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Rice seedling establishment as affected by cultivar, seed coating with calcium peroxide, sowing depth, and water level
ELSEVIER
Field Crops Research 41 (1995) 123-134
Field Crops Research
Rice seedling establishment as affected by cultivar, seed coating with calcium peroxide, sowing depth, and water level M. Yamauchi a'*, P.V. Chuong b a International Rice Research Institute (IRRI), P.O. Box 933, Manila, Philippines b Vietnam Agricultural Science Institute, Hanoi, Viet Nam
Received 8 August 1994; accepted 20 February 1995
Abstract
Inconsistent seedling establishment is a constraint to the adoption of direct seeding of lowland rice ( O r y z a sativa L.) in the tropics. Rice cultivars with superior seedling establishment in flooded soil have been recently identified. The establishment of these tolerant cultivars was compared with a control cultivar with and without calcium peroxide-coated seed under various combinations of water level and sowing depth. Water level had little effect on seedling establishment when seed were sown on the soil surface, but establishment was reduced by raising the water level when seed were sown below the soil surface. Calcium peroxide-coated seed established better than the tolerant cultivars at 13- and 25-mm sowing depths, but their seedlings were shorter and less vigorous than those of tolerant cultivars. Tolerant cultivars and coated seed had longer mesocotyls than controls. Sowing tolerant cultivars beneath a flooded soil surface at less than 13 mm assists achievement of consistent seedling establishment in lowland rice production. Keywords: Calcium peroxide; Crop establishment; Management practices; Rice; Variety trials
1. Introduction Rice can be established by transplanting young seedlings, sowing nongerminated seed into dry soil, or sowing pregerminated seed onto the surface of watersaturated, drained, or flooded soil (wet sowing). Although wet sowing is becoming popular among tropical rice farmers, inconsistent seedling establishment is still a major drawback. Rice seedlings can establish on the surface of flooded soil (Jones, 1933) because the standing water in ricefields contains enough dissolved oxygen (Chapman * Correspondingauthor. Present address: Chugoku National Agricultural Experiment Station, Nishifukatsu 6-12-1, Fukuyama 721, Japan. 0378-4290/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10378-4290(95)00008-9
and Peterson, 1962; Chapman and Mikkelsen, 1963). The seed are exposed to an aerobic environment as long as they are on the soil surface. However, it is difficult to place seed uniformly on the surface of flooded or drained (water-saturated) soil because the physical condition of the surface is heterogeneous and is changed by many factors, e.g., intensity of puddling and leveling, soil properties, time of sowing after puddling, water level, solar radiation, and rainfall. The contact o f broadcast seed with soil differs according to physical conditions. Seed sown on the soil surface are exposed to various disturbances and abiotic stresses, making their establishment uncertain. They are often damaged by birds and rats. When germinating seed are exposed to air, they dry and die in parts o f fields with inadequate soil
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M. YamauchLP.V. Chuong/ Field Crops Research 41 (1995) 123-134
moisture. In standing water, roots have difficulty penetrating the soil because of buoyancy (Mitsuishi, 1975) and drift, depending on water movement and wind direction. Heavy rain splashes germinating seed and causes the seedlings to lodge. Seedlings grow slowly in anaerobic flooded soil (Jones, 1933) and so establishment is erratic but can be improved by coating seed with calcium peroxide which releases oxygen (Yamada, 1952; Ota and Nakayama, 1970; Park et al., 1986), Yamauchi et al. (1993, 1994) recently found that seedlings of some cultivars are tolerant of hypoxia and can be established in flooded soil. The tolerant cultivars were selected based on establishment, height, and leaf development of seedlings planted into flooded soil, although their detailed performance has not been clarified compared with that of nontolerant (control) cultivars and calcium peroxidecoated seed. Seedling growth might be controlled by sowing depth and water level. In this, a long mesocotyl is an important characteristic for successful establishment of seed sown deep in the soil (Turner et al., 1982). Clarifying the interrelationships among sowing depth, water level, and the performance of cultivars and calcium peroxide-coated seed is a prerequisite for developing sowing techniques and for breeding suitable cultivars for direct sowing culture. Our study compared the seedling establishment of tolerant cultivars with that of control cultivars and calcium peroxide-coated seed at various water and sowing depths. The cultivars were tested by broadcast sowing on the surface of flooded and drained soil at different times after land preparation (to simulate current practices of farmers) and by sowing beneath the surface of flooded soil.
2. Material and methods 2.1. Plant material
Seed obtained from the International Rice Germplasm Center (IRGC) and the International Network for Genetic Evaluation in Rice, International Rice Research Institute (IRRI), and the National Institute of Agrobiological Resources, Tsukuba, Japan, were multiplied during the 1991 wet and 1992 dry seasons at IRRI. IR50, a cultivar released in the Philippines for
transplanting culture, was used as a control. The tolerant cultivars were previously identified in screening studies (Yamauchi et al., 1993) and have not been used commercially. Dormancy was broken by keeping the seed at 50°C for 5 d (Jennings and de Jesus, 1964), after which seed were stored below 5°C until sown. 2.2. Standard method
Seed were soaked in water for 24 h at 30°C, incubated for another 24 h in petri dishes, and then sown. Calcium peroxide-coated seed were prepared as follows. Soaked seed were combined with twice the seed dry weight of Calper ( 16% calcium peroxide product of Hodogaya Chemical Co., Tokyo) powder in a petri dish ( 14 cm diameter). The seed were stirred in Calper powder by shaking the petri dish while they were sprayed with water mist. Stirring and spraying continued until all the powder was attached to the seed. Coated seed were kept at room temperature (25-30°C) for 18 h to dry and harden the coat, and then sown. Seed were sown in either the experimental lowland ricefields at IRRI, Los Bafios, Laguna, Philippines, or in soil collected from the plow layer of flooded or upland fields at IRRI. The soils were clay with pH 6-7, cation exchange capacity (CEC) of 30-50 meq/100 g soil (Ponnamperuma, 1977). Established seedlings were pulled from the soil 14 d after sowing. The number of seedlings was recorded and establishment percentage was calculated based on the number of seed sown. Leaf score (1 = 1st leaf emerged, 2 = 2nd leaf emerged, etc.), seedling height, shoot dry weight, and mesocotyl length of at least 20 randomly selected seedlings per replication were determined. Soil hardness was measured with a crust hardness meter (DIK-5560, Daiki-rika, Tokyo) assembled with a spring (1 kg/40 mm) and a cone (8 mm diameter × 50 m m long). The detection limit was 0.03 kg/ cm 2. Redox potentials of soil and water were measured by inserting a platinum electrode (PTS-2019C, TOA Electronics, Tokyo) about 20 mm deep into the soil or water. 2.3. Experiment 1: Broadcast sowing into standing water
The experimental design was a split-plot with four replications. Main plot was the water level and the
M. Yamauchi,P.V. Chuong/ Field CropsResearch41 (1995)123-134
125
subplots were cultivar and seed coating. Each subplot was 15 cm in diameter and contained 40 seed. Upland soil, which passed through a 2-mm sieve, was placed in tanks ( 9 0 X 7 0 X 4 5 cm) to a height of 10 cm. Corn starch was added ( 1.3 g/kg soil) to lower the redox potential. The soil was flooded to 10 or 25 cm. Eight tanks corresponded to the main plots. Soil and water in the tanks were mixed 1 h before sowing. Seed were dropped from a height of 1 m above the water surface through a 15-cm-diameter PVC tube. Ten tolerant cultivars- ASD1 (IRGC Accession no. 6267, origin, India), Taothabi (13746, India), Chinchan (4882, China), Caloro/Blue Rose (16910, USA), CO 25 (3697, India), IR41996-50-2-1-3 (Philippines ), IR52341-60-1-2-1 (Philippines), IR3180248-2-2-2 (Philippines), Rikuu 132 (Japan), and Kuhei 2 (Japan) - and one control cultivar, IR50 with and without calcium peroxide coating, were sown into turbid standing water. Redox potential of the soil and water were 30 and 22 mV at sowing and 49 and 170 mV at 14 d after sowing, respectively (average of tanks). At sowing, soil pH was 6.6 and water pH was 6.9.
with four replications. Seeds of Taothabi, ASD 1, IR50, and calcium peroxide-coated IR50 were sown in field microplots. The water level was 25 m m below ( - 25 ram), above (25 mm), or at (0 mm) the soil surface. The water level was controlled by changing the level of soil surface in a steel quadrate frame ( 100 X 100 × 20 cm) that had holes (8 mm diameter, 380 holes/m 2) on the side walls. The frames were installed after puddling the field (the same field used for Experiment 2), and then submerged for 17 h before sowing. The field was drained and 50 seed were sown in a plot at a depth of 25 mm using forceps (row 100 cm long). Thereafter, water levels were controlled until sampling. Redox potential was 89 mV for the soil and 356 mV for water at sowing. Soil hardness for the water levels 0 and 25 mm was less than the detection limit, whereas that for the water level 25 mm below the soil surface was 0.03, 0.27, 0.50, and 0.62 k g / c m 2 at 2, 4, 7, and 14 d after sowing (average of plots), respectively.
2.4. Experiment 2: Broadcast sowing onto drained soil
The experimental design was a split-plot (main plot = water level; subplot = cultivar, seed coating, and sowing depth) with four replications. The subplot was 70 cm long and planted with 35 seed. The distance between subplots was 3 cm. Soil was collected from a flooded experimental field (pH 6.8, clay texture, CEC 43.9 meq/100 g), passed through a 5-mm sieve, and placed in tanks (70 X 40 × 15 cm). Each tank represented a main plot. Water level was controlled by changing soil height so that excess water overflowed. Seed of ASD 1, Taothabi, IR50, and calcium peroxide-coated IR50 were sown at depths of 0, 13, and 25 mm and with water levels of 0, 13, 25, and 38 ram. Seed were placed at a given depth using forceps. At sowing, redox potential was 168 mV for soil and 246 mV for water (average of tanks).
The experimental design was a split-plot (main plot = time of sowing, subplot = cultivar and calcium peroxide coating) with four replications. Subplot size was 1 m 2. Seed of Aswina (26289, Bangladesh), JC178 (9080, India), Taothabi, ASD1, IR50, and calcium peroxide-coated IR50 were broadcast at 400/plot from a height of 1 m onto the soil at 2 h, and 1, 2, and 3 d after puddling, leveling, and draining the soil (pH 7.0, clay texture, CEC 34.3 meq/100 g, 1.19% organic C). The experiment was conducted during the 1992 dry season in the IRRI field and received no rainfall. The field was flush-irrigated (irrigation immediately followed by drainage) 10 d after sowing. Soil redox potential in the field after puddling was 61 mV. Soil hardness of the field was 0.60, 0.97, 1,47, and 5.55 kg/ cm 2 at 9, 10, 12, and 14 d after sowing, respectively.
2.6. Experiment 4: Interaction between water level and sowing depth
3. Results
2.5. Experiment 3: Sowing beneath the soil surface
3.1. Broadcast sowing into standing water
The experimental design was a split plot (main plot = water level, subplot = cultivar and seed coating)
When seed were broadcast into standing water of 10 or 25 cm depth (Experiment 1 ), there were few floating
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M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
Table 1 Analysis of variance for establishment and growth characters of seedlings sown under various combinations of water level and sowing depth Experiment and source of variation
df
Mean squares Establishment
Height
Dry weight
L e a f score
Mesocotyl length
Experiment 1 Replication Water level ( W ) a Error ( a ) Cultivars/seed coating ( C ) W×C Error ( b )
3 1
171 6
26084 2497
18 57
0.25 1.31
3 11 11 66
248 258" * 45 46
52373 13915 * * 938 979
63 21 * * 3* 1
0.39 0.09* * 0.14"* 0.03
3 3
501 16819* *
627 983 *
9 5 15 60
633 1004" * 443 * * 174
3 2 6 3
Experiment 2 Replication Sowing time ( T ) Error ( a ) Cultivars/seed coating ( C ) T× C Error ( b )
84 116
0.14 1.60" *
235 6446* * 225 190
38 268" * 81 * * 31
0.20 0.23" * 0.05 0.06
1007 10178* 1512 1476" *
161 670 652 13055 * *
13 336 111 750* *
0.18 0.18 0.18 0.01
8.0 2.3 3.1 14.0" *
1 1 6 27
4389* * 35 63 305
3605 * * 828" * 53 87
128" * 482" * 23 12
0.00 0.02 0.05 0.03
8.9* 17.3 * * 3.6 2.1
3 3 9 2 3 1 1 1 6 6 9 18 132
651 2006" * 201 60338** 9008" * 23646** 2938" * 434 2091 * * 477" * 77 194 118
1230 2823 883 26819** 75856* ° 27568* * 91047" * 108952* * 4648* * 1095 665 923* 524
12 117 * * 16 440* * 606" * 78* 411 * * 1329" * 57* * 11 8 12 11
Experiment 3 b Replication W a t e r level ( W ) Error ( a ) Cultivars/seed coating ( C ) Contrast 1 Contrast 2 W XC Error ( b )
Experiment 4 ~ Replication W a t e r level ( W ) Error ( a ) Depth ( D ) Cultivars/seed coating ( C ) Contrast 1 Contrast 2 Contrast 3 D×C W × D W × C W×DxC Error ( b )
0.08 0.10 0.04 1.70"* 2.42" * 0.71 * * 5.02* * 1.57" * 0.31 * * 0.07 0.08 0.11" * 0.05
4.8 0.1 2.7 290.9** 59.5 * * 136.1 * * 36.3" * 6.1 * 19.1" * 2.2" 0.8 0.9 1.0
*, * *Significant at the 5 and 1% levels, respectively. a Insufficient error df for reliable F-test. b Contrast 1 = ASD1 and IR50 + Calper vs Taothabi and IR50; contrast 2 = ASD1 vs IR50 + Calper. c Contrast 1 = IR50 + Calper vs ASD1, Taothabi, and IR50; contrast 2 = IR50 vs ASD1 and Taothabi; contrast 3 = ASD1 vs Taothabi.
seedlings, presumably because seed were sown into turbid water created by mixing soil and water before sowing where, unlike the field, there was no water
movement. The seed were covered by a thin layer of soil (about 1-3 mm thick), which might have counteracted the upward rooting force. Significant differences
M. Yamauchi, P.V. Chuong /Field Crops Research 41 (1995) 123-134 Woter
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in seedling establishment, seedling height, shoot dry weight, and leaf score were noted among 11 cultivars and coated seed (Table 1 ). There was, however, little elongation of the mesocotyl. Variabilities in establishment and leaf score were less than those in dry weight and height (Fig. 1 ). Treatment ASD 1 gave the highest establishment (97-98 %) among the cultivars and coated seed, although the difference between ASD1 and control IR50 was not statistically significant. Seed coating had no effect on the establishment of IR50. The establishment of all cultivars and coated seed was the same (90%) at 10 and 25 cm water depth, suggesting that water level has little effect on establishment. ASD1 showed the best performance in dry weight and height among the cultivars and coated seed (Fig. 1). The simple linear correlations of dry weight and height between the water levels of 10 and 25 cm were significant at the 1% level with coefficients of 0.898 for dry weight and 0.877 for height, suggesting that such characters were controlled genetically and by seed coating.
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(c) Fig. 1. Seedling establishment, seedling height, and shoot dry weight of 11 rice cultivars and Calper (calcium peroxide) coated IR50 (Experiment 1). Germinated seed were broadcast onto standing water of 10 or 25 cm depth. Bars having the same letter within the identical water level are not statistically different at the 5% level by Duncan's multiple range test.
Time of broadcasting after puddling, leveling, and draining the field affected the settling of seed into the soil (Experiment 2). Seed broadcast 2 h after puddling sank 1-3 mm into the soil. Little elongation of the mesocotyl occurred. The seed sown 1 d after puddling settled on the soil surface such that half of each seed was in the soil and the other half was exposed to the atmosphere. Seed sown 2 and 3 d after puddling remained on top of the soil with little contact. Some of the seed sown 3 d after puddling dried out. "Percentage establishment and leaf score were more affected by time of sowing than by cultivar and seed coating, whereas seedling height and dry weight were related more to cultivar and seed coating than to time of sowing ( Table 1, Fig. 2). Establishment was greatest when seed were sown 1 d after puddling (88%). When seed were sown 2 h after puddling, establishment was greatest with calcium peroxide-coated seed, followed by cultivars ASD1 and Aswina. Sowing 2 and 3 d after puddling significantly reduced seedling establishment. Cultivars Taothabi and Aswina, two deep-water rices from Bangladesh, showed greater establishment than
128
M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
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Fig. 2. Effect of time of broadcasting seed after puddling, leveling, and draining the field on seedling establishment, seedling height, shoot dry weight, and leaf score of Aswina ( i ) , JC178 ([3), ASD1 (©), Taothabi ( 0 ) , IR50 (~7), and calcium peroxide-coated IRS0 ( • ) rice (Experiment 2). Points having the same letter at each time of broadcasting are not significantly different at the 5% level by Duncan's multiple range test. The LSD value is for comparison between times of broadcasting within a cultivar or seed coating (no LSD value indicates no significant effect of time of broadcasting). the other cultivars and coated seed, suggesting that they are tolerant o f dryness after germination. L e a f score increased as t i m e o f broadcasting increased (Fig. 2). T h e l o w e r l e a f score at 2-h s o w i n g
m i g h t i m p l y that d e v e l o p m e n t was delayed due to seed settlement under the soil surface. S h o o t dry w e i g h t r e s p o n d e d differently to time of broadcasting than the other characters measured, and
M. Yamauchi, P.V. Chuong/ Field Crops Research 41 (1995) 123-134
there was little consistency in the response among cultivars and coated seed ( Fig. 2). The seedlings of ASD 1 and Taothabi had greater dry weight and height than IR50 and calcium peroxide-coated IR50, particularly when the time of broadcasting was between 2 h and 1 d. 3.3. Sowing beneath the soil surface
At 25-mm sowing depth, the variability of seedling establishment due to water level was more than the variability due to cultivar and seed coating (Table 1, Experiment 3). On the other hand, seedling height, dry weight, and mesocotyl length varied more with cultivar and seed coating than with water level. Leaf score was not significantly affected by water level, cultivar, or seed coating. Seedling establishment of ASD1 was the same as that of calcium peroxide-coated IR50 and greater than that of IR50 and Taothabi (Table 1, Fig. 3 ). Dry weight and height of ASD1 and Taothabi were greater than those of IR50 and calcium peroxide-coated IR50. Calcium peroxide coating increased the mesocotyl length of IR50 regardless of water level. The greater mesocotyl length of Taothabi was negated by the increased water level. Lowering the water level increased soil hardness up to 0.6 kg/cm 2 at 14 d after sowing. Because seedling establishment was increased by lowering the water level, the increased soil hardness was apparently not detrimental. 3.4. Interaction between water level and sowing depth
Variations produced by sowing depth, cultivar, and seed coating were greater than those caused by water level (Table 1, Experiment 4). Although the effects of water level on seedling establishment and dry weight were significant, the variations induced by water level were smaller than those of sowing depth or cultivar and seed coating. Water level did not affect seedling height, leaf score, and mesocotyl length, but sowing depth and cultivar and seed coating did. The variations in height and dry weight were more pronounced by cultivar and seed coating than by sowing depth. On the other hand, seedling establishment was more variable with sowing depth than with cultivar and seed coating.
129
The interaction between water level and sowing depth was significant for establishment and mesocotyl length. The interaction between sowing depth and cultivar and seed coating was significant for all characters measured. Because there was no significant interaction between water level and cultivar and seed coating, the mean of cultivar and coated seed at each sowing depth versus water level and the mean of water level for each cultivar and seed coating versus sowing depth are presented (Figs. 4 and 5). When seed were sown on the soil surface, seedling establishment was high and not affected by water level (Fig. 4). Establishment was decreased, however, by the increase in water level when seed were sown at 13and 25-mm depths. Shoot dry weight decreased, regardless of sowing depth. Seedling establishment, height, dry weight, and leaf score decreased with greater sowing depth (Fig. 5). Seed coating effectively increased establishment, followed by use of tolerant cultivars. Taothabi produced taller and larger seedlings at all sowing depths. Seed coating reduced the height and dry weight of surface sown IR50 (Fig. 5). On the other hand, seed coating increased seedling height and dry weight at sowing depths of 13 and 25 mm, indicating that coating was beneficial when seed were sown deep in the soil. Regardless of sowing depth, seedlings of the tolerant cultivar Taothabi were taller and heavier than the coated IR50, implying that genetic potential is more effective than calcium peroxide coating for increasing seedling height and dry weight. The analysis of single degree of freedom contrast (Gomez and Gornez, 1976) for cultivar and seed coating clarified that the performance of calcium peroxidecoated IR50 significantly differed from those of ASD 1, Taothabi, and IRS0 (Table 1). ASD1 and Taothabi performed differently from IR50 (Table 1). The difference between ASD1 and Taothabi was significant except for establishment. Mesocotyls elongated little when seed were sown on the surface (0 mm depth) (Fig. 4). But at sowing depths of 13 and 25 ram, mesocotyls elongated and there were differences among cultivars and between seed coating (Fig. 5, Table 1). Seed coating increased mesocotyl elongation in IRS0. The tolerant cultivars had longer mesocotyls than IR50. The mesocotyl of IRS0 did not elongate in response to increased sowing depth (from 13 to 25 mm) but those of calcium per-
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oxide-coated IR50 and the tolerant cultivars did. Correlation coefficients between seedling establishment and mesocotyl length of 0.879 at sowing depth of 25 mm and 0.641 at 13 mm were statistically significant at the 1% level. Longer mesocotyls might have enhanced establishment of seed sown beneath the surface.
4. Discussion The stresses imposed on seedlings may comprise the complex combination of hypoxia, soil physical pressure, and toxic substances produced in the soil at low redox potential. In the present study, stress intensity was changed by combinations of water level and sow-
131
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ing depth. Deep sowing reduced seedling establishment more than did increased water level (Experiment 4). Once the seedling was successfully established, shoot dry weight and seedling height were more controlled by cultivar than by stress. The use of taller and more vigorous cultivars would be advantageous especially in competing with weeds.
Seed is sown into standing water in some parts of Asia, but it is less common in the tropics than in the temperate regions (De Datta, 1981). This method places seed on the soil surface. The results of Experiment 1 confirm the report of Jones (1933) that seedlings can establish in standing water. In addition, establishment was little affected by water level between
M. Yamauchi, P. V. Chuong /Field Crops Research 41 (1995) 123-134
132
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M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
0 and 25 cm and by cultivar (Experiments 1 and 4). Chapman and Peterson (1962) and Chapman and Mikkelsen (1963) reported that in California, seedling growth was not limited by dissolved 02. In addition, microbial photosynthetic activity supersaturated the standing water with 02. In the tropics, we also detected the supersaturation o f standing water in the field where rice seed were broadcast (unpublished). The lack o f response in establishment to the addition o f calcium peroxide (Experiment 1) also suggests that 02 does not limit establishment when seed are sown on the soil surface. The feasibility o f this sowing method would depend on the availability of water at the time of sowing, seedling anchorage, and presence of biotic stresses (snails, fish, insects, diseases) that destroy seedlings. More information on the improvement of agronomic practices is needed for seed sown on the surface of flooded soil. Sowing onto drained puddled soil surfaces is widely practiced in tropical Asia. Sowing is best timed so that seed land on the soil surface where both moisture and oxygen are available. Farmers know that seed settled beneath the soil surface develop poorly so they intentionally delay the time of broadcasting particularly when the soil is soft. Most farmers in the Philippines and Vietnam sow seed 1 d after puddling. Because the soil surface conditions may be determined by various factors, it might be difficult for farmers to achieve consistent seedling establishment. The results o f Experiment 2 demonstrated that although broadcast sowing just after puddling placed seed under the soil surface and delayed seedling development, more than 60% of seedlings established. Use of calcium peroxide coating or tolerant cultivars may improve establishment. Broadcast sowing just after puddling would ease the problems associated with surface sowing. In addition, combining the practice of broadcast sowing just after puddling with current farmers' practice (sowing seed 1 d after puddling) increases the time o f sowing, thus giving farmers more flexibility in farm operations. Sowing seed of tolerant cultivars into anaerobic soil at depths less than 13 m m resulted in establishment greater than 60% (Fig. 4). Seed can be coated with calcium peroxide to improve establishment (Yamada, 1952; Ota and Nakayama, 1970; Mitsuishi, 1975; Park et al., 1986). It may be difficult, however, for farmers to adopt the technology because seed coating requires
133
investments in chemicals and machinery, and knowledge o f seed coating procedures. Although seed coating is more effective in increasing establishment, the use of tolerant germplasm can substitute for coating when seed is sown at less than 13 m m depth. Compared with coating, tolerant cultivars have the advantage inasmuch as investments and knowledge of coating are, in this case, not required. Further, because seedlings may be taller and more vigorous, they might be more competitive with weeds. Breeding tolerant cultivars with appropriate agronomic characters might be important.
Acknowledgements This work is part o f a collaborative project between the Government of Japan and IRRI on the development of stabilization technology for rice double cropping in the tropics.
References Chapman, A.L. and Mikkelsen, D.S., 1963. Effect of dissolved oxygen supply on seedling establishment of water-sown rice. Crop Sci., 3: 392-397. Chapman, A.L. and Peterson, M.L., 1962. The seedling establishment of rice under water in relation to temperature and dissolved oxygen. Crop Sci., 2: 391-395. De Datta, S.K., 1981. Principles and Practices of Rice Production. John Wiley and Sons, New York. Gomez, K.A. and Gomez, A.A., 1976. Statistical Procedures for AgriculturalResearch with Emphasis on Rice. InternationalRice Research Institute, Manila, Philippines, 294 pp. Jennings, P.R. and de Jesus, J. Jr., 1964. Effect of heat on breaking seed dormancy in rice. Crop Sci., 4: 530-533. Jones, J.W., 1933. Effect of reduced oxygen pressure on rice germination. J. Am. Soc. Agron., 25: 69-81. Mitsuishi, S., 1975. Study on the direct underground sowing in the submerged field of rice. Special Bulletin. Ishikawa Prefecture College of Agriculture, Ishikawa, Japan. Ota, Y. and Nakayama,M., 1970. Effect of seed coating with calcium peroxide on germinationunder submergedconditionin rice plant. Proc. Crop Sci. Soc. Jpn., 39: 535-536. Park, S.H., Lee, C.W., Yang, W.H. and Park, R.K., 1986. Direct seeding cultivationon submergedpaddy in rice. I. Seedlingemergence and early growth under different temperature and seedling depth. Korean J. Crop Sci., 31: 204-213. Ponnamperuma, F.N., 1977. Physicochemical properties of submerged soils in relation to fertility. IRRI Res. Pap. Ser., 5: 1-32.
134
M. Yamauchi, P.V. Chuong / Field Crops Research 41 (1995) 123-134
Turner, F.T., Chen, C.C. and Bollich, C.N., 1982. Coleoptile and mesocotyl lengths in semidwarf rice seedlings. Crop Sci., 22: 43--46. Yamada, N., 1952. Calcium peroxide as an oxygen supplier for crop plants. Proc. Crop Sci. Soc. Jpn., 21: 65-66.
Yamauchi, M., Aguilar, A.M., Vaughan, D.A. and Seshu, D.V., 1993. Rice (Oryza sativa L.) germplasm suitable for direct sowing under flooded soil surface. Euphytiea, 67: 177-184. Yamauchi, M., Herradura, P.S. and Aguilar, A.M., 1994. Genotype difference in rice postgermination growth under hypoxia. Plant Sci., 100: 105-113.
Field Crops Research 41 (1995) 123-134
Field Crops Research
Rice seedling establishment as affected by cultivar, seed coating with calcium peroxide, sowing depth, and water level M. Yamauchi a'*, P.V. Chuong b a International Rice Research Institute (IRRI), P.O. Box 933, Manila, Philippines b Vietnam Agricultural Science Institute, Hanoi, Viet Nam
Received 8 August 1994; accepted 20 February 1995
Abstract
Inconsistent seedling establishment is a constraint to the adoption of direct seeding of lowland rice ( O r y z a sativa L.) in the tropics. Rice cultivars with superior seedling establishment in flooded soil have been recently identified. The establishment of these tolerant cultivars was compared with a control cultivar with and without calcium peroxide-coated seed under various combinations of water level and sowing depth. Water level had little effect on seedling establishment when seed were sown on the soil surface, but establishment was reduced by raising the water level when seed were sown below the soil surface. Calcium peroxide-coated seed established better than the tolerant cultivars at 13- and 25-mm sowing depths, but their seedlings were shorter and less vigorous than those of tolerant cultivars. Tolerant cultivars and coated seed had longer mesocotyls than controls. Sowing tolerant cultivars beneath a flooded soil surface at less than 13 mm assists achievement of consistent seedling establishment in lowland rice production. Keywords: Calcium peroxide; Crop establishment; Management practices; Rice; Variety trials
1. Introduction Rice can be established by transplanting young seedlings, sowing nongerminated seed into dry soil, or sowing pregerminated seed onto the surface of watersaturated, drained, or flooded soil (wet sowing). Although wet sowing is becoming popular among tropical rice farmers, inconsistent seedling establishment is still a major drawback. Rice seedlings can establish on the surface of flooded soil (Jones, 1933) because the standing water in ricefields contains enough dissolved oxygen (Chapman * Correspondingauthor. Present address: Chugoku National Agricultural Experiment Station, Nishifukatsu 6-12-1, Fukuyama 721, Japan. 0378-4290/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10378-4290(95)00008-9
and Peterson, 1962; Chapman and Mikkelsen, 1963). The seed are exposed to an aerobic environment as long as they are on the soil surface. However, it is difficult to place seed uniformly on the surface of flooded or drained (water-saturated) soil because the physical condition of the surface is heterogeneous and is changed by many factors, e.g., intensity of puddling and leveling, soil properties, time of sowing after puddling, water level, solar radiation, and rainfall. The contact o f broadcast seed with soil differs according to physical conditions. Seed sown on the soil surface are exposed to various disturbances and abiotic stresses, making their establishment uncertain. They are often damaged by birds and rats. When germinating seed are exposed to air, they dry and die in parts o f fields with inadequate soil
124
M. YamauchLP.V. Chuong/ Field Crops Research 41 (1995) 123-134
moisture. In standing water, roots have difficulty penetrating the soil because of buoyancy (Mitsuishi, 1975) and drift, depending on water movement and wind direction. Heavy rain splashes germinating seed and causes the seedlings to lodge. Seedlings grow slowly in anaerobic flooded soil (Jones, 1933) and so establishment is erratic but can be improved by coating seed with calcium peroxide which releases oxygen (Yamada, 1952; Ota and Nakayama, 1970; Park et al., 1986), Yamauchi et al. (1993, 1994) recently found that seedlings of some cultivars are tolerant of hypoxia and can be established in flooded soil. The tolerant cultivars were selected based on establishment, height, and leaf development of seedlings planted into flooded soil, although their detailed performance has not been clarified compared with that of nontolerant (control) cultivars and calcium peroxidecoated seed. Seedling growth might be controlled by sowing depth and water level. In this, a long mesocotyl is an important characteristic for successful establishment of seed sown deep in the soil (Turner et al., 1982). Clarifying the interrelationships among sowing depth, water level, and the performance of cultivars and calcium peroxide-coated seed is a prerequisite for developing sowing techniques and for breeding suitable cultivars for direct sowing culture. Our study compared the seedling establishment of tolerant cultivars with that of control cultivars and calcium peroxide-coated seed at various water and sowing depths. The cultivars were tested by broadcast sowing on the surface of flooded and drained soil at different times after land preparation (to simulate current practices of farmers) and by sowing beneath the surface of flooded soil.
2. Material and methods 2.1. Plant material
Seed obtained from the International Rice Germplasm Center (IRGC) and the International Network for Genetic Evaluation in Rice, International Rice Research Institute (IRRI), and the National Institute of Agrobiological Resources, Tsukuba, Japan, were multiplied during the 1991 wet and 1992 dry seasons at IRRI. IR50, a cultivar released in the Philippines for
transplanting culture, was used as a control. The tolerant cultivars were previously identified in screening studies (Yamauchi et al., 1993) and have not been used commercially. Dormancy was broken by keeping the seed at 50°C for 5 d (Jennings and de Jesus, 1964), after which seed were stored below 5°C until sown. 2.2. Standard method
Seed were soaked in water for 24 h at 30°C, incubated for another 24 h in petri dishes, and then sown. Calcium peroxide-coated seed were prepared as follows. Soaked seed were combined with twice the seed dry weight of Calper ( 16% calcium peroxide product of Hodogaya Chemical Co., Tokyo) powder in a petri dish ( 14 cm diameter). The seed were stirred in Calper powder by shaking the petri dish while they were sprayed with water mist. Stirring and spraying continued until all the powder was attached to the seed. Coated seed were kept at room temperature (25-30°C) for 18 h to dry and harden the coat, and then sown. Seed were sown in either the experimental lowland ricefields at IRRI, Los Bafios, Laguna, Philippines, or in soil collected from the plow layer of flooded or upland fields at IRRI. The soils were clay with pH 6-7, cation exchange capacity (CEC) of 30-50 meq/100 g soil (Ponnamperuma, 1977). Established seedlings were pulled from the soil 14 d after sowing. The number of seedlings was recorded and establishment percentage was calculated based on the number of seed sown. Leaf score (1 = 1st leaf emerged, 2 = 2nd leaf emerged, etc.), seedling height, shoot dry weight, and mesocotyl length of at least 20 randomly selected seedlings per replication were determined. Soil hardness was measured with a crust hardness meter (DIK-5560, Daiki-rika, Tokyo) assembled with a spring (1 kg/40 mm) and a cone (8 mm diameter × 50 m m long). The detection limit was 0.03 kg/ cm 2. Redox potentials of soil and water were measured by inserting a platinum electrode (PTS-2019C, TOA Electronics, Tokyo) about 20 mm deep into the soil or water. 2.3. Experiment 1: Broadcast sowing into standing water
The experimental design was a split-plot with four replications. Main plot was the water level and the
M. Yamauchi,P.V. Chuong/ Field CropsResearch41 (1995)123-134
125
subplots were cultivar and seed coating. Each subplot was 15 cm in diameter and contained 40 seed. Upland soil, which passed through a 2-mm sieve, was placed in tanks ( 9 0 X 7 0 X 4 5 cm) to a height of 10 cm. Corn starch was added ( 1.3 g/kg soil) to lower the redox potential. The soil was flooded to 10 or 25 cm. Eight tanks corresponded to the main plots. Soil and water in the tanks were mixed 1 h before sowing. Seed were dropped from a height of 1 m above the water surface through a 15-cm-diameter PVC tube. Ten tolerant cultivars- ASD1 (IRGC Accession no. 6267, origin, India), Taothabi (13746, India), Chinchan (4882, China), Caloro/Blue Rose (16910, USA), CO 25 (3697, India), IR41996-50-2-1-3 (Philippines ), IR52341-60-1-2-1 (Philippines), IR3180248-2-2-2 (Philippines), Rikuu 132 (Japan), and Kuhei 2 (Japan) - and one control cultivar, IR50 with and without calcium peroxide coating, were sown into turbid standing water. Redox potential of the soil and water were 30 and 22 mV at sowing and 49 and 170 mV at 14 d after sowing, respectively (average of tanks). At sowing, soil pH was 6.6 and water pH was 6.9.
with four replications. Seeds of Taothabi, ASD 1, IR50, and calcium peroxide-coated IR50 were sown in field microplots. The water level was 25 m m below ( - 25 ram), above (25 mm), or at (0 mm) the soil surface. The water level was controlled by changing the level of soil surface in a steel quadrate frame ( 100 X 100 × 20 cm) that had holes (8 mm diameter, 380 holes/m 2) on the side walls. The frames were installed after puddling the field (the same field used for Experiment 2), and then submerged for 17 h before sowing. The field was drained and 50 seed were sown in a plot at a depth of 25 mm using forceps (row 100 cm long). Thereafter, water levels were controlled until sampling. Redox potential was 89 mV for the soil and 356 mV for water at sowing. Soil hardness for the water levels 0 and 25 mm was less than the detection limit, whereas that for the water level 25 mm below the soil surface was 0.03, 0.27, 0.50, and 0.62 k g / c m 2 at 2, 4, 7, and 14 d after sowing (average of plots), respectively.
2.4. Experiment 2: Broadcast sowing onto drained soil
The experimental design was a split-plot (main plot = water level; subplot = cultivar, seed coating, and sowing depth) with four replications. The subplot was 70 cm long and planted with 35 seed. The distance between subplots was 3 cm. Soil was collected from a flooded experimental field (pH 6.8, clay texture, CEC 43.9 meq/100 g), passed through a 5-mm sieve, and placed in tanks (70 X 40 × 15 cm). Each tank represented a main plot. Water level was controlled by changing soil height so that excess water overflowed. Seed of ASD 1, Taothabi, IR50, and calcium peroxide-coated IR50 were sown at depths of 0, 13, and 25 mm and with water levels of 0, 13, 25, and 38 ram. Seed were placed at a given depth using forceps. At sowing, redox potential was 168 mV for soil and 246 mV for water (average of tanks).
The experimental design was a split-plot (main plot = time of sowing, subplot = cultivar and calcium peroxide coating) with four replications. Subplot size was 1 m 2. Seed of Aswina (26289, Bangladesh), JC178 (9080, India), Taothabi, ASD1, IR50, and calcium peroxide-coated IR50 were broadcast at 400/plot from a height of 1 m onto the soil at 2 h, and 1, 2, and 3 d after puddling, leveling, and draining the soil (pH 7.0, clay texture, CEC 34.3 meq/100 g, 1.19% organic C). The experiment was conducted during the 1992 dry season in the IRRI field and received no rainfall. The field was flush-irrigated (irrigation immediately followed by drainage) 10 d after sowing. Soil redox potential in the field after puddling was 61 mV. Soil hardness of the field was 0.60, 0.97, 1,47, and 5.55 kg/ cm 2 at 9, 10, 12, and 14 d after sowing, respectively.
2.6. Experiment 4: Interaction between water level and sowing depth
3. Results
2.5. Experiment 3: Sowing beneath the soil surface
3.1. Broadcast sowing into standing water
The experimental design was a split plot (main plot = water level, subplot = cultivar and seed coating)
When seed were broadcast into standing water of 10 or 25 cm depth (Experiment 1 ), there were few floating
126
M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
Table 1 Analysis of variance for establishment and growth characters of seedlings sown under various combinations of water level and sowing depth Experiment and source of variation
df
Mean squares Establishment
Height
Dry weight
L e a f score
Mesocotyl length
Experiment 1 Replication Water level ( W ) a Error ( a ) Cultivars/seed coating ( C ) W×C Error ( b )
3 1
171 6
26084 2497
18 57
0.25 1.31
3 11 11 66
248 258" * 45 46
52373 13915 * * 938 979
63 21 * * 3* 1
0.39 0.09* * 0.14"* 0.03
3 3
501 16819* *
627 983 *
9 5 15 60
633 1004" * 443 * * 174
3 2 6 3
Experiment 2 Replication Sowing time ( T ) Error ( a ) Cultivars/seed coating ( C ) T× C Error ( b )
84 116
0.14 1.60" *
235 6446* * 225 190
38 268" * 81 * * 31
0.20 0.23" * 0.05 0.06
1007 10178* 1512 1476" *
161 670 652 13055 * *
13 336 111 750* *
0.18 0.18 0.18 0.01
8.0 2.3 3.1 14.0" *
1 1 6 27
4389* * 35 63 305
3605 * * 828" * 53 87
128" * 482" * 23 12
0.00 0.02 0.05 0.03
8.9* 17.3 * * 3.6 2.1
3 3 9 2 3 1 1 1 6 6 9 18 132
651 2006" * 201 60338** 9008" * 23646** 2938" * 434 2091 * * 477" * 77 194 118
1230 2823 883 26819** 75856* ° 27568* * 91047" * 108952* * 4648* * 1095 665 923* 524
12 117 * * 16 440* * 606" * 78* 411 * * 1329" * 57* * 11 8 12 11
Experiment 3 b Replication W a t e r level ( W ) Error ( a ) Cultivars/seed coating ( C ) Contrast 1 Contrast 2 W XC Error ( b )
Experiment 4 ~ Replication W a t e r level ( W ) Error ( a ) Depth ( D ) Cultivars/seed coating ( C ) Contrast 1 Contrast 2 Contrast 3 D×C W × D W × C W×DxC Error ( b )
0.08 0.10 0.04 1.70"* 2.42" * 0.71 * * 5.02* * 1.57" * 0.31 * * 0.07 0.08 0.11" * 0.05
4.8 0.1 2.7 290.9** 59.5 * * 136.1 * * 36.3" * 6.1 * 19.1" * 2.2" 0.8 0.9 1.0
*, * *Significant at the 5 and 1% levels, respectively. a Insufficient error df for reliable F-test. b Contrast 1 = ASD1 and IR50 + Calper vs Taothabi and IR50; contrast 2 = ASD1 vs IR50 + Calper. c Contrast 1 = IR50 + Calper vs ASD1, Taothabi, and IR50; contrast 2 = IR50 vs ASD1 and Taothabi; contrast 3 = ASD1 vs Taothabi.
seedlings, presumably because seed were sown into turbid water created by mixing soil and water before sowing where, unlike the field, there was no water
movement. The seed were covered by a thin layer of soil (about 1-3 mm thick), which might have counteracted the upward rooting force. Significant differences
M. Yamauchi, P.V. Chuong /Field Crops Research 41 (1995) 123-134 Woter
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in seedling establishment, seedling height, shoot dry weight, and leaf score were noted among 11 cultivars and coated seed (Table 1 ). There was, however, little elongation of the mesocotyl. Variabilities in establishment and leaf score were less than those in dry weight and height (Fig. 1 ). Treatment ASD 1 gave the highest establishment (97-98 %) among the cultivars and coated seed, although the difference between ASD1 and control IR50 was not statistically significant. Seed coating had no effect on the establishment of IR50. The establishment of all cultivars and coated seed was the same (90%) at 10 and 25 cm water depth, suggesting that water level has little effect on establishment. ASD1 showed the best performance in dry weight and height among the cultivars and coated seed (Fig. 1). The simple linear correlations of dry weight and height between the water levels of 10 and 25 cm were significant at the 1% level with coefficients of 0.898 for dry weight and 0.877 for height, suggesting that such characters were controlled genetically and by seed coating.
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Time of broadcasting after puddling, leveling, and draining the field affected the settling of seed into the soil (Experiment 2). Seed broadcast 2 h after puddling sank 1-3 mm into the soil. Little elongation of the mesocotyl occurred. The seed sown 1 d after puddling settled on the soil surface such that half of each seed was in the soil and the other half was exposed to the atmosphere. Seed sown 2 and 3 d after puddling remained on top of the soil with little contact. Some of the seed sown 3 d after puddling dried out. "Percentage establishment and leaf score were more affected by time of sowing than by cultivar and seed coating, whereas seedling height and dry weight were related more to cultivar and seed coating than to time of sowing ( Table 1, Fig. 2). Establishment was greatest when seed were sown 1 d after puddling (88%). When seed were sown 2 h after puddling, establishment was greatest with calcium peroxide-coated seed, followed by cultivars ASD1 and Aswina. Sowing 2 and 3 d after puddling significantly reduced seedling establishment. Cultivars Taothabi and Aswina, two deep-water rices from Bangladesh, showed greater establishment than
128
M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
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Fig. 2. Effect of time of broadcasting seed after puddling, leveling, and draining the field on seedling establishment, seedling height, shoot dry weight, and leaf score of Aswina ( i ) , JC178 ([3), ASD1 (©), Taothabi ( 0 ) , IR50 (~7), and calcium peroxide-coated IRS0 ( • ) rice (Experiment 2). Points having the same letter at each time of broadcasting are not significantly different at the 5% level by Duncan's multiple range test. The LSD value is for comparison between times of broadcasting within a cultivar or seed coating (no LSD value indicates no significant effect of time of broadcasting). the other cultivars and coated seed, suggesting that they are tolerant o f dryness after germination. L e a f score increased as t i m e o f broadcasting increased (Fig. 2). T h e l o w e r l e a f score at 2-h s o w i n g
m i g h t i m p l y that d e v e l o p m e n t was delayed due to seed settlement under the soil surface. S h o o t dry w e i g h t r e s p o n d e d differently to time of broadcasting than the other characters measured, and
M. Yamauchi, P.V. Chuong/ Field Crops Research 41 (1995) 123-134
there was little consistency in the response among cultivars and coated seed ( Fig. 2). The seedlings of ASD 1 and Taothabi had greater dry weight and height than IR50 and calcium peroxide-coated IR50, particularly when the time of broadcasting was between 2 h and 1 d. 3.3. Sowing beneath the soil surface
At 25-mm sowing depth, the variability of seedling establishment due to water level was more than the variability due to cultivar and seed coating (Table 1, Experiment 3). On the other hand, seedling height, dry weight, and mesocotyl length varied more with cultivar and seed coating than with water level. Leaf score was not significantly affected by water level, cultivar, or seed coating. Seedling establishment of ASD1 was the same as that of calcium peroxide-coated IR50 and greater than that of IR50 and Taothabi (Table 1, Fig. 3 ). Dry weight and height of ASD1 and Taothabi were greater than those of IR50 and calcium peroxide-coated IR50. Calcium peroxide coating increased the mesocotyl length of IR50 regardless of water level. The greater mesocotyl length of Taothabi was negated by the increased water level. Lowering the water level increased soil hardness up to 0.6 kg/cm 2 at 14 d after sowing. Because seedling establishment was increased by lowering the water level, the increased soil hardness was apparently not detrimental. 3.4. Interaction between water level and sowing depth
Variations produced by sowing depth, cultivar, and seed coating were greater than those caused by water level (Table 1, Experiment 4). Although the effects of water level on seedling establishment and dry weight were significant, the variations induced by water level were smaller than those of sowing depth or cultivar and seed coating. Water level did not affect seedling height, leaf score, and mesocotyl length, but sowing depth and cultivar and seed coating did. The variations in height and dry weight were more pronounced by cultivar and seed coating than by sowing depth. On the other hand, seedling establishment was more variable with sowing depth than with cultivar and seed coating.
129
The interaction between water level and sowing depth was significant for establishment and mesocotyl length. The interaction between sowing depth and cultivar and seed coating was significant for all characters measured. Because there was no significant interaction between water level and cultivar and seed coating, the mean of cultivar and coated seed at each sowing depth versus water level and the mean of water level for each cultivar and seed coating versus sowing depth are presented (Figs. 4 and 5). When seed were sown on the soil surface, seedling establishment was high and not affected by water level (Fig. 4). Establishment was decreased, however, by the increase in water level when seed were sown at 13and 25-mm depths. Shoot dry weight decreased, regardless of sowing depth. Seedling establishment, height, dry weight, and leaf score decreased with greater sowing depth (Fig. 5). Seed coating effectively increased establishment, followed by use of tolerant cultivars. Taothabi produced taller and larger seedlings at all sowing depths. Seed coating reduced the height and dry weight of surface sown IR50 (Fig. 5). On the other hand, seed coating increased seedling height and dry weight at sowing depths of 13 and 25 mm, indicating that coating was beneficial when seed were sown deep in the soil. Regardless of sowing depth, seedlings of the tolerant cultivar Taothabi were taller and heavier than the coated IR50, implying that genetic potential is more effective than calcium peroxide coating for increasing seedling height and dry weight. The analysis of single degree of freedom contrast (Gomez and Gornez, 1976) for cultivar and seed coating clarified that the performance of calcium peroxidecoated IR50 significantly differed from those of ASD 1, Taothabi, and IRS0 (Table 1). ASD1 and Taothabi performed differently from IR50 (Table 1). The difference between ASD1 and Taothabi was significant except for establishment. Mesocotyls elongated little when seed were sown on the surface (0 mm depth) (Fig. 4). But at sowing depths of 13 and 25 ram, mesocotyls elongated and there were differences among cultivars and between seed coating (Fig. 5, Table 1). Seed coating increased mesocotyl elongation in IRS0. The tolerant cultivars had longer mesocotyls than IR50. The mesocotyl of IRS0 did not elongate in response to increased sowing depth (from 13 to 25 mm) but those of calcium per-
130
M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134 100
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oxide-coated IR50 and the tolerant cultivars did. Correlation coefficients between seedling establishment and mesocotyl length of 0.879 at sowing depth of 25 mm and 0.641 at 13 mm were statistically significant at the 1% level. Longer mesocotyls might have enhanced establishment of seed sown beneath the surface.
4. Discussion The stresses imposed on seedlings may comprise the complex combination of hypoxia, soil physical pressure, and toxic substances produced in the soil at low redox potential. In the present study, stress intensity was changed by combinations of water level and sow-
131
M. Yamauchi, P. V. Chuon g / Field Crops Research 41 (1995) 123-134 I
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Water level ( m m ) Fig. 4. Effect of water level on seedling establishment, shoot dry weight, and mesocotyl length at 0-, 13-, and 25-mm sowing depths (Experiment 4). The points are the mean of ASD1, Taothabi, IR50, and calcium peroxide-coated IR50. Points having the same letter at each water level are not significantly different at the 5% level by Duncan's multiple range test. The LSD value is for the comparison between water levels (no LSD value indicates no significant effect of water level).
ing depth. Deep sowing reduced seedling establishment more than did increased water level (Experiment 4). Once the seedling was successfully established, shoot dry weight and seedling height were more controlled by cultivar than by stress. The use of taller and more vigorous cultivars would be advantageous especially in competing with weeds.
Seed is sown into standing water in some parts of Asia, but it is less common in the tropics than in the temperate regions (De Datta, 1981). This method places seed on the soil surface. The results of Experiment 1 confirm the report of Jones (1933) that seedlings can establish in standing water. In addition, establishment was little affected by water level between
M. Yamauchi, P. V. Chuong /Field Crops Research 41 (1995) 123-134
132
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M. Yamauchi, P. V. Chuong / Field Crops Research 41 (1995) 123-134
0 and 25 cm and by cultivar (Experiments 1 and 4). Chapman and Peterson (1962) and Chapman and Mikkelsen (1963) reported that in California, seedling growth was not limited by dissolved 02. In addition, microbial photosynthetic activity supersaturated the standing water with 02. In the tropics, we also detected the supersaturation o f standing water in the field where rice seed were broadcast (unpublished). The lack o f response in establishment to the addition o f calcium peroxide (Experiment 1) also suggests that 02 does not limit establishment when seed are sown on the soil surface. The feasibility o f this sowing method would depend on the availability of water at the time of sowing, seedling anchorage, and presence of biotic stresses (snails, fish, insects, diseases) that destroy seedlings. More information on the improvement of agronomic practices is needed for seed sown on the surface of flooded soil. Sowing onto drained puddled soil surfaces is widely practiced in tropical Asia. Sowing is best timed so that seed land on the soil surface where both moisture and oxygen are available. Farmers know that seed settled beneath the soil surface develop poorly so they intentionally delay the time of broadcasting particularly when the soil is soft. Most farmers in the Philippines and Vietnam sow seed 1 d after puddling. Because the soil surface conditions may be determined by various factors, it might be difficult for farmers to achieve consistent seedling establishment. The results o f Experiment 2 demonstrated that although broadcast sowing just after puddling placed seed under the soil surface and delayed seedling development, more than 60% of seedlings established. Use of calcium peroxide coating or tolerant cultivars may improve establishment. Broadcast sowing just after puddling would ease the problems associated with surface sowing. In addition, combining the practice of broadcast sowing just after puddling with current farmers' practice (sowing seed 1 d after puddling) increases the time o f sowing, thus giving farmers more flexibility in farm operations. Sowing seed of tolerant cultivars into anaerobic soil at depths less than 13 m m resulted in establishment greater than 60% (Fig. 4). Seed can be coated with calcium peroxide to improve establishment (Yamada, 1952; Ota and Nakayama, 1970; Mitsuishi, 1975; Park et al., 1986). It may be difficult, however, for farmers to adopt the technology because seed coating requires
133
investments in chemicals and machinery, and knowledge o f seed coating procedures. Although seed coating is more effective in increasing establishment, the use of tolerant germplasm can substitute for coating when seed is sown at less than 13 m m depth. Compared with coating, tolerant cultivars have the advantage inasmuch as investments and knowledge of coating are, in this case, not required. Further, because seedlings may be taller and more vigorous, they might be more competitive with weeds. Breeding tolerant cultivars with appropriate agronomic characters might be important.
Acknowledgements This work is part o f a collaborative project between the Government of Japan and IRRI on the development of stabilization technology for rice double cropping in the tropics.
References Chapman, A.L. and Mikkelsen, D.S., 1963. Effect of dissolved oxygen supply on seedling establishment of water-sown rice. Crop Sci., 3: 392-397. Chapman, A.L. and Peterson, M.L., 1962. The seedling establishment of rice under water in relation to temperature and dissolved oxygen. Crop Sci., 2: 391-395. De Datta, S.K., 1981. Principles and Practices of Rice Production. John Wiley and Sons, New York. Gomez, K.A. and Gomez, A.A., 1976. Statistical Procedures for AgriculturalResearch with Emphasis on Rice. InternationalRice Research Institute, Manila, Philippines, 294 pp. Jennings, P.R. and de Jesus, J. Jr., 1964. Effect of heat on breaking seed dormancy in rice. Crop Sci., 4: 530-533. Jones, J.W., 1933. Effect of reduced oxygen pressure on rice germination. J. Am. Soc. Agron., 25: 69-81. Mitsuishi, S., 1975. Study on the direct underground sowing in the submerged field of rice. Special Bulletin. Ishikawa Prefecture College of Agriculture, Ishikawa, Japan. Ota, Y. and Nakayama,M., 1970. Effect of seed coating with calcium peroxide on germinationunder submergedconditionin rice plant. Proc. Crop Sci. Soc. Jpn., 39: 535-536. Park, S.H., Lee, C.W., Yang, W.H. and Park, R.K., 1986. Direct seeding cultivationon submergedpaddy in rice. I. Seedlingemergence and early growth under different temperature and seedling depth. Korean J. Crop Sci., 31: 204-213. Ponnamperuma, F.N., 1977. Physicochemical properties of submerged soils in relation to fertility. IRRI Res. Pap. Ser., 5: 1-32.
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Turner, F.T., Chen, C.C. and Bollich, C.N., 1982. Coleoptile and mesocotyl lengths in semidwarf rice seedlings. Crop Sci., 22: 43--46. Yamada, N., 1952. Calcium peroxide as an oxygen supplier for crop plants. Proc. Crop Sci. Soc. Jpn., 21: 65-66.
Yamauchi, M., Aguilar, A.M., Vaughan, D.A. and Seshu, D.V., 1993. Rice (Oryza sativa L.) germplasm suitable for direct sowing under flooded soil surface. Euphytiea, 67: 177-184. Yamauchi, M., Herradura, P.S. and Aguilar, A.M., 1994. Genotype difference in rice postgermination growth under hypoxia. Plant Sci., 100: 105-113.