Yoshida - 1981 - Fundamentals of Rice Crop Science

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24 FUNDAMENTALS OF RICE CROP SCIENCE 1.4.4. Functions of individual leaves a. Physiologically active centers. The rice plant at any given growth stage is composed of leaves of physiologically different ages and this suggests that those leaves are different in their contribution to the growth of the whole plant. Accumulation of 32 P in plant tissues is usually considered an indication of high metabolic activity. When 32 P is absorbed through the roots of solutioncultured rice plants, an accumulation is found in the topmost leaf and the third and fourth leaves (Fig. 1.23). In the topmost leaf that is still elongating, a 32 P accumulation is associated with a high respiratory rate. This leaf, however, is low in photosynthetic activity and depends on the lower leaves for assimilates. In other words, the topmost elongating leaf is dependent on the lower leaves. The third or fourth fully developed leaf from the top has the highest photosynthetic activity among the leaves and exports assimilates to upper leaves. These photosynthetically active leaves are considered the most important for growth of the whole plant and are called physiologically active centers (Tanaka 1961a). Any leaf serves as a physiologically active center at some point in the plant’s life cycle. Since new leaves develop upward as lower leaves die, the physiologically active centers move upward as growth advances. Thus, the physiologically active centers may be 7/0 leaf during the tillering stage, move to 9/0 and 10/0 leaves after panicle primordia initiation, and to 10/0 and 11/0 leaves during milky stage. b. Division of work among leaves. Leaves are different not only in age but in position relative to panicles or roots. Physical distance appears to control the direction of assimilate movement. At the milky stage, the flag leaf, 12/0, exports assimilates mainly to panicles, whereas the lowest leaf, 8/0, exports a large amount of assimilates to the roots (Table 1.7). Thus, grain growth appears to be largely dependent on the upper leaves, and root activity is maintained by assimi- 1.23. Distribution of 32 P, photosynthetic activity, and respiration of individual leaves of the rice plant (Tanaka 1960).
GROWTH AND DEVELOPMENT OF THE RICE PLANT 25 Table 1.7. Fate of 14 C assimilated by leaves of different positions at milky stage. a Position Assimilating leaves Plant parts Panicles Leaf blades Internodes of leaves – 12/0 11/0 10/0 9/0 8/0 Panicle–12/0 12/0–11/0 11/0–10/0 10/0–9/0 Roots – 12/0 10/0 8/0 2562 b 26,124 – – – – 620 376 24 16 – 370 160 130 46,560 34 – 240 70 70 70 a Tanaka (1958). b Counts per minute per 10-mg sample. – 60 106 66 50 40 51,360 114 112 40 40 444 lates sent by the lower leaves. This relationship, however, should not be taken as the fixed assignment of individual leaves. There is evidence in other plants that the division of work among leaves is flexible (King et al 1967, Rawson and Hofstra 1969). If, for some reason, the lower leaves are not properly functioning or die, the upper leaves may supply assimilates to the roots. c. Function of leaf sheath. The leaf sheath contributes very little to photosynthesis but it performs other important functions. Until internode elongation starts at around panicle primordia initiation, the culm remains very small, about 1 cm long, and the leaf sheath supports the whole plant. Even after the completion of internode elongation, the leaf sheath contributes to the breaking strength of the shoot by 30 – 60% (Chang 1964). Thus, it functions as a mechanical support to the whole rice plant. In addition, it serves as a temporary storage site for starch and sugar before heading (Murayama et al 1955, Togari et al 1954). The accumulated carbohydrate in the leaf sheath and culm translocates into the grains after heading. The estimated contribution of the accumulated carbohydrate to the grain carbohydrate normally ranges from 0 to 40%, depending on the rate of nitrogen application and growth duration (see Chapter 7). The accumulated carbohydrate also serves as a carbohydrate source for developing tiller buds. In contrast with the leaf sheath, leaf blades of healthy rice plants do not accumulate starch; those of plants infected with tungro virus accumulate an appreciable amount of starch, which can be easily demonstrated by the iodine reaction (Ling 1968). 1.5 CULM 1.5.1. Morphology The culm is composed of a series of nodes and internodes (Fig. 1.24). It is enclosed within the sheath before heading, and a small portion of the culm right below the panicle becomes exposed after heading.
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Page 5 and 6: CONTENTS Foreword Preface GROWTH AN
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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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