Yoshida - 1981 - Fundamentals of Rice Crop Science

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MINERAL NUTRITION OF RICE 165 3.29. Relationship between relative dry matter of shoots and percentage of sulfur in leaf blades at tilleriny stage (Yoshida and Chaudhry 1979). The critical N-S ratio in straw for the maximum dry weight also varies from 23 at tillering to 13 at maturity. The critical N-S ratio required to achieve 50% of the maximum dry weight ranges from 63 at flowering to 40 at maturity. Thus, both critical sulfur content and critical N-S ratios vary with the growth stage of the rice plant. Since determining the N-S ratio requires extra cost to the single sulfur analysis, the sulfur content in plant tissues is considered a better tool for diagnosing sulfur status in the rice plant. 3.14. SILICON 3.14.1. Occurrence of deficiency Essentiality of silicon in rice is still open to question. Increased absorption of silicon, however, appears to increase rice’s resistance to some environmental Table 3.32. Critical sulfur contents and N-S ratios at different growth stages. a Critical sulfur (%) Critical N-S ratio Growth Leaf blades Straw Leaf blades Straw stage DC 50 DC 100 DC 50 DC 100 DC 50 DC 100 DC 50 DC 100 Tillering 0.06 0.16 0.06 0.16 45 29 44 23 Flowering 0.05 0.10 0.04 0.07 52 24 63 23 Harvest 0.04 0.07 0.03 0.06 25 16 40 13 a Yoshida and Chaudhry (1979). DC 50 is the critical sulfur content, or critical N-S ratio, in plant tissue required to obtain 50% of the maximum dry weight. DC 100 is the critical sulfur content, or critical N-S ratio, in plant tissue required to obtain maximum dry weight.
166 FUNDAMENTALS OF RICE CROP SCIENCE stresses (Yoshida 1975b). The application of a silicate slag to degraded paddy soils and peaty soils in Japan and Korea has been found beneficial to rice yields. Yield increases are usually around 10% but may exceed 30% when blast disease is severe. 3.14.2. Silicon in soil solution Silicon dissolved in an aqueous solution exists as orthosilicic acid, Si (OH) 4 . Because silica sol or silica gel derives from the condensation polymerization of orthosilicic acids, orthosilicic acid is sometimes called a monomer or a monomeric form of silica. The term silica refers to SiO 2 . The concentration of silicon in the soil solution generally increases with flooding and soils with a high organic matter content give the highest increases (Ponnamperuma 1965). The concentration of silicon extractable with dilute acids shows a more rapid increase with flooding than that of silicon in the soil solution. Readily soluble silicon in the soil is perhaps present as absorbed on or as combined with amorphous aluminum and ferrous hydroxide (Imaizumi and Yoshida 1958, McKealgue and Cline 1963). Soils derived from volcanic ash have high levels of soluble silicon and high silicon-supplying power. 3.14.3. Functions of silicon The functions of silicon in rice growth are still not clearly understood (Yoshida 1975b). First, the level of silicon in the plant at which growth is improved by additional silicates is much higher in the field than in solution culture. In the field, an application of silicates is considered beneficial when the silica content of straw is below 11% (Imaizumi and Yoshida 1958). In solution culture, however, the addition of silicon has little effect on vegetative growth when silica content in the leaves is 0.07% (Yoshida et al 1959); it has no effect on rice growth and yield when the silica content of green leaves is above 1.25% (Tanaka and Park 1966). Second, low-silicon plants are susceptible to various kinds of diseases and attacks by insects and mites (Yoshida et al 1962). It is difficult to keep such plants free from these hazards. Third, no direct evidence of the participation of silicon in essential physiological and biochemical processes has yet been found. Although the results and interpretation of several studies differ, silicon appears to have the following functions in rice growth: • Increased silicon absorption protects plants against infestations of fungi and insect attacks (Yoshida et al 1962). A thick cuticle-silicon layer (Fig. 3.30) serves as a barrier against fungi, insects, and mites because of its physical hardness. Entomologists have found that the mandibles of rice borer larvae fed on rice plants with a high silicon content were damaged. • Increased silicon absorption maintains erect leaves (Yoshida et al 1969; Cock and Yoshida 1970). The importance of leaf angle to photosynthesis by a crop canopy is well recognized; erect leaves are desirable in a high yielding rice variety. Although leaf angle is mainly a varietal characteristic, it is affected by
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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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