Yoshida - 1981 - Fundamentals of Rice Crop Science

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38 FUNDAMENTALS OF RICE CROP SCIENCE 1.36. A line grid for indirect measurement of total root length in complex line patterns (adapted from March 1971). used to estimate the length of the root. Knowing the number of intercepts, the total root length is given as: R = p NA (1.7) 2H where R = the total length of root, N = the number of intercepts between the root and the straight lines, A = the area of the square or rectangle, and H = the total length of the straight lines. In Fig. 1.36 there are 11 horizontal and 11 perpendicular straight lines. Thus, the equation 1.7 becomes: R= 3.14 × N x 14 × 14(cm 2 ) 2(14 × 11 + 14 × 11) (cm)
GROWTH AND DEVELOPMENT OF THE RICE PLANT 39 (1.8) In other words, the number of intercepts gives an estimation of the total root length in centimeters. This method can also be used for estimating the length of any irregular shape of lines. Root length per unit volume of soil is called root density: (1.9) Thus, the root density has a dimension of cm -2 . It is used as a measure of root growth in a field-grown crop. The root density of rice crops ranges from less than 1 cm -2 in a deep soil horizon to more than 30 cm -2 around the plant base. 1.7.7. Root characteristics related to drought resistance The rice root system can be studied using several techniques: 1) root box, 2) core sampling in the field, and 3) monolith. Since drought occurs when there is an imbalance between water absorption and transpiration, greater root growth can help the plant perform better under a limited water supply. The root-shoot ratio of rice varieties can be studied by the root box technique. The ratios range from 0.05 to 0.3 at heading, and average about 0.1. Under drought conditions, the soil starts drying from the surface but the deep soil horizon may remain wet and able to supply water to the plant’s roots. Consequently, deep root portions may be more meaningful than shallow root portions when the drought resistance of a variety is to be examined. For this reason, the deep root-shoot ratio is considered a better measure for drought resistance in the field. The deep root-shoot ratio of rice varieties is defined as the weight, in milligrams, of roots that are deeper than 30 cm/gram shoot. It ranges from less than 10 to greater than 80 mg/g. Table 1.10. Vertical root density distribution of 7 rice varieties grown on an upland field, IRRI, 1977. a Variety Depth (cm) 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 Lowland IR20 14.4 2.8 0.9 0.4 0.1 IR2035-117-3 22.7 5.8 0.8 0.1 0.1 IR442-2-58 Upland 16.8 7.1 1.2 0.3 0.1 OS4 12.6 1.4 0.8 0.9 0.8 0.5 0.5 0.2 Moroberekan 11.8 2.3 0.9 0.8 0.6 0.8 0.4 0.2 Salumpikit 16.2 5.5 1.9 1.4 0.8 0.6 0.3 0.1 20 A 19.8 2.6 0.9 0.8 0.9 0.9 0.6 0.4 a IRRI (1978).
Page 3 and 4: FOREWORD Rice is vital to more than
Page 5 and 6: CONTENTS Foreword Preface GROWTH AN
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Page 11 and 12: 1 .1. OUTLINE OF THE LIFE HISTORY T
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5.1. PHOTOSYNTHESIS 5.1.1. Photosyn
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PHOTOSYNTHESIS AND RESPIRATION 197
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Photosynthetic characteristics Meas
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A traditional tall variety vs an im
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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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