Yoshida - 1981 - Fundamentals of Rice Crop Science

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CLIMATIC ENVIRONMENT AND ITS INFLUENCE 67 Geographers define the tropics as the region between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S). It is commonly said that the tropics is where winter never comes, implying that the temperature remains high throughout the year. However, at high altitudes the weather is cool and the common definition no longer holds. Thus, it would be more meaningful to define the tropics in terms of temperature. One such definition proposed by Koppen in 1936 uses the threshold monthly mean temperature for the coldest month of the year (Trewartha 1968). The monthly mean temperature of 18°C for the coldest month is used for a climatic boundary of the tropics. Another temperature-based definition, proposed in 1945 by the plant ecologist Kira, uses a heat index — a summation of monthly mean temperature. The heat index, which involves the minimum temperature for plant growth as a threshold temperature, is defined as: (2.1) where t i is the monthly average temperature in centigrade, and the heat index (W) is for 12 months. The subtropics and tropics are defined as regions where the heat index value is greater than 180° and 240°C, respectively. When such classification is applied to the subtropics and tropics, the heat index can be converted to the annual mean temperature because monthly mean temperatures in these regions are always higher than the threshold temperature. For instance, an annual mean temperature of 20°C gives: (2.2) Similarly, an annual mean temperature of 25°C gives a heat index value of 240°C. Thus, heat index values of 180° and 240°C correspond to annual mean temperatures of 20° and 25°C, respectively. This modification defines the tropics as the region where annual mean temperature is above 25°C. Vegetation characteristics can also be considered as a convenient basis by which to identify the tropics, subtropics, and temperate regions of Asia. In Asia, an annual mean temperature of 20°C is considered as the northern limit for vegetation that includes palm (palmae) and screw-pines (pandanus), and 25°C is the northern limit for dipterocarps (Dipterocarpaceae). Karachi, Pakistan (25°N), and New Delhi, India (28°N), are north of the Tropic of Cancer but have annual mean temperatures of 25.8°C and 25.3°C, respectively. Hence, they have a tropical climate by Kira's definition. By Koppen's definition, however, New Delhi does not have a tropical climate because the monthly mean temperature for the coldest months (December, January, and February) is below 18°C. 2.2. EFFECTS OF CLIMATE The average rice yields in rice-growing countries range from less than 1 to more than 6 t/ha. There are a number of biological, environmental, and socioeconomic
68 FUNDAMENTALS OF RICE CROP SCIENCE Table 2.2. Climatic influence on rice production. Temperate Tropics Crop period Productivity Stability Temperature Sunlight Temperature Rainfall Sunlight Typhoons Rainfall Sunlight Rainfall Typhoons reasons for the large differences in rice yield. Low yields are associated with upland rice, rainfed lowland rice, deepwater rice, and the poor socioeconomic conditions in the tropics, whereas high yields are associated with irrigated lowland rice and the good socioeconomic conditions in the temperate regions. Temperature, solar radiation, and rainfall influence rice yield by directly affecting the physiological processes involved in grain production, and indirectly through diseases and insects. In the field, these factors are often difficult to separate from one another. From a crop physiologist’s point of view, crop period, productivity, and stability are important aspects of rice cultivation. Climatic factors affect each of them in different ways (Table 2.2). In the temperate regions, irrigated rice cultivation starts when spring temperatures are between 13° and 20°C; the crop is harvested before temperatures drop below 13°C in the autumn. In the tropics where temperature is favorable for rice growth throughout the year and irrigation is not available in most places, cultivation starts with the rainy season. The average dates of the onset and withdrawal of the monsoons are known for particular regions of South Asia (Fig. 2.1). The starting time and duration of rainfed rice cultivation is largely determined by these two dates. In both the tropics and the temperate regions, rice yield per hectare is primarily determined by the level of incident solar radiation. In the tropics, when adequately 2.1. Average dates of the onset (left) and withdrawal (right) of the southwest monsoon, and the dates of the start of the northeast monsoon over Indo-China and Malaya (right) (from Tropical Climatology by S. Nieuwolt. Copyright © John Wiley 1977. Reprinted by permission of John Wiley & Sons, Ltd.).
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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 25 and 26: GROWTH AND DEVELOPMENT OF THE RICE
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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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