown with considerate reduction in the input of agrochemical under sustainable conditions.

Rice is also an important food as well as a cash crop of Pakistan, which occupies 11 per cent of the country's cropped area. During 2002-2003, Pakistan exported 2.5 million tons of rice and earned $ 5.6 billion in foreign exchange. Pakistan enjoys a monopoly in the export of world famous fine aromatic Basmatic variety. Rice, therefore, occupies a very significant position in the national economy. Pakistan is among the four major rice exporting countries of the world, but producing only 5.1 m.tons compared to 21.1, 16.6 and 88.5 m.tons by Vietnam, Thailand and India, which are the other three exporters. Demand of rice, the largest staple food is increasing with the increasing population of the world. It is expected that consumption of rice will increase from 560 m.tons in 1995 to 786 m.tons by 2020 for meeting the requirements of the population explosion. It is grown in fertile as well as in salt-affected soils. It is very common food of the people of Asian countries. Reclamation of saline soil involves reducing soluble salt content to a level at which, the salts will not seriously interfere with plant growth.

Pakistan has been endowed by nature with vast potentialities for growing rice on large scale, the relatively leveled terrain, heavy soils with good water holding capacity, good sunny days, congenial climatic conditions and abundant supplies of farm labour. But, scarcity of irrigation water and salinity of soil as well as ground water are the major limiting factors. A vast rice growing area of the country is under the influence of marginal to severe salinity. It has been reported that 6.3 mha of arable lands of Pakistan are affected by salinity. Research must find ways and means for utilization of saline lands and brackish water by exploiting genetic resources and improved agronomic practices. To provide food for ever increasing population, the interests have been intensified by the economists, agriculturists and government planning agencies to utilize various types of unproductive lands. Particularly in countries having arid or semi-arid climates, one opportunity for meeting future needs lies in reclamation of salt-affected soils. On these soils, some crops can not make normal growth owing to the presence of soluble salts in the soil solution. The presence of exchangeable sodium on the surface of the soil of the soil particles, or the presence of both characteristics (saline sodic soils). It has been found that in extreme cases, there may be no plant growth at all. Rainwater percolating through the soil of one area can pick up salt and transport it to another area, and there salt may accumulate and become concentrated as a result of evaporation. This phenomenon has been occurring all over the world for many centuries and is still occurring on flat lands and in the lakebeds and enclosed basins.

For any crop, a high water table is a serious salinity hazard. Evaporation from surface soil or absorption of water by crop roots near the soil surface tends to produce a moisture gradient that causes water to move upward from the water table. Such movement of water carries additional salt into the upper layers of the soil and causes them to become more saline. The use of irrigation water containing too much salt, or poor management of even good irrigation water, can cause soils to become saline. All irrigation water contains some salts. Heavy irrigation water is necessary to leach down the excess salt at the bottom layer of the soil. Usually, the salt concentration in the soil solution is indicated in terms of parts per million or of the electrical conductivity of the soil solution. The salt tolerance of crops has generally been expressed as the yield decrease expected for a given level of soluble salts in the root medium as compared with yield under non-saline conditions. There are several factors, which influence salt tolerance in plants. The most important among these are species, temperature, salt composition, the growth stage of plant, salinity level, environmental variables, soil fertility and cultivars.

Salinity affects plants at all stages of development and for some crops, sensitivity varies from one growth stage to the next. Several studies have shown that rice is tolerant during germination, but becomes very sensitive during early seedling growth. It again becomes more tolerant during vegetative growth with the sensitivity returning again during pollination and fertilization and finally it again becomes more tolerant at maturity. According to US salinity laboratory staff, the definition of salinity is the presence of an excessive concentration of soluble salts that suppresses plant growth. The suppression increases as the salt concentration increase. There exists a threshold (salinity at initial yield decline) that varies with species and with cultivar within species. Literature reveals that the salinity threshold for rice, based on some published data, was around 3 dSm-1 a 25oC. The soluble salts that effectively contribute to soil salinity consists mostly of various proportions of the cations calcium, magnesium and sodium and the anions chloride, sulphate, bicarbonate and sometimes carbonates. For most species studied the effect of salts on growth suppression seems to be non-specific, depending more on the total concentration of soluble salts than on specific ions. It seems to make little difference whether the soil solution consists of salt, e.g. NaCl, Na2S O4, or CaCl2, that commonly are present in excess in saline soils.

The climate may significantly influence the plant response to salinity. Soil salinization is generally associated under climatic conditions, where evaporation is greater than precipitation and in coastal areas subject to tides. In addition to precipitation, temperature and relative humidity influenced salt tolerance significantly. Many crops seem to be less salt tolerant when grown under hot dry conditions than under cool humid ones. Crop tolerance for salinity may vary with soil fertility. Crops grown on infertile soils generally seem to have abnormally high salt tolerance as compared with crops grown on fertile soils. But, it has also been reported that unless salinity causes a specific nutritional inbalance, additional fertilization generally has little effect on the salt tolerance of crops. The resistance of salinity by rice is not determined by a single heritable character, but is a complex whole plant phenomenon involving many interacting processes. Consequently, it has been suggested that the resistance to saline conditions can be increased beyond the existing phenotypic range by selecting individual physiological traits that contribute to resistance and combining them in a breeding programme. While some of the characters determining resistance at the seedling stage have been described, rather little is known about those characters that might be important for grain yield under saline conditions. More is known about seedlings than mature plants primarily because results can be obtained more quickly and easily with small vegetative plants than with large flowering plants. This is especially so for characters, where there is great plant to plant variation within a supposedly homozygous population requiring the use of large numbers of replicates in an experiment. Experiments on seedlings have been justified since without seedling establishment there would be no plants to flower.

It has long been known that grain yield in rice is more sensitive to salinity than the later stages of vegetative growth, although very young seedlings are particularly sensitive to salinity. Thus, salinity imposed at different stages during the life cycle might be expected to have different effects on yield. It has recently been observed that when the application of salinity co-indicates with the stage of panicle initiation, there is a particularly marked effect on the reproductive growth of rice. Salinity causes a delay in germination and reduces the final percentage of germination. Sand-culture experiments conducted at U.S. Salinity laboratory indicates that a 50 per cent reduction in the germination of rice within 10 days after planting was associated with an electrical conductivity of somewhat more than 20 millimohs. per centimeter. Among 12 individual rice varieties studied, only moderate differences were found in this regard.

Applying 825 pounds of salt per acre to a non-saline soil before planting rice reduced the grain yield by 15 per cent. This was due to reduction in germination of rice seedlings. Studies at the U.S. Salinity Laboratory also have produced evidence that, beyond the early seedling stage, the salt tolerance of rice increases with age. Seedlings grown in non-saline soil for 3 and for 6 weeks survived, when transplanted in saline soils having electrical conductivity values up to 9 and 14 millimohs/cm, respectively. In general, it may be said that rice is very tolerant of salt during germination but very sensitive to it during the early seedling stage, gains tolerance during progressively during the tillering stage, again becomes sensitive when flowering and is tolerant during the period of grain maturation. A crop is classed as salt sensitive if its yield is reduced 50 per cent by a seasonal average level of salinity represented by an electrical conductivity value of 8 millimohs/cm. Extreme salinity makes it impossible to grow any variety of rice profitably. CORH2, a rice hybrid produced an average of 5.1 t/ha on high salinity soils. The yield was 23% higher than existing salt tolerant line CO43. It was also described that pokkali (rice variety) as the best rice cultivar growing in saline soils.

It was concluded from the above fact that rice, a water loving plant can grow successfully in a saline environment, because submergence of a soil substantially reduces the toxic effects of saline water.