Yoshida - 1981 - Fundamentals of Rice Crop Science

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PHYSIOLOGICAL ANALYSIS OF RICE YIELD 237 At an optimum combination of spacing and nitrogen level, the spikelet number per unit land area is strongly influenced by levels of solar radiation and temperature during reproductive growth (see Table 2.10, 2.11, and Fig. 2.13). 7.3.3. Filled-spikelet percentage Factors such as weather, soil, fertilizer application, and incidence of diseases and insects affect filled-spikelet or sterility percentages. Sterility percentage often refers to the percentage of unfertilized spikelets plus the percentage of partially filled spikelets. Hence, the term sterility is not always strictly used. The iodinepotassium iodide method can distinguish unfertilized spikelets from partially filled spikelets (Matsushima and Tanaka 1960). The sterility percentage determined with the iodine-potassium iodide method is usually low under normal weather conditions. About 9% was the average sterility percentage of 4 crops at Los Baños, Philippines. Crops shaded during ripening had a low percentage of filled spikelets because of increased numbers of partially filled spikelets (see Table 2.11). Except under extremely adverse weather conditions, an unusually low percentage of filled spikelets is attributed to an increased percentage of partially filled spikelets. A brief account of each factor that affects filled-spikelet percentage follows (Yoshida and Parao 1976): a. High levels of nitrogen application. Some varieties may have lower filledspikelet percentages than other varieties at high levels of nitrogen. b. Lodging or bending. At high levels of nitrogen, lodging or culm bending decreases filled-spikelet percentage. Lodging reduces the cross-sectional area of vascular bundles and disturbs the movement of assimilates and absorbed nutrients via the roots. It also disturbs the leaf display and increases shading. c. Low solar radiation. The percentage of filled spikelets often decreases as the number of spikelets per square meter increases (Wada 1969). Filled-spikelet percentage is determined by the source activity relative to the sink size (spikelet number), the ability of spikelets to accept carbohydrates, and the translocation of assimilates from leaves to spikelets. When solar radiation is low, the source activity may be insufficient to produce enough carbohydrates to support the growth of all the spikelets. As a result, the numbers of unfilled spikelets may increase. Under a given solar radiation, the sink size relative to the source activity affects filled-spikelet percentage. This is shown by an increased percentage of filled spikelets when the spikelets are partially removed (Matsushima 1970, Wada 1969). d. Low temperature. Air temperatures below 20°C may cause high percentages of sterility if they persist for more than a few days at booting or heading. This occurs in temperate regions at high latitudes and in the tropics at high altitudes. It may also occur in the tropics during the dry season. e. High temperature. High temperatures shorten the grain-filling period. Persistent cloudy weather will be more detrimental to grain-filling under high temperatures because of a shorter ripening period. Temperatures higher than 35°C at
238 FUNDAMENTALS OF RICE CROP SCIENCE anthesis may cause high percentages of sterility. f. Strong winds. Strong winds can cause sterility at flowering by desiccating the plant, impair grain growth by causing mechanical damage to the grain surface, and cause the crop to lodge. A hot, dry air called foehn frequently causes white head, particularly when it occurs during panicle exsertion (Hitaka and Ozawa 1970). Strong winds from the coast often contain brackish water and cause sterility. g. Soil salinity. High salinity in the soil causes a high percentage of sterility. h. Drought. Drought at flowering often causes high percentages of sterility in rainfed lowland and upland rices. 7.3.4. Grain weight Grain size is rigidly controlled by hull size. When a piece of vinyl film was placed inside the palea and lemma at anthesis, grain filling proceeded normally but the grain weight decreased from 22 mg (untreated) to 18 mg (treated) (Matsushima 1970). Under most conditions, the 1,000-grain weight of field crops is a very stable varietal character (see Table 2.11, Fig. 2.3, and Soga and Nozaki 1957). The CV of 1,000-grain weight ranged from 2.2 to 4.4%, with a mean value of 3.3% for data collected from 9 varieties for 19 years at Konosu, Japan. The mean CVs of panicle number and yield were 12.4% and 14.1%, respectively (Matsushima 1970). Heavy shading before heading changed the hull size and decreased 1,000-grain weight from about 26 to 21 g (Matsushima 1970). As will be discussed later, CO 2 enrichment also affects the grain weight to some extent. 7.4. Frequency curve of individual grain weights of Koshiji-wase at 30 days after heading (drawn from data of Nakayama 1969).
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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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Page 257 and 258: Table 7.6. An example of high yield
Page 261 and 262: REFERENCES AGRICULTURAL POLICY STUD
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Yoshida - 1981 - Fundamentals of Rice Crop Science

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