Effects of air temperature and light on grain filling of an indica and a japonica rice (Oryza sativa L.) under controlled environmental conditions

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1 Soil Science and Plant Nutrition ISSN: (Print) (Online) Journal homepage: Effects of air temperature and light on grain filling of an indica and a japonica rice (Oryza sativa L.) under controlled environmental conditions Shouichi Yoshida & Tetsuo Hara To cite this article: Shouichi Yoshida & Tetsuo Hara (1977) Effects of air temperature and light on grain filling of an indica and a japonica rice (Oryza sativa L.) under controlled environmental conditions, Soil Science and Plant Nutrition, 23:1, , DOI: / To link to this article: Published online: 29 Mar Submit your article to this journal Article views: 659 View related articles Citing articles: 48 View citing articles Full Terms & Conditions of access and use can be found at

2 Soil Sci. Plant Nut,., 23 (I), , 1977 EFFECTS OF AIR TEl\iPERATURE AND LIGHT ON GRAIN FILLING OF AN INDICA AND A japonica RICE (Uryza sativa L.) UNDER CONTROLLED ENVIRONl\IENTAL CONDITIONS Shouichi YOSHIDA and Tetsuo HARAl The International Rice Research Institute, Manila, Philippines Received July 19, Th~. effects of air temperature and light on the grain filling of an indica (IR20) and ajaponica flce (FuJlsaka 5) was studied in artificially lighted cabinets. Within the daily mean temperature range of 16 to 28 C, the higher the temperature, the faster the grains filled and matured. At 28 C, the upper grains of IR20 rice took 13 days to reach the maximum weight, whereas those of Fujisaka 5 took 18 days. The optimum daily mean temperature range to achieve maximum weight per grain was 19 to 25 C for IR20 and 16 to 22 C for Fujisaka 5. Apparently, IR20 rice is better adapted to higher temperatures during the ripening period than is Fujisaka 5 rice. More chalky grains occurred when the temperature was above or below the optimum range. Both day and night temperatures affected grain weight and grain quality. The daily mean temperature was found to be the most meaningful expression for describing the effect of temperature on grain filling. Low light intensity appeared to cause a slight delay in the grain filling of the whole panicle and a reduction in the percentage of filled grains on the lower branches. A combination of high light intensity and low temperature gave the best ripening grade (grain weight X percent filled grains). During the ripening of rice, the major effects of air temperature on grain yield is largely on the duration of the grain filling period and on the maximum weight per grain achieved; it also affects grain quality. Studies on japonica varieties of rice in Japan yielded evidence that the optimum air temperature for the ripening of rice is about 20 to 22 0 and temperatures higher or lower than this optimum may impair the grain yield (1-3,7-9,21). Such findings may be relevant to the tropics where temperature is higher than 20 to 22 0 during the ripening period of rice. Even in a cool region of Japan, adoption of early planting practices has shifted the ripening period to the hottest month of the year, and thus, the rice crop is exposed to day temperatures as high as 33 C during ripening (13). Studies with IRS rice at Los Banos, Philippines showed no difference in the 1,000 grain weight 1 Presently, College of Agriculture, Gifu University, Japan. 93

3 94 S. YOSHIDA and T. HARA between the wet season and the dry season crop (28), although the temperature was higher during ripening in the dry season than in the wet season (29). The grain filling period of a field rice crop was estimated to range between about 30 days at Los Banos, Philippines to 65 days at Sapporo, Japan and Yanco, Australia (20), based on visual observations of the maturity time. Presumably, these large differences in the length of the grain filling period are caused by differences in the air temperature. However, there is a dearth of reports on the effect of air temperature on the grain filling period of indica rice studied under a well-defined environment. Solar radiation is the most influential climatic factor determining grain yield during ripening of rice in the tropics (29). Most data on the effect of light on grain yield deals mainly with photosynthesis, i.e., supply of assimilates to the rice grains, and not with the length of the grain filling period, for which there is no reported experimental evidence at present. The present study examines the effects of air temperature and light on the length of the grain filling period, the final grain weight, and grain quality of one indica (IR20) and one japonica (Fujisaka 5) rice. MATERIALS AND METHODS Both IR20 (indica) and Fujisaka 5 (japonica) rices were used in all experiments of this study. Controlled environmentalfacilities. All experiments were conducted in the bioclimatic laboratory of the International Rice Research Institute at Los Banos, Philippines. The plants were grown in a glasshouse room until 3 days after anthesis, after which they were transferred to artificially lighted cabinets, Koitotron KG-I06 SHL-D cabinets. The relative humidity of the glasshouse room was maintained above 70 percent while the temperature was kept at 29 C between 0900 and 1700 hr and 21 C during the night. Daylength was according to the natural conditions. In the artificially lighted cabinets, light was provided by 11 X 400 W Toshiba Yoko lamps DR 400/T and 12 x 40 W Mitsubishi white fluorescent lamps. The spectral composition of the radiation is similar in the visible region to that of sunlight. However, the proportion of photosynthetically active radiation is only 25% of total short wave radiation for the artificially lighted cabinet compared to about 50 percent for sunlight. Photosynthetically active radiation was 114 cal cm- 2 day-l at the flag leaf level unless specified otherwise. Daytime was from 0600 to 1800 hr. The relative humidity in the artificially lighted cabinets was maintained at 70 percent during daytime and 80 percent during the night. Plant culture. To obtain a large number of panicles that would flower simultaneously, 20 pregerminated seeds were sown in a circular pattern in 4-liter plastic pots (19). Each pot contained 3 kg Maahas clay soil fertilized with 1 g N, 0.5 g P 2 0 6, and 0.5 g KIO. Sampling and measuring procedures. Grain samples were collected from the panicles

4 Temperature and Light on Rice Grain Filling 95 of the main culm at anthesis, 3 days later, and then every five days. To measure the length of the grain filling period, nine grains were sampled from the 3rd, 4th, and 5th positions from the top on three upper primary branches with the same flowering date. It was replicated three times. The panicles from which the nine grains were sampled were allowed to ripen to maturity and were used for measurement of yield components. The sampled grains were weighed immediately after harvest to record the fresh weight, then oven dried for 3 days at 70 0, and finally weighed for dry weight. Total nitrogen was measured with the Kjeldahl method. At maturity, 15 selected panicles were harvested for yield components and grain quality. The grains were separated by hand into filled and unfilled grains, unfilled grains were further divided by the iodine test into unfertilized and partially filled grains ( 5). Each of the 200 randomly sampled grains were de hulled and examined visually for chalky grains. Experiment 1. Effect oj the daily mean temperature on grain filling. The daily mean temperature was varied from 16 to 28 0 with a constant day-night temperature difference of 8 0. Thus, at a daily mean temperature of 16 0, the day temperature was 20 0 and the night temperature was 12 0; this is expressed as 20 /12 0. Experiment 2. Effect of night temperature on grain weight and grain quality. The night temperature. was varied from 14 to 32 0 with a constant day temperature of 32 0; thus, the dally mean temperature ranged from 23 to Experiment 3. Effect oj day temperature on grain weight and grain quality. The day Table 1. Effect of daily mean temperature on grain weight and grain quality of IR20 and Fujisaka 5 rices. Day Night Daily mean Grain Filled Partially Grain quality tempera- tempera- tempera- Unfertilized wtl) grains filled grains Chalky' Green ture ture ture (mg/grain) grains (%) (%) grains grains (0C) (OC) (OC) (%) (%) (%) IR ( 86)' II ( 98) (100) ( 98) ( 93) Fujisaka (100) (100) (100) ( 95) ( 89) LSD value at 5% level is 0.2 for both varieties. II Including chalky, white center, white belly and white back grains. I. Figures in parenthesis indicate relative values taking the maximum as 100.

5 96 s. YOSHIDA and T. HARA temperature was varied from 16 to 34 0 with a constant night temperature of 16 0; thus, the daily mean temperature ranged between 16 and Experiment 4. Effict oj temperature and light on grain filling. Two levels of light intensity (low light=36 cal cm- 2 day-i; high light=1l4 cal cm- I day-i) were combined with two levels of temperature (23 /15 0 and 32 /24 0). The low light intensity was achieved by placing a mesh screen beneath the bank of lights. RESULTS Effect oj the daily mean temperature on grain filling Grain growth curve. Five daily mean temperatures (Table 1) were used, but in this coverage data will be presented for only three daily mean temperatures (16, 22, and 28 0). The slope in the grain growth curve suggested that the rate of grain filling was faster at the higher temperature in both varieties (Fig. 1). Additionally, the grain filling period was shorter at the higher temperatures for both varieties of rice. The leaves of IR20 rice turned yellow within 24 hours only after the plants were subjected to daily mean temperature of 16 0, indicating that day temperature of 20 0 is subnormal for leaf activity, i.e., photosynthesis (14) and others. Grain filling at this temperature was very slow, but the final grain weight was similar to that achieved under the 22 0 and the 28 0 regimes. The duration of grain filling period, defined as number of days required to reach the maximum weight, was 13 days at 28 0 and 33 days at 16 0 for IR20 rice c::.oj... &. tlo 15 E '-'....s::: tlo 'iii ~ c::.iij fujisaka o Days after flowering Days Ifter flowerini Fig Effect of temperature on grain growth of IR20 and Fujisaka riel's. Leaves of Fujisaka 5 rice remained green even at a daily mean temperature of 16 C. The grain growth curve of Fujisaka 5 at 16 0 was characterized by a long lag-phase following a linear phase. At a daily mean temperature of 28 C, the grain weight declined after the peak weight was reached, and the final grain weight was about 15

6 Temperature and Light on Rice Grain Filling 97 percent less than that at 16 C. The grain filling period was 18 days at a daily mean temperature of 28 C and 43 days at 16 C. Thus, temperature appears to affect markedly the grain filling period. The length of the grain filling period differed between the varieties; at a daily mean temperature of 28 C, Fujisaka 5 required five additional days to reach the maximum grain weight over the time required by IR20 rice. Accumulation ofnitrogen in the rice grains. The accumulation of nitrogen in rice graim can be assessed in two ways, i.e., changes in the percent nitrogen and in the total nitrogen per grain. During the initial stages of grain filling, the percent nitrogen was relatively high, indicating that the accumulation of nitrogen relative to the starch accumulation in the grains was faster at this stage than at later stages (Fig. 2). At maturity, within a, o~--~--~~--~--~~~--~--~~~~--~~ Doy.afftr~ Fig. 2. Nitrogen uptake by IR20 and Fujisaka 5 rices at successive stages of grain growth under three temperature regimes. FUJISAKA5 ~... '" -c: '" 'E 0 u ~ ~ ~ _ 9' Days after flowering Days after flowering Fig. 3. Changes in the water content at successive stages of grain growth of two varieties of rice under three temperature regimes. 50

7 98 S. YOSHIDA and T. HARA daily mean temperature range of 16 to 28 C, the higher was the temperature, the higher was the percent of nitrogen. Curves for the accumulation of nitrogen per grain are basically similar to those for dry weight; the higher was the temperature, the faster the nitrogen accumulated in the rice grains. The variation in the percent nitrogen among the three temperature regimes was greater than that of the nitrogen accumulated per grain. However, because the weight per grain ofir20 was lower than that of Fujisaka 5, IR20 rice contained less nitrogen per grain, but contained a higher percent nitrogen than did Fujisaka 5. Changes in water content in the grains. During the initial phases of the grain filling period the water content of the grains was about 58 percent and then declined to less than 17 percent during the later stages of development (Fig. 3). The rate of this decrease in water content with maturity increased with the increase in the temperature regime. The water content of grains at the time of maximum grain weight, in general, ranged from 19 to 24 percent, but it was 29 percent for IR20 rice grains when the daily mean temperature was 16 C. Grain weight and grain quality. Based on data collected from whole panicle grains, the optimum daily mean temperature for maximum weight per grain appeared to range between 19 and 25 C for IR20 rice and 16 to 22 C for Fujisaka 5 (Table 1). In IR20 rice the grain weight of upper branches at a daily mean temperature of 16 C did not differ appreciably from that obtained at a daily mean temperature of 22 C (Fig. 1), but the average grain weight of the whole panicle at 16 C was significantly lower than that at 22 C. This was because at 16 C the grains of the upper branches matured normally but those of the lower branches were not mature at harvest. The lowest weight per grain for IR20 rice occurred with a daily mean temperature of 16 C, while that for Fujisaka 5 occurred with a daily mean temperature of 28 C. IR20 rice appeared to be sensitive to a low daily mean temperature while Fujisaka 5 appeared to be sensitive to a high daily mean temperature. Similarly, the maximum amount of chalky grains in IR20 rice occurred at 16 C, while this was true for Fujisaka 5 rice at 28 C. A lower weight per grain seemed to be associated with a higher percentage of chalky grains. Green grains were found with all three temperature regimes with IR20 rice, but they were found only at 16 C with Fujisaka 5 rice. Additionally, the brown rice of IR20 became red brown in color when the daily mean temperature ranged between 16 and 22 C. Effect oj night temperature on grain weight and grain quality Within a range of night temperatures of 14 to 32 C with a constant day temperature of 32 C, the grain weight among the different temperature regimes was similar, except when the night temperature for Fujisaka 5 rice was HOC (Table 2). The daily mean temperature of the 32 /14 C temperature regime was 23 C which should have been optimum for maximum grain weight of Fujisaka 5 rice (Table 1). Apparently, the large difference (l8 C) between the day and night temperatures had an adverse effect on the ripening of Fujisaka 5 rice plants. The large difference in the day and night temperatures had little effect on the grain weight of IR20 rice.

8 Temperature and Light on Rice Grain Filling 99 Table 2. Effect of night temperature on grain weight and grain quality of 1R20 and Fujisaka 5 rices. Grain quality Night Daily mean Grain temperature ll temperature wtt) Chalky Green (0C) (0C) (mg/grain) grains grains (%) (%) IR ( 98) (100) ( 98) ( 95) 81 5 Fujisaka ( 90) (100) ( 98) ( 94) ) Day temperature was kept at 32 C. I) LSD value at 5% level is 0.2 for both varieties. I) Figures in parenthesis indicate relative values taking the maximum as 100. Most chalky grains were found at the low (14 0) and high night (32 0) temperatures. In general, more chalky grains occurred than was expected when compared with the previously described experiment with similar daily mean temperatures. Since night temperatures as low as 15 C or 18 C did not seem to cause a high incidence of chalky grains (see Table 1), it was probably the high day temperature that affected the increased percentage of chalky grains at 32 /20 o e. For instance, 12 percent ofth~ir20" grains was chalky when the daily mean temperature was 25 C with a temperat~~ t regime of 29 /21 e. However, in this experiment, when the daily mean temperature was 26 C with a temperature regime of 32 j200e, 24 percent of the IR20 grains was chalky. In Fujisaka 5 rice, a high incidence of chalky grains was found not only at 32 /32 C but also at 32 /14 e. At 32 /14 C, either night temperature or day temperature or daily mean temperature should not cause the high incidence of chalky grains. Consequently, it must be the large difference in temperature between day and night which is responsible for the high incidence of chalky grains. Effect oj day temperature on grain weight and grain qualiry The weight ofir20 rice grains was the lowest when the day temperature was 16 C (Table 3), which is below the optimum for leaf photosynthesis of IR20 rice. The grain weight of Fujisaka 5 rice was the lowest when the day temperature was 34 C. IR20 rice appeared to be sensitive to a low day temperature and Fujisaka 5 rice was sensitive to a high day temperature. The largest percentage (99%) of Fujisaka 5 grains Were chalky at the highest day temperature (34 0), while the incidence of chalky grains of IR20 rice occurred at both

9 100 S. YOSHIDA and T. HARA Table 3. Effect of day temperature on grain weight and grain quality of IR20 and Fujisaka 5 rices. Grain quality Day Daily mean Grain tempera turell temperature wt" Chalky Green (0C) (OC) (mg/grain) grains grains (%) (%) IR ( 82) ( 98) (100) ( 93) 82 Fujisaka ( 97) (100) ( 98) ( 81) 99 1) Night temperature was kept at 16 C. I) LSD value at 5% level is 0.2 for both varieties. I) Figures in parenthesis indicate relative values taking the maximum as Grain wi (1IlQ/~ain I 20 o o~--':k)---:':2o:---...i30i.--...i40 0 K) 20 Days oller flower"" Fig. 4. Grain growth of upper grains and whole panicle in IR20 rice under two levels of light intensity and temperature.

10 Temperature and Light on Rice Grain Filling 101 the lowest (16 0) and the highest (34 0) day temperature, 87 and 82 percent, respectively. Effect of temperature and light on grain filling Grain growth curve. While light intensity did not affect the length of the grain filling period in the upper grains, it affected the grain weight of IR20 rice (Fig. 4) and of Fujisaka 5 rice (Fig. 5). The length of the grain filling period in the whole panicle grains was increased by 5 days at a high light intensity for IR20 rice only, but it was extended by 10 and 5 days at a low light intensity for IR20 and Fujisaka 5, respectively. This suggests that low light intensity may delay the ripening of the whole panicle grains, probably because of a limited supply of assimilates to grains on lower branches. The effect of light intensity on grain weight was more pronounced on the grains of the whole panicle than on the upper grains, because all of the grains (filled and unfilled) were used to measure the average weight of the whole grains and the low light intensity increased the percentage of unfilled grains at the lower branches (Fig. 6). To understand better how light intensity affected grain filling of the whole panicle, sample grains were collected from primary branches at different positions. Similar UPPER GRAtlS 2~~======::! ~=====:: 20 I~ 10 O~--~~--~20~--~~--~~~ O~--~O~--20~---~~ Days orlef rlcooorinq Fig. 5. Grain growth of upper grains and whole panicle in Fujisaka 5 rice under two levels of light intensity and temperature.

11 102 S. YOSHIDA and T. HARA results were obtained for both IR20 and Fujisaka 5 at two temperatures, hence the data are presented for only IR20 at a daily mean temperature of 28 C (Fig. 6). Low light intensity consistently decreased grain weight to a small extent on all the branches. On the other hand, low light intensity decreased the percentage of filled grains markedly on the lower branches of the panicle. These effects oflight intensity on grain filling are in good agreement with our previous findings that low light intensity during ripening period affects grain yield through decreasing the percentage of filled grains but it has little effect on grain weight (29). Grain weight and percentage oj filled grains. Since light affects both grain weight (W) and percent fllled grains (F), the overall effect of the light-temperature interaction on grain yield can be best assessed by computing W X F, i.e., ripening grade. The combi. Xl 1.s:: g 2 as li 3 ~4 E '~ 5 '0 6 Grain weight (mg) Filled grains (%) Grain number o O'{-_-i5~_.!.Ir-0_--=-15;; 'in 8 ~ 9' ' ~~ ~ Fig, 6. Effect of light on grain filling ofir20 riee at 32/24 0. (Low light-36 cal. em-" day-i; high light=1l4cal. cm-. day-i PAR.) (PAR: Photosynthetically active radiation). Table 4. Effect of light and temperature on grain growth of IR20 and Fujisaka 5 rices. Grain Filled Ripening Lightll Temperatures, weight (W)I' grains (F) grade (mg) (%) (WxF) IR20 Low Low ( 71)11 Low High )0 ( 64) High Low (100) High High ( 96) Fujisaka 5 Low Low ( 90) Low High 2M ( 82) High Low (100) High High ( 96) 1) Low: 36cal. em-i. day-l PAR (photosynthetically active radiation), High: 114cal. cm- day-' PAR. II Low: 23/15 0, High: 32/24 0. I' LSD value at 5% level is 0.2 for both varieties. "Figures in parenthesis indicate relative values with 100 serving as the maximum.

12 Temperature and Light on Rice Grain Filling 103 nation of high light intensity and a low temperature gave the maximum ripening grade for both rice varieties (Table 4). Taking the maximum value as 100 for each variety, the ripening grade was reduced by 36 percent in IR20 and 18 percent in Fujisaka 5 when the light intensity was low and the temperature was high. Apparently, grain filling, and hence, the yield of IR20 is more likely to be reduced by adverse climatic environments than are those of Fujisaka 5 rice. DISCUSSION The length of the grain filling period markedly affects the yield of rice because it affects the maximum amount of solar energy available for grain filling. Additionally, it constitutes the last part of the rice plant's life cycle; this knowledge about the grain filling period can contribute to the overall understanding of the total growth duration of rice varieties. Existing information on the effect of environmental factors and (or) plant physiological factors on the length of the grain filling period of rice is scarce. An.n et al. (1) reported that while the grain filling of a japonica rice was completed in about 25 days after anthesis at a daily mean temperature of 25 C, it took much longer when the daily mean temperature was 21 C, and was not complete even after 75 days from anthesis when the daily mean temperature was 17 C. Their experiment was conducted in temperature-controlled glasshouse rooms where day and night temperatures were held constant. NAGATO and EBATA (11, 12) also measured the rate of grain filling under different temperature regimes. When YAMAKAWA (24) planted 12 rice varieties in the field in Japan at different times so that these rice crops would be exposed to different daily mean temperatures during the ripening period, he showed that japonica rice ripened over 30 days when the daily mean temperature was about 29 C, while ripening took 53 days when the daily mean temperature was 18 e. The time for maturity was determined by visual examination in this experiment. In the present study, rice grains were found to mature faster than was expected, only 13 days were required for the upper grains of IR20 rice to reach the maximum weight when the daily mean temperature was 28 C, and 18 days for Fujisaka 5 rice. When the daily mean temperature was lower, 16 C, the upper grains of IR20 rice achieved maximum weight in 33 days, whereas those of Fujisaka 5 rice required 43 days. The critical low temperature for photosynthesis has been reported as IS.5 C for japonica rice (23) and between 20 and 25 e for indica rice (14). In the experiment of Aimi et al., the day and night temperatures were both 17 0 which is a subnormal temperature for photosynthesis of japonica rice, In our experiment, however, at a daily mean temperature of 16 0, the day temperature was held at 20 C, which is above the critical low temperature for photosynthesis of japonica rice. This difference in the temperature regimes may explain the variation in the results obtained in the present study and those of Aimi et ai, It is normally assumed that light intensity has little or no effect on the length of the

13 104 S. YOSHIDA and T. HARA grain filling period. The present findings indicate that low light intensity extends the grain filling period of the grains of the whole panicle by about 5 days by affecting the filling of the grains on the lower branches of the rice panicle. Light intensity did not affect the length of the grain filling period of the grains on the upper branches of the rice panicle. For field rice crops in the tropics, where the daily mean temperature varies between 25 and 30 C, the duration of ripening is reported to be about 30 days after flowering (20). The duration of ripening is considered the sum of the length of the grain filling period for the first-flowering spikelet, difference in days between the first and last flowering spikelets on the same panicle, and the duration of panicle emergence. In the present study, the percentage fertility was 94 to 92 percent in IR20 rice and 81 to 9(} percent in Fujisaka 5 rice, when the plants were allowed to remain in the glasshouse room at 25 C for 3 days after the first spikelet flowering in each panicle. This indicates. that 3 days are about enough between the time of anthesis of the first spikelet and fertilization of the last spikelet on the same panicle. Panicle emergence takes 14 days in the field crop ofir20 rice at Los Banos, Philippines. Thus, the sum of each term, i.e., ( ), becomes 30 days for an IR20 rice crop at Los Banos. However, when flowering (or heading) or a rice crop in the field is. defined in agronomic terms as the time when 50 percent of the panicles of a rice crop has emerged, the length of the grain filling period for IR20 rice as a field crop at Los. Banos is 23 days. The optimum daily mean temperature of 20 to 22 C (J, 3) reported for the ripening of japonica rice agrees with the results of statistical analysis of the effects of climatic factors on rice yield (2, 7-9). Despite the fact that different varieties of rice, locations~ and techniques were used, the optimum temperatures reported were remarkably similar. This may be because in Japan there may not be any differences among japonica rice varieties in response to temperature during ripening. Based on this premise, the results of the present study can be compared with those reported in Japan. In this study, the optimum daily mean temperature for grain filling was found torange from 19 to 25 C for IR20 rice and from 16 to 22 C in Fujisaka 5 rice. There appears to be no sharp optimum temperature for grain filling, only a temperature range that is optimum for grain filling. In their statistical analyses of effects of temperature on rice yield, MURATA (9), HANYU et al. (2), and MURAKAMI (8) developed similar parabolic equations in which the daily mean temperature and solar radiation or sunshine hours are used as climatic parameters. :Murata's equation is: r =s [ (t-21.5)2] where Y is grain yield (kg/lo a), Sand t are solar radiation (calc m- 2 day-i), and temperature (0C), respectively, in August and September. When 2BoC is substituted for t, a 74 percent reduction in yield is obtained as compared with the maximum yield achieved with a daily mean temperature of21.5 C. However, our experimental results. show that the grain weight of Fujisaka 5 rice was reduced by only about 11 percent at 28 C as compared with the maximum weight achievable. Recently, MURATA (JO}

14 Temperature and Light on Rice Grain Filling 105 reported the results of one field experiment in which the grain weight varied from 24 g at a daily mean temperature of 22 C to about 21 g at 28 C, about a 13 percent reduction. Probably, the equation obtained by statistical analysis may help to explain yield differ -ences attributable to climatic components in certain ecological regions, but apparently, the equation is not appropriate for the general relationship between parameters used and yield. Some unexpressed or unrecognized factors associated with temperature ~hanges might be involved in the temperature term of the equation. One complicating factor in temperature studies is the possibility that the day or the night temperature has a specific physiological role (22), in which case the daily mean temperature may not be meaningful. Day temperature is obviously important for photosynthesis. As noted earlier, the difference in the duration of grain filling between the present results at a daily mean temperature of 16 C and those of Aimi et al. at a daily mean temperature of 17 C may be better understood in terms of the differ -ence in day temperature. High night temperature has been shown to have deleterious effects on the ripening -of rice (4, 11,25,26), such as lowering the final grain weight and increasing the in ~idence of chalky grains and other poor quality grains. While a high night temperature increases respiration, which might account for the impaired ripening (27), Moss.et al. (6) demonstrated that a high night temperature increased respiration in the night, and that this in turn, caused an increase in photosynthesis during the day. Hence, it is difficult to ascribe the adverse effect of high night temperature on grain filling to increased respiration. Furthermore, high day temperatures may cause results similar to those of high night temperatures _"",,- Fig :::: 20.s:; 110 "; c: 18.~ D Experimer1 123 IR20.,. Fu;SOI<o 5 0 V D D v /.-:'-.-'~ Mean temperature ('C) Relationship between 1,OOO-grain weight and mean temperature for two varieties of rice. D

15 106 S. YOSHIDA and T. HARA To better understand how temperature during the grain filling period affects grain weight, the grain weight was plotted against the daily mean temperature, regardless of the day and night temperatures (Fig. 7). There was a smooth and consistent pattern in the changes in grain weight as affected by the daily mean temperature in both IR2(} and Fujisaka 5 rices, except with Fujisaka 5 when the difference between the day and night temperature was The maximum grain weight of Fujisaka 5 rice was. achieved at the low temperature range, while the grain weight of IR20 remained high and stable at the high temperature range. This implies that temperature, regardless. of day or night, affects grain weight, and such effect of temperature is best related tathe daily mean temperature. Thus, within the temperature range between 14 and 34 0, the daily mean temperature appears to be the most meaningful expression by which to describe the effect of temperature on ripening of IR20 and Fujisaka 5 rices. In the continent type of climate, large differences in the day and night temperatures are common (15). A night temperature of 15 0 has been reported to prevent or delay panicle initiation of rice even when the day temperature is maintained at 30 C or 33 0 (16-18). In the present experiment, however, the large difference of lboe between day and night temperatures appeared to have an adverse effect on the ripening of Fujisaka 5 rice rather than a low night temperature per se. At Los Banos, Philippines, high yields are normally achieved in experimental fields in the dry season when rice matures in April or May. These two months are characterized by high temperatures and high solar radiation. In terms of yield components, high yields in the dry season crop are clearly associated with a large grain number per square meter (29). In addition, however, high temperature does not appear to have any noticeable adverse effects on grain filling, probably because IR2() rice and other indica varieties are well adapted to high temperatures during the grain filling period (Fig. 7). REFERENCES 1) Anu, R., SAWAMURA, H., and KmolNo, S., Physiological studies on the mechanism of crop plants. The effect of the temperature upon the behavior of carbohydrates and related enzymes during the ripening of rice plant, Proc. Crop Sci. Soc. Japan, 27 (4), (1959) 2) llanyu,j., UCHIJIMA, T., and SUGAWAJlA, S., Studies on the agro-climatological method for expressing the paddy rice products (Part I). An agro-climatic index for expressing the quantity of ripening of" the paddy rice, Bull. Nat. Tolwku Agr. Exp. Sta., 34, (1966) 3) MATSUSHIMA, S., MANAKA, K., and TSUNODA, K., Analysis of developmental factors determining yield and yield prediction and culture improvement of lowland rice. XXXIX. On mechanism of ripening (5). XL. On the mechanism of ripening (6), Proc. Crop Sci. Soc. Japan, 25, (1957) 4) MATSl.JSHIMA, S. and TSUNODA, K., Analysis of developmental facton determining yield and its application to yield prediction and culture improvement of lowland rice. XLV. Effects of temperature and its daily range in different growth stages upon the growth, grain yield and its constitutional factors. Proc. Crop Sci. Soc. Japan, 26, (1958) 5) MATSUSHIMA, S. and TANAKA, T., Analysis of developmental factors determining yields and its appli-

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