Yoshida - 1981 - Fundamentals of Rice Crop Science

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210 FUNDAMENTALS OF RICE CROP SCIENCE Our knowledge of maintenance respiration is much less certain than our knowledge of growth respiration. The most potential maintenance processes in plants are (Penning de Vries 1975a): • Protein turnover. • Active transport processes to maintain certain ion concentrations in the cells. Proteins in living organisms undergo constant renewal by a process referred to as protein turnover. The average turnover rate of leaf proteins may be about 100 mg protein/g protein per day at normal temperatures in leaves assimilating at moderate light intensities. In other words, about 10% of leaf protein are resynthesized everyday. This process consumes 28–53 mg glucose/g protein per day, which corresponds to 7–13 mg glucose/g dry weight per day in leaves. Such a consumption implies that the complete removal of protein turnover would reduce maintenance costs by about 10 mg glucose/g dry weight per day or by about 10 kg glucose/day for every 1 t dry weight. The cost of maintaining the ion concentration in leaves is estimated at about 6–10 mg glucose/g dry weight per day. In addition, more energy is used to break down and resynthesize the lipids that constitute cell membranes. The cost of membrane maintenance is estimated at 1.7 mg glucose/g dry matter per day. Adding these figures gives a maintenance respiration cost of 15–25 mg glucose/g dry matter per day. This estimated maintenance respiration cost agrees with reported values (compare with equation 5.18), but is lower than other measured values for plants grown under high light intensities. For example, values as high as 50–150 mg glucose/g dry weight per day have been reported for the leaves of Hordeum sp. and Triticum sp. (Penning de Vries 1975a). There is also a clear difference in the costs of maintenance respiration between species: the maintenance respiration of sorghum is about one-third that of white clover (McCree 1974). The existence of uncoupled or idling respiration in plants has been suggested to explain unexpected high rates (Beevers 1970) and low yields (Tanaka 1972b). It is also possible that part of the protein turnover represents a process of little use. 5.2.5. Bioenergetics of crop yields Crop yields can be compared agronomically in several ways: yield per crop, yield per day, and yield per year. From a physiologist’s point of view, a more meaningful comparison of crop yields can be made in terms of the amount of glucose or energy from which a harvested organ is produced. To simplify the pathway of yield formation, consider that glucose is the product of photosynthesis, different compounds such as lipids, proteins, and carbohydrates are synthesized from glucose, and harvested organs of different crops are composed of different proportions of these three. When glucose is converted into proteins, lipids, and carbohydrates, the conversion efficiency can be defined as: grams of a compound produced Conversion efficiency = (5.21) 1 gram of glucose
PHOTOSYNTHESIS AND RESPIRATION 211 Table 5.7. Relative yielding ability of various crops with different chemical compositions. a Chemical content (%) Yielding ability Crop Protein Lipid Carbo- Ash g/100 g Relative hydrate glucose value Rice 8.8 2.7 87.0 1.5 73 100 Maize 9.5 5.3 83.7 1.5 70 96 Wheat 12.1 2.3 83.7 1.9 71 97 Barley 11.6 2.2 83.4 2.8 71 98 Soybeans 39.0 19.9 35.4 5.7 45 62 Field bean 24.1 2.6 69.0 4.3 62 86 Potato 9.2 0.5 86.3 3.9 75 103 Sesame 21.2 54.7 18.4 5.7 37 51 a Yamaguchi (1978). The measured conversion efficiency on dry weight basis is 0.38 for proteins, 0.31 for lipids, and 0.84 for carbohydrates (Yamaguchi 1978). The definition of conversion efficiency is identical with that of the production value discussed in the preceding section (Table 5.5). The conversion efficiency and the theoretically calculated production value agree quite well for lipids and carbohydrates, but not for proteins. This discrepancy may be attributed to the complex nature of protein synthesis. The yields of different crops on the same basis, i.e., glucose, can be calculated using the conversion efficiency for different compounds when the chemical composition of the harvested organs is known (Table 5.7). Thus, 1 t rice yield is comparable to 0.62 t soybean and 0.51 t sesame. In general, the grain crops and potatoes have similar productivity because grains and tubers have similar chemical composition. The relative yields of soybean and sesame are much lower because the seed of these crops contain higher percentages of lipids and proteins, which require larger amounts of energy for biosynthesis. A similar calculation can be made using the production values shown in Table 5.5. When nitrate is the nitrogen source, 1 g photosynthate produces yields of 0.75 g for barley and rice; 0.64–0.67 for peas, beans, and lentils; 0.50 g for soybeans; and 0.42–0.43 g for rape and sesame (Sinclair and De Wit 1975). Thus, relative yields of different crops estimated by two different methods appear to be in good agreement.
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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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Yoshida - 1981 - Fundamentals of Rice Crop Science

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