Yoshida - 1981 - Fundamentals of Rice Crop Science

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PHYSIOLOGICAL ANALYSIS OF RICE YlELD 233 7.2. The contribution of accumulated carbohydrate in the vegetative parts at heading to grain yield (Cock and Yoshida 1972). after flowering (Murayama et al 1955). This indicates that when the current supply of assimilate is insufficient for rapid grain growth, the accumulated carbohydrate can be easily translocated into the grain. This translocation may prevent the occurrence of the unfilled spikelets (spikelets that are fertilized but fail to grow at early stages of grain filling) and is important when photosynthesis during ripening is temporarily restricted by cloudy or rainy weather in monsoon Asia. Grain carbohydrate depends more on accumulated carbohydrate when light intensity after heading is low because photosynthesis during ripening is reduced (Soga and Nozaki 1957). At low nitrogen levels, large amounts of carbohydrate accumulate in the vegetative parts before heading and contribute substantially to the grain carbohydrate (Murayama et al 1955, Wada 1969, Yoshida and Ahn 1968). 7.2. CURRENT PHOTOSYNTHESIS DURING RIPENING The contribution of accumulated carbohydrate to grain carbohydrate ranges from 0 to 40% under most conditions. It follows that the photosynthesis during ripening contributes to grain carbohydrate by 60–100% under most conditions. The photosynthetic contribution by different plant parts to the grain—difficult to assess because of technical problems (Yoshida 1972) — is affected by the potential photosynthetic activity, longevity of the tissue during ripening, and light environment in a crop canopy. The potential photosynthetic activity of different plant parts can be estimated by measuring dark respiration and photosynthesis when light is not limited (e.g.
234 FUNDAMENTALS OF RICE CROP SCIENCE Table 7.1. Photosynthetic activity of leaves, sheaths, and panicles at a light intensity of 60 klx at milky stage. a Leaf blade 1 c 2 3 4 All leaves Leaf sheath plus culm Panicle Total Surface area (cm 2 ) 24 54 51 37 Assimilation b Rate (mg CO 2 /h) (mg CO 2 /dm 2 per h) P n R d P g P n R d 8.3 (30) 0.44 (9) 8.7 (27) 34 1.8 10.1 (36)0.52 (11) 10.6 (33) 19 1.0 5.6 (20)0.46 (11) 6.1 (19) 11 0.9 2.2 (8) 0.34 (7) 2.5 (8) 6 0.9 26.2 (94) 1.76 (38) 27.9 (87) 1.2 (4) 0.60 (14) 1.8 (5) – – 0.6 (2) 2.05 (48) 2.7 (8) – – 28.0(100) 4.46(100) 32.4(100) – – a Yoshida and Cock (1971); variety IR22 grown in the field. b P n = net photosynthesis, R d = dark respiration, P g = gross photosynthesis. Figure in parentheses indicate percentage of the total. c Counted from the top. Table 7.1). The potential net photosynthesis of the leaves was 94% of the total. The fourth leaf had a very low net photosynthesis both on a per-leaf basis and on a per-unit-area basis. The top leaf had the highest net photosynthesis per unit area but the second leaf was larger in size and had a greater potential net photosynthesis on a per-leaf basis. In rice, all the leaves from the flag leaf down to the third leaf from the top export assimilates to the panicle. Lower leaves send their assimilates to the roots (Tanaka 1958). The top 3 leaves of plants in an IR8 crop made up 74% of the total leaf area when LAI was 5.5 at heading. Flag leaves were only 19% of the total leaf area, and the second and third leaves made up 55% (Yoshida and Cock 1971). The net photosynthesis of the leaf sheath and panicle is extremely small. Even in terms of gross photosynthesis ( P g ), the panicle (8%) and the leaf sheath (5%) have extremely low photosynthetic capacity. Hence, both can be considered nonphotosynthetic tissues (Tsuno et al 1975). The longevity of the green tissue must be considered when assessing the photosynthetic contribution of different plant parts during ripening. The panicle becomes yellow relatively early in ripening but the leaves remain green much longer. The flag leaf remains green and maintains active photosynthesis until maturity (Takeda and Maruta 1956). At later stages of ripening, however, photosynthesis may not make much of a net contribution to grain production because rapid grain growth occurs earlier and slows down toward maturity. The light environment of different plant parts in a crop canopy is extremely important for determining the real photosynthetic activity of a given part. Panicles of improved indica varieties tend to bend and become positioned below the flag leaf where they are heavily shaded by the leaf canopy, particularly when the LAI is large. Panicles of these varieties are unable to make a significant contribution to grain filling because of their low potential photosynthetic activity and low light
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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 203 and 204: 5.1. PHOTOSYNTHESIS 5.1.1. Photosyn
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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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