Yoshida - 1981 - Fundamentals of Rice Crop Science

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MINERAL NUTRITION OF RICE 175 which is soluble in water, and the iodide concentration in the soil solution increases. The concentration of iodide in soil solution was found to be as high as 0.7–3 ppm when iodine toxicity symptoms were observed. This concentration is far greater than the 0.05 ppm normally found in river and sea waters. In water culture experiments, iodine toxicity symptoms were induced at a concentration of about 1 ppm (Watanabe and Tensho 1970). 3.17.3. Plant’s response to iodine toxicity Visible symptoms of iodine toxicity begin to appear a few weeks after transplanting, and they are similar to those of zinc deficiency and iron toxicity (see Chapter 4). The alkali soluble iodine content of the affected plants is about 30–40 ppm; healthy plants usually contain less than 10 ppm. Clear varietal differences in tolerance for iodine toxicity have been recorded (Watanabe and Tensho 1970). 3.18. SALINITY 3.18.1. Occurrence of salinity Salinity is defined as the presence of excessive concentrations of soluble salts in the soil. Major ionic species of salts are sodium, calcium, magnesium, chloride, and sulfate. Among those, sodium chloride is predominant. Salinity occurs in two distinctly different regions: coastal regions and arid and semiarid regions. In the coastal regions, salinity is induced by inundation of sea water; the salinity is often associated with low soil pH. In the arid and semiarid regions, salinity occurs mostly in canal-irrigated areas. Evapotranspiration is very high in the arid and semiarid regions and, as a consequence, water movement is upward, resulting in the accumulation of salts in the root zone. Such saline soils can be easily recognized by the white salt crusts on the soil surface. This type of salinity is often associated with high pH. In relation to salinity, two soil types are recognized in the arid and semiarid regions.. Saline soils normally refer to soils with pH lower than 8.5 and electric conductivity of the saturation extract greater than 4 mmho/cm at 25°C. Sodic soils are defined in terms of exchangeable sodium percentage (ESP). A soil may be considered sodic if ESP is more than 6, and strongly sodic if the ESP is greater than 15 (Bhumbla and Abral 1978). 3.18.2. Salts in soil solution Electric conductivity of either the saturation extract or soil solution collected from the root zone is normally measured to quantify the degree of salinity. For rice growing in flooded soils. the two conductivities can be considered comparable. For upland crops and when soil moisture content is about field capacity or below, electric conductivity of the soil solution will be about twice as great as that of the saturation extract (Pearson 1959). When salinity is moderate, electric conductivity of the saturation extract (ECe) is related to osmotic potential by the following formula:
176 FUNDAMENTALS OF RICE CROP SCIENCE Osmotic potential (in atm) = 0.36 × ECe (in mmho/cm). (3.47) 3.18.3. Physiological nature of salinity injury When salinity is increased suddenly, water uptake by the plant may be temporarily impaired due to the low osmotic potential of the soil solution. However, the plant is able to reduce the osmotic potential of the cells to avoid dehydration and death. This process is called osmotic adjustment. Both tolerant and sensitive plants seem to adjust osmotically to saline solutions, but still their growth is suppressed in proportion to the osmotic potential of the solution. At present, the physiological causes of growth suppression by salinity are not well understood (Maas and Nieman 1978). 3.18.4. Critical salinity level and varietal difference in salinity tolerance Rice is very tolerant of salinity during germination but very sensitive at the 1- to 2-leaf stage. Its salt tolerance progressively increases during tillering and elongation and decreases at flowering. Ripening appears to be little affected by salinity. For example, the California variety Caloro germinated at an electric conductivity of 20 mmho/cm, but sprouted seed failed to grow at 4 mmho/cm. Three- and 6-week-old seedlings survived even at 9 and 14 mmho/cm, respectively. The grain yield of Caloro was not appreciably affected by salinity at 4 mmho/cm, but was reduced by 50% at 8 mmho/cm (Pearson 1959, 1961). A considerable degree of varietal difference in salinity tolerance appears to exist. Examples of salttolerant varieties are Johna 349, Kala rata, Pokkali, Nonabokra, and Benisail (IRRI 1967, Datta 1972, Ponnamperuma 1977). Available data, however, indicate that the varietal response to different salinity values is complex (Table 3.37). At 4.5 mmho/cm, the relative yield of TN1 or IR8 is as good as or even greater than that of the salt-tolerant variety Nonabokra. At 12.5 mmho/cm, however, the relative yield of Nonabokra is significantly higher than that of either TN1 or IR8. Table 3.37. Yields a of 12 rice varieties at 4 levels of salinity. b Variety Electric conductivity of soil solution (mmho/cm) 4.5 9.5 12.5 15.5 Patnai Benisail Kumaigare Rupsail Bokra Nonabokra Getu O.C 1393 TN1 IR8 BPI 76 SR26B 74 89 59 43 54 63 47 68 73 63 65 44 71 47 32 38 24 58 43 49 45 36 49 36 22 39 9 20 2 35 18 4 8 16 19 3 0 6 0 0 0 7 2 0 0 0 0 0 a Yields are relative to control (nonsaline plot) in each variety. b Computed from De Datta (1972).
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FOREWORD Rice is vital to more than
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CONTENTS Foreword Preface GROWTH AN
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3.2. Adaptation to Submerged Soils
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4.5. Corrective Measures for Nutrit
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1 .1. OUTLINE OF THE LIFE HISTORY T
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GROWTH AND DEVELOPMENT OF THE RICE
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Table 7.6. An example of high yield
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REFERENCES AGRICULTURAL POLICY STUD
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Yoshida - 1981 - Fundamentals of Rice Crop Science

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