Penetration of hardpans by rice lines in the rainfed lowlands

Field Crops Research 76 (2002) 175±188

Penetration of hardpans by rice lines in the rainfed lowlands B.K. Samsona,1, M. Hasanb, L.J. Wadea,* a

Crop, Soil and Water Sciences Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines b Bangladesh Rice Research Institute, Rajshahi Regional Station, Rajshahi 6212, Bangladesh

Abstract Rice roots are commonly shallow in rainfed lowland conditions. Mechanical impedance is one factor that may restrict access of roots to deeper soil layers, thereby reducing the capacity of the root system to extract water from depth during late-season drought. The capacity of rice roots to penetrate hardpans was examined in experiments at Rajshahi, Bangladesh, in the 1994 wet season. Eight lines (CT9993, IR52561, IR58821, IR62266, KDML105, Mahsuri, Namsagui19, and IR20) were grown in three experiments: irrigated early, rainfed early, and rainfed late. As drought intensi®ed from heading to dough stage, soil penetration resistance increased to 3.0 MPa at 15±25 cm depth in both rainfed experiments. A high proportion of the total root length was found in the surface layer, particularly in IR20. CT9993 and IR58821 had thicker roots than other lines. Root length density (RLD) increased in deeper soil layers in rainfed with time, but lines differed in their capacity to penetrate the compacted layer as drought intensi®ed after heading. Only IR58821 and Mahsuri were able to increase RLD below 15 cm depth after heading to values greater than 1.6 cm cm 3, and only in the rainfed early experiment. In rainfed late, soil penetration resistance tended to increase after heading in IR20, IR52561 and IR62266, implying these lines were able to extract water below 15 cm depth, but without the concomitant increase in RLD. The greater penetration ability of Mahsuri and IR58821 was expressed in both rainfed environments at high soil penetration resistance. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hardpan penetration ability; Rice; Roots; Water stress

1. Introduction Rainfed lowland rice is grown in approximately 40 million hectares in south and southeast Asia by some of the poorest subsistence farmers in the world. Drought is the major constraint (Widawsky and O'Toole, 1990), and yields average only 2.3 t/ha (International Rice Research Institute, 1997). Rainfed lowland rice is grown in bunded ®elds without irrigation, so hydrologic conditions during the growing season may ¯uctuate from submergence to drought, * Corresponding author. E-mail address: [email protected] (L.J. Wade). 1 Present address: 124 Malcha Marg, Chanakyapuri, New Delhi 110021, India.

with major consequences for root growth, nutrient availability and weed competition (Wade et al., 1998). Various systems of crop establishment are employed, from dry direct seeding to transplanting, and capacity to withstand water stress is in¯uenced by soil preparation and establishment method (Boling et al., 1998). Roots are commonly shallow in the rainfed lowlands (Ahmed et al., 1996; Pantuwan et al., 1996, 1997; Naklang et al., 1996; Samson et al., 1995). Selection of cultivars with extensive and deep root systems has been proposed as one of several strategies to mitigate the adverse effects of water stress on growth, development and yield (O'Toole and Bland, 1987; Fukai and Cooper, 1995). Yoshida and Hasegawa (1982) showed that upland rice extracted water

0378-4290/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 2 ) 0 0 0 3 8 - 2

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from the shallow soil layers ®rst, which is consistent with the high root density there. As the soil near the surface dried, moisture was then extracted from middle soil layers. They suggested that an extensive and deep root system may allow the crop to better exploit soil moisture in deeper layers of the soil pro®le. Lilley and Fukai (1994a,b,c) have demonstrated that increased root length density (RLD) was associated with greater water uptake during drought, more favorable plant water relations, and increased crop growth rate during recovery from water stress in upland rice. The evidence for an extensive and deep root system being bene®cial is less clear in rainfed lowland rice. Variation in root density near the surface may not affect water uptake, because the top layer of the soil is often dry and root length there is very large (Pantuwan et al., 1997). Recently, by imposing drought stress in pots in simulated rainfed-lowland conditions, Azhiri-Sigari et al. (2000) and Kamoshita et al. (2000) showed genetic variation in the capacity of rice lines to develop roots and extract water from deep layers. In contrast, Pantuwan et al. (1997) identi®ed small genotypic differences in water extraction from 40 to 50 cm soil layer in the ®eld for a 10-day drying period at ¯owering. As drought progresses, soil strength also increases as the soil shrinks (Dexter and Woodhead, 1985), magnifying the effects of soil hard pans (Sharma and De Datta, 1985) that result from the tillage of wet soil. Increased soil strength impedes the growth of roots (Hasegawa et al., 1985; Thangaraj et al., 1990). The adverse effects of hardpans on root elongation to depth may be lessened by agronomic measures, by earlier root penetration before the drought intensi®es, or by an enhanced ability to penetrate hardpans. Mechanical rupturing of the hardpan has been observed to increase yield by about 0.5 t/ha in rainfed lowland rice in Bangladesh (Ahmed et al., 1996). Perforation of the hardpan by a pre-rice legume such as Sesbania provided similar bene®ts to rice yield. But such interventions are labor intensive and delay sowing of rice, thereby reducing availability of water for a post-rice crop. Root penetration of the hardpan before drought intensi®es would be bene®cial, but oxygen supply via aerenchyma restricts roots to surface layers while water is ponded and the soil remains essentially anaerobic (Ingram et al., 1994). Consequently, in practice, root systems must be able to enter and grow through hardpans of drought-hardened

soils, in order to capture resources from deeper soil layers. Thangaraj et al. (1990) reported that IR36 roots grew longer and deeper into soil as the soil dried and became harder. Because traditional, drought-resistant dryland rice cultivars had thick roots, Yoshida and Hasegawa (1982) surmised that thick roots were associated with deep root systems. Yu et al. (1995) reported a technique for examining genotypic variation in ability of rice roots to penetrate a hardpan, by using wax±petrolatum layers in pots. Lines differed in their ability to penetrate the wax±petrolatum layer (Yu et al., 1995), with thicker roots reported to have greater penetration ability (Materechera et al., 1992; Ray et al., 1996). It is evident that ®eld data are lacking on the capacity of rice cultivars to extend roots into the subsoil, especially when drought occurs and soil penetration resistance increases. This paper reports data from experiments conducted in Rajshahi, Bangladesh, which examined the capacity of rice lines to penetrate a hardpan in the ®eld. Lines used by Yu et al. (1995), Lilley and Ludlow (1996), and Sarkarung et al. (1995) were grown in irrigated and rainfed conditions, and penetration was examined in relation to soil strength during late-season drought. The results are discussed in relation to other parameters, including crop phenology, dry matter production, and seasonal conditions. 2. Materials and methods 2.1. Site and soil conditions The experiments were conducted in the 1994 wet season (July±December) at Rajabari, Rajshahi, in northwest Bangladesh (latitude 248210 N, longitude 888180 E). Soil was a dark gray clay (CEC 9.1±10.2 meq/100 g soil; 40.8±44.2% clay) of the Ammura series, aeric haplaquepts (UNDP/FAO/Pakistan, 1968), that softens when submerged for several days and hardens and develops deep cracks upon drying. Soils (0±15 cm) were mildly acidic (pH 5.8±6.3), and low in organic C (0.76±1.00%), total N (0.05±0.08%), Olsen-P (8.0± 10.0 mg/kg), exchangeable K (0.14±0.19 cmol/kg), and available S (10.0±11.0 mg/kg). The undulating topography had a range in elevation of about 5 m, which affected duration of ponded water and rice productivity. Plant available water content in the top 30 cm of soil

B.K. Samson et al. / Field Crops Research 76 (2002) 175±188

ranged from 50 mm at high to 73 mm at low positions (Wade et al., 1999). 2.2. Experimental There were three experiments comparing penetration ability of eight rice lines, comprising an irrigated and a rainfed site at the same sowing time, and a second rainfed site with delayed sowing to enhance the likelihood of late-season drought. Experiment 1 (irrigated early) was located in a low position, while experiments 2 (rainfed early) and 3 (rainfed late) were located about 1.2 m higher in elevation. Water was applied regularly to the irrigated experiment to maintain a minimum of 3 cm depth of ponded water throughout the experiment. Rainfed experiments were initially irrigated to assist land preparation and seed germination; but were then dependent on rainfall to harvest. Experiments were laid out in a randomized complete blocks design with ®ve replications. The dimension of each plot was 4 m  12 m. Eight diverse rice lines were used: CT9993-5-10-1M (CT9993), IR58821-23-B-1-2-1 (IR58821), IR52561-UBN-1-1-2 (IR52561), IR62266-42-6-2 (IR62266), Mahsuri, Nam Sa Gui 19 (NSG19), Khao Dawk Ma Li 105 (KDML105) and IR20. These lines were reported to differ in osmotic adjustment (Lilley and Ludlow, 1996), hardpan penetration ability against a wax layer (Yu et al., 1995), and size of root system (Sarkarung et al., 1995). 2.3. Cultural details The eight lines were sown on different dates in 1994 so that ¯owering would occur at about the same time within each experiment. Late maturing lines (IR52561, CT9993, Mahsuri, KDML105 and IR58821) were sown on 12, 13 and 28 July, and early maturing lines (NSG19, IR20 and IR62266) were sown on 25 and 26 July and 7 August in experiments 1, 2 and 3, respectively. Seeds, pre-soaked for 24 h, were direct-seeded on wet, puddled soil in hills spaced 0.25 m by 0.25 m apart. Thinning to two to three plants per hill was done on 14, 18 and 21 August in experiments 1, 2 and 3, respectively. Seeds of CT9993 germinated poorly, so observations for this entry are incomplete. Urea, triple superphosphate and muriate of potash were applied to supply 90 kg ha 1 N, 42 kg ha 1 P

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and 40 kg ha 1 K in all three experiments. Gypsum and zinc sulfate were applied at the locally recommended rates of 20 and 11 kg ha 1, respectively. All of the P, K, S and Zn were incorporated into the soil during the last tillage operation. N was split into two equal applications, at 27 days after sowing of late maturing lines and at panicle initiation. Weeds were removed by hand at 23, 38, 54 and 77 days after sowing. Thrips were controlled by spray application of NOGOS 100EC (dichlorovos-o-(2,2dichlorovinyl)o,-o-dimethyl phosphate) at the rate of 560 ml ha 1 on 8 August. CURATERR 5G (2,3-dihydro-2,2-dimethyl-benzofuranyl-methylcarbamate), a broad-spectrum systemic insecticide, was applied at the rate of 10 kg ha 1 on 11 September to control other insect pests. 2.4. Measurements Daily solar radiation was measured at the Bangladesh Rice Research Institute, Regional Station, about 20 km from the experiment site. Other weather data (rainfall, class A pan evaporation, and maximum, minimum, wet, and dry bulb temperatures) were collected daily from a weather station at the site. Aboveground biomass was sampled from a randomly chosen 0.5 m2 (eight hills) area within each plot at tillering, heading and dough stages. Samples were oven dried at 70 8C for 72 h. Plant height and tiller numbers were determined from a subsample of three hills. Root samples were taken over a hill within each biomass sampling area with a 10 cm diameter sharpened steel pipe. Samples were taken at tillering (43 days after sowing; 24 and 25 August and 19 September), heading (10, 11 and 27 October) and dough (5, 6 and 21 November) stages, for irrigated early, rainfed early and rainfed late experiments, respectively. Maximum depth of sampling was 30, 35 and 45 cm at tillering, heading and dough stages, respectively. Soil cores were divided into 5 cm depth increments from the soil surface, except the distal 10 cm that was taken as a single section. Hence, at tillering, cores were divided into ®ve increments: 0±5, 5±10, 10±15, 15±20, and 20±30 cm. Cores at heading and dough stages were divided into six and eight sections, respectively. Individual sections of the soil core were placed in separate labeled plastic bags and frozen at 10 8C if

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they were not immediately washed. For washing, cores were placed on a double layer of 1.5 mm wire mesh screen and soil was removed from roots with a stream of water. No chemical pre-treatment was necessary to separate roots from soil. Washed roots were frozen at 10 8C if not immediately measured. Roots were cleaned of debris and root length determined manually using a grid system based on Tennant's (1975) modi®cation of Newman's (1966) lineintercept technique. RLD was calculated as the quotient of total root length and soil volume from which the roots were extracted. Total root biomass was determined as the sum of root oven dry weight (70 8C for 72 h) from each section of the soil core. Root thickness (diameter) was measured with calipers to the nearest 0.1 mm, for a minimum of three (at deep soil layers) to a maximum of ten (surface soil layers) roots, randomly drawn from each section of the core prior to oven drying. At maturity, grain yield was determined from a 5 m2 (80 hills) area in the center of the plot and corrected to 14% moisture content. Harvest index and yield components were determined from 12 hills taken adjacent to the sample for grain yield. Water table depth was monitored daily from 31 October, using cloth-covered piezometers installed in each plot of blocks 1, 3 and 5 of the three experiments. These piezometers were made from 95 cm long PVC tubes of 3 cm diameter that were perforated for the lower 30 cm. Soil strength was measured from the soil surface to 0.5 m depth with a recording cone penetrometer, whenever root samples were taken. The penetrometer was equipped with a 308 angle steel cone, 1.28 cm in diameter. Soil strength (MPa) was recorded at 5 cm depth intervals from each plot.

2.5. Statistical analysis Analysis of variance and mean separation were conducted on data from rice lines in each experiment using BSTAT (McLaren, 1996). Trends in shoot and root growth and development were plotted using CoPlot (CoHort Software, 1990). 3. Results 3.1. Site conditions Cumulative rainfall exceeded pan evaporation until the end of September, but there was then less than 10 mm to the end of the cropping season (Table 1). Mean daily maximum and minimum temperature and solar radiation and monthly total pan evaporation declined from the time that rainfall ceased. Pan evaporation and minimum temperature each declined to about 50% of earlier values. In the rainfed experiments, ponded water was present at tillering and heading. Water remained ponded in rainfed plots until 19 October when all lines had ¯owered, except for Mahsuri, KDML105 and IR58821 in rainfed early, and for IR20, IR62266, Mahsuri, KDML105 and IR58821 in rainfed late. By dough stage, shallow cracks (<10 cm deep) had developed in rainfed plots. Data from the piezometers revealed that mean ground water level was within 5 cm of the soil surface in irrigated early until 20 November, 4 days after the last application of water. Subsequently, mean water table depth in that treatment declined rapidly to about 65 cm below the soil surface. Soils in rainfed experiments began to dry from mid-October. In rainfed early, piezometers were dry from 31 October onwards (mean

Table 1 Mean daily solar radiation, maximum and minimum temperature, relative humidity, monthly total pan evaporation and rainfall at Rajshahi, Bangladesh, during the 1994 wet season

2

Solar radiation (MJ m per day) Maximum temperature (8C) Minimum temperature (8C) Relative humidity (%) Pan evaporation (mm) Rainfall (mm)

July

August

September

October

November

December

15.8 32.1 26.1 91.2 130 156

15.7 32.5 26.5 94.7 138 275

14.8 32.5 25.7 95.9 126 145

16.0 31.5 22.3 93.9 116 0

13.3 29.1 18.1 92.6 78 9

12.9 25.6 12.9 95.9 63 0

B.K. Samson et al. / Field Crops Research 76 (2002) 175±188

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Table 2 Dates of ¯owering and maturity for eight rice lines in irrigated early, rainfed early and rainfed late experiments at Rajshahi, Bangladesh, during the 1994 wet seasona Lines

Dates and days to flowering Irrigated early

IR20 NSG19 IR62266 IR52561 CT9993 Mahsuri KDML105 IR58821

Dates and days to maturity

Rainfed early

20 October (87) 17 October (84) 17 October (84) 2 October (82) 6 October (86) 21 October (101) 31 October (111) 1 November (112)

20 October (86) 17 October (83) 17 October (83) 14 October (93) 20 October (99) 20 October (99) 30 October (109) 1 November (111)

Rainfed late 23 October 10 October 24 October 10 October n.a. 30 October 23 October 30 October

(77) (64) (78) (74) (94) (87) (94)

Irrigated early b

n.a. n.a. n.a. 1 November (112) 5 November (116) 20 November (131) 4 December (145) 15 December (156)

Rainfed early

Rainfed late

21 17 17 17 17 20 30 13

3 December (118) 17 November (102) 15 December (130) 1 December (126) n.a. 30 November (125) 6 December (131) 25 December (150)

November (118) November (114) November (114) November (127) November (127) November (130) November (140) December (153)

a

The values in the parentheses are the number of days from sowing to ¯owering and maturity, respectively.

b

Not available.

water table depth was below 85 cm). Mean water table depth in rainfed late was about 45 cm on 31 October and declined to about 65 cm at harvest. 3.2. Phenology, plant height, total aboveground biomass and grain yield In irrigated early, IR20, NSG19, IR62266, IR52561 and CT9993 ¯owered in 82±87 days, while Mahsuri, KDML105 and IR58821 ¯owered in 101±112 days (Table 2). In rainfed early, most lines ¯owered in 83± 99 days, except KDML105 and IR58821 that ¯owered in 109±111 days. Flowering ranged from 64 to 94 days after sowing in rainfed late. In rainfed early, maturity was delayed by 11±15 days in IR52561 and CT9993

relative to irrigated early. In rainfed late, by contrast, maturity was delayed by 16 days in IR62266 and hastened by 12 days in NSG19. Plants were generally taller in irrigated early than in rainfed late (Table 3), except for IR20. For most lines, total aboveground biomass at harvest was greater in irrigated early than in rainfed early, which in turn was greater than in rainfed late (Table 3). Exceptions were IR52561 with a greater biomass in rainfed early and IR20 in rainfed late. Grain yields in irrigated early and rainfed early were around 3.6 t/ha, about 1 t/ha greater than in rainfed late (Table 3). For the late maturing lines, Mahsuri, KDML105 and IR58821, grain yields were halved to 1.8 t/ha from irrigated early to rainfed early to rainfed late.

Table 3 Plant height, total aboveground biomass and grain yield of eight rice lines in three experiments at Rajshahi, Bangladesh, during the 1994 wet season Lines

IR20 NSG19 IR62266 IR52561 CT9993 Mahsuri KDML105 IR58821 5%LSD a

Plant height (cm)

Total biomass (t/ha)

Grain yield (t/ha)

Irrigated early

Rainfed early

Rainfed late

Irrigated early

Rainfed early

Rainfed late

Irrigated early

Rainfed early

Rainfed late

97 159 105 148 115 141 176 135

93 146 105 149 110 144 165 139

104 133 111 118 n.a.a 111 137 110

7.5 9.1 8.4 7.2 n.a. 10.0 11.2 15.0

7.4 7.4 7.9 8.5 n.a. 8.6 9.8 11.9

8.2 7.0 7.4 6.5 n.a. 6.9 7.0 9.4

3.7 3.7 3.8 3.1 n.a. 3.5 3.3 4.4

3.7 3.4 3.6 3.4 n.a. 2.9 2.9 3.5

3.4 3.4 2.6 2.8 n.a. 2.0 1.8 1.7

10

11

15

1.5

1.6

1.3

0.6

0.5

0.4

Not available.

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B.K. Samson et al. / Field Crops Research 76 (2002) 175±188

3.3. Soil strength In irrigated early, mean soil strength was close to zero in the puddled surface layer (Fig. 1), increasing almost linearly with soil depth at tillering, heading and dough

stages. Mean soil strength at heading was slightly higher than at either of the other sampling times, due to a brief period of 4 days when irrigation was not available when the sample was taken. At 20 cm depth, mean soil strength was about 1.0 MPa at heading in irrigated early.

Fig. 1. Depth pro®les of soil strength (MPa), root diameter (mm) and RLD (cm cm 3) averaged over eight rice lines at tillering (*), heading (~) and dough (Y) stages in irrigated early, rainfed early and rainfed late experiments at Rajshahi, Bangladesh, during the 1994 wet season. For RLD, roots in the 0±5 cm soil layer were plotted against a 0±75 cm cm 3 scale, roots in 5±10 cm against 0±37.5 cm cm 3, and roots below 10 cm against a 0±15 cm cm 3 scale. The symbols ,  and  indicate statistical signi®cance at P ˆ 0:05, 0.01 and 0.001, respectively.

B.K. Samson et al. / Field Crops Research 76 (2002) 175±188

During tillering and heading stages in rainfed early, mean soil strength increased curvilinearly from close to zero at the soil surface to 0.8±1.0 MPa at 20 cm soil depth, then remained at similar values throughout the soil pro®le (Fig. 1). At dough

181

stage, mean soil strength followed a similar pattern, but soil strength was about three times greater than in the previous two growth stages. Mean soil strength increased in a linear manner from the soil surface to about 3.0 MPa at 20 cm depth, before

Fig. 2. Depth pro®les of root diameter (mm) for each of eight rice lines (IR20 (~), NSG19 (^), IR62266 (Y), IR52561 (&), CT9993 (*), Mahsuri (^), KDML105 (~), IR58821 (*)) at heading and dough stages in irrigated early, rainfed early and rainfed late experiments at Rajshahi, Bangladesh, during the 1994 wet season. Component ®gures present values for four lines at each sampling occasion in each experiment. The symbols ,  and  indicate statistical signi®cance at P ˆ 0:05, 0.01 and 0.001, respectively, across all eight lines at each sampling occasion in each experiment.

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dec-lining curvilinearly to about 2.0 MPa at 40 cm soil depth. In rainfed late, soil strength increased linearly with increasing soil depth at tillering and heading stages

(Fig. 1), with higher values at tillering than at heading. At dough stage, soil strength was curvilinear with depth, with mean soil strength being highest at 20 cm depth (3.0 MPa) before declining to 2.0 MPa at 40 cm.

Fig. 3. Depth pro®les of RLD (cm cm 3) for each of eight rice lines (IR20 (~), NSG19 (^), IR62266 (Y), IR52561 (&), CT9993 (*), Mahsuri (^), KDML105 (~), IR58821 (*)) at heading and dough stages in irrigated early, rainfed early and rainfed late experiments at Rajshahi, Bangladesh, during the 1994 wet season. For RLD, roots in the 0±5 cm soil layer were plotted against a 0±75 cm cm 3 scale, roots in 5±10 cm against 0±37.5 cm cm 3, and roots below 10 cm against a 0±15 cm cm 3 scale. Component ®gures present values for four lines at each sampling occasion in each experiment. The symbols ,  and  indicate statistical signi®cance at P ˆ 0:05, 0.01 and 0.001, respectively, for all eight lines at each sampling occasion in each experiment.

B.K. Samson et al. / Field Crops Research 76 (2002) 175±188

3.4. Root diameter Root diameter increased from tillering to heading, and declined with soil depth (Fig. 1). At tillering, root thickness did not differ among rice lines in irrigated early, but in the rainfed experiments, root thickness differed by almost 0.5 mm at 5±10 cm soil

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depth (not shown). Differences in root diameter among rice lines generally increased as the crop progressed to heading and dough stages, with IR58821 and CT9993 having signi®cantly larger root diameters than other lines (about 1.0 mm versus 0.5 mm at 15 cm depth in rainfed early at dough stage; Fig. 2).

Table 4 Mean RLD of eight rice lines at 0±15 cm and greater than 15 cm soil depths at tillering, heading and maturity in three experiments conducted at Rajshahi, Bangladesh, during the 1994 wet season Mean RLD (cm cm 3) IR20

NSG19

IR62266

IR52561

CT9993

Mahsuri

KDML105

IR58821

Mean

5%LSD

0±15 cm soil depth Irrigated early Tillering Heading Maturity Mean 5%LSD

8.4 23.2 25.8 19.2 6.5

7.5 14.0 13.6 11.7 4.2

9.3 14.4 14.7 12.8 7.7

7.0 17.3 15.6 13.3 4.6

5.0 14.1 12.1 10.4 2.4

6.2 16.4 16.1 12.9 3.6

6.0 18.4 12.7 13.4 5.7

6.2 25.9 20.7 17.6 6.0

7.0 18.0 16.4

3.1 4.9 5.2

Rainfed early Tillering Heading Maturity Mean 5%LSD

10.4 23.7 23.9 19.3 4.3

11.0 20.2 18.9 16.7 6.5

8.5 21.6 17.2 15.8 6.0

10.8 21.6 12.9 15.1 7.7

6.4 21.7 14.7 13.8 5.4

10.8 22.1 21.1 18.0 8.7

8.6 22.7 21.5 17.6 6.7

12.2 25.8 29.3 22.4 9.2

9.7 22.4 19.9

4.4 7.5 6.4

Rainfed late Tillering Heading Maturity Mean 5%LSD

10.6 29.4 20.9 20.3 4.5

10.8 16.8 18.3 13.8 10.0

8.3 20.8 17.8 15.6 2.7

6.0 16.9 18.3 12.9 5.1

7.5 ± ± ± ±

5.0 20.3 17.4 14.3 5.7

4.4 18.2 18.3 13.6 4.4

5.3 25.8 25.2 18.8 8.0

7.2 21.1 19.0

2.9 5.3 6.0

Greater than 15 cm soil depth Irrigated early Tillering 0.6 Heading 1.2 Maturity 0.9 Mean 0.9 5%LSD 0.4

0.8 0.7 0.8 0.8 0.6

0.6 0.8 0.7 0.7 0.4

0.6 1.1 0.7 0.8 0.4

0.5 0.8 0.8 0.7 0.4

0.6 0.9 1.0 0.9 0.5

0.5 1.3 0.7 0.8 0.5

0.6 1.3 1.2 1.0 0.5

0.6 1.0 0.8

0.2 0.4 0.4

Rainfed early Tillering Heading Maturity Mean 5%LSD

0.6 1.1 0.9 0.9 0.3

1.1 1.2 0.9 1.1 0.7

0.7 1.5 0.9 1.0 0.6

0.7 2.0 0.8 1.2 0.7

0.7 1.4 0.9 1.0 0.4

1.1 1.4 1.8 1.4 0.9

1.1 1.4 1.3 1.3 0.6

1.2 1.6 2.1 1.6 1.0

0.9 1.4 1.2

0.6 0.7 0.6

Rainfed late Tillering Heading Maturity Mean 5%LSD

0.5 1.2 1.3 1.0 0.51

0.5 1.2 1.0 0.8 1.08

0.5 1.0 1.2 0.9 0.66

0.3 0.8 0.9 0.7 0.23

0.6 ± ± ± ±

0.5 0.9 1.2 0.8 0.26

0.3 1.2 0.7 0.7 0.21

0.4 1.0 1.3 0.8 0.51

0.4 1.0 1.1

0.3 0.5 0.6

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3.5. RLD and root dry weight (RDW) RLD was closely related to RDW (RLD ˆ 8:25  RDW, R2 ˆ 0:85) so RDW data are not presented. Patterns in RLD are shown in Fig. 1 for the means across lines, and in Fig. 3 for each line individually. Table 4 summarizes RLD for the eight lines at tillering, heading and dough stages, for the surface (0± 15 cm) and deeper (15‡ cm) soil layers. On average, RLD increased from tillering to heading and dough stages. Rooting declined sharply with depth, so that RLD below 15 cm (around 1.0 cm cm 3) was about 5% of values in the surface layer (around 20.0 cm cm 3). IR20 and IR58821 produced more roots than other lines in the surface layer at heading. RLD was generally maintained from heading to dough stage for the surface layer, except in KDML105 and IR58821 in irrigated early, in IR52561 and CT9993 in rainfed early, and in IR20 in rainfed late. RLD generally increased with water stress, and to a greater extent in the deeper soil layer. From heading to dough stage below 15 cm depth, RLD increased signi®cantly in Mahsuri and IR58821 to 1.81 and 2.10 cm cm 3 in rainfed early, and non-signi®cantly to 1.18 and 1.34 cm cm 3 in rainfed late (Table 4). 3.6. Change in RLD and soil strength with time In rainfed early, RLD increased consistently from heading to dough stage in IR58821, and to a lesser extent in Mahsuri for depths below 10 cm (Fig. 4). Soil penetration resistance increased from 1.0 to 2.0 MPa for most lines at most soil depths from heading to maturity, but to 2.5 MPa for IR62266, and 3.0±4.0 MPa for IR58821 and Mahsuri. Changes in RLD and soil strength were generally not statistically signi®cant among lines in rainfed late (Fig. 5). Nevertheless, RLD tended to increase from heading to maturity in IR62266 at 15±25 cm depth, in IR52561 at 10±20 cm depth, in IR20 and IR62266 at 25±30 cm depth, and below 10 and 15 cm depths in IR58821 and Mahsuri. At 10±20 cm soil depth at dough stage, soil penetration resistance increased to about 3.5 MPa in IR20, to about 3.0 MPa in IR52561 and IR58821, and to about 2.0±2.5 MPa in Mahsuri and KDML105. At 25±30 cm depth, soil penetration resistance increased to about 2.5 MPa in IR20 and IR62266.

Fig. 4. Changes in RLD (cm cm 3) and in soil penetration resistance (MPa) for each of eight rice lines (IR20 (~), NSG19 (^), IR62266 (Y), IR52561 (&), CT9993 (*), Mahsuri (^), KDML105 (~), IR58821 (*)) in each of six soil depth increments (0±5, 5±10, 10±15, 15±20, 20±25, and 25±30 cm) in the rainfed early experiment at Rajshahi, Bangladesh, in 1994 wet season. The symbols ,  and  indicate statistical signi®cance at P ˆ 0:05, 0.01 and 0.001, respectively.

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4. Discussion 4.1. Crop growth and development

Fig. 5. Changes in RLD (cm cm 3) and in soil penetration resistance (MPa) for each of eight rice lines (IR20 (~), NSG19 (^), IR62266 (Y), IR52561 (&), CT9993 (*), Mahsuri (^), KDML105 (~), IR58821 (*)) in each of six soil depth increments (0±5, 5±10, 10±15, 15±20, 20±25, and 25±30 cm) in the rainfed late experiment at Rajshahi, Bangladesh, in 1994 wet season. The symbols *, ** and *** indicate statistical signi®cance at P ˆ 0:05, 0.01 and 0.001, respectively.

In rainfed early, lines ¯owered up to 2 days earlier than in irrigated early, except for IR52561 and CT9993, in which ¯owering and maturity were delayed. For the other lines for which data were available, days to maturity were slightly reduced in rainfed early. Compared with rainfed early, lines ¯owered earlier after sowing in rainfed late, but maturity was either hastened (NSG19), unaffected (IR20), or delayed (IR62266). As water remained ponded in rainfed plots only until 19 October, later maturing lines were subjected to increasing drought stress as grain-®lling progressed. Jearakongman et al. (1995) have also reported a greater reduction in grain yield for lines that ¯owered after ponded water disappeared. IR58821 yielded most in irrigated early, where the ready availability of water allowed its longer duration and greater biomass potential to be expressed. Grain yields averaged about 3.5 t/ha for the remaining lines in irrigated early, and for IR20, NSG19, IR62266 and IR52561 in rainfed early, where grain-®ll duration averaged about 32 days and the lines escaped the adverse consequences of late-season drought. NSG19 and IR20 yielded similarly at 3.4 t/ha in rainfed late, but for different reasons. Photoperiod sensitivity restricted vegetative growth duration and biomass in NSG19, allowing a longer grain-®ll duration before the onset of severe water stress. Phenological development, plant height and biomass were not reduced in IR20, allowing it to maintain its yield in rainfed late. In contrast, maturity was severely delayed in another early maturing line in rainfed late, IR62266, resulting in lower yields of 2.6 t/ha. The late maturing lines, Mahsuri, KDML105 and IR58821, were shorter in rainfed late, and produced less biomass and less grain in rainfed early and especially in rainfed late, due to increased exposure to terminal drought stress. Consequently, drought stress and its collateral effects on soil strength should have been more signi®cant for Mahsuri, KDML105 and IR58821 in rainfed early and rainfed late, and possibly for IR62266 and IR52561 in rainfed late. 4.2. Variation in root parameters Highly signi®cant differences among rice lines in RLD and root diameter were observed in all

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experiments. This is in agreement with earlier reports of genotypic variation in rice root traits (Yu et al., 1995; Mambani and Lal, 1983a,b; Armenta-Soto et al., 1983; Ekanayake et al., 1985a,b). The range in RLD was greater than reported elsewhere for rice in irrigated or drought-stressed conditions (Ekanayake et al., 1985a,b; Hasegawa et al., 1985; Thangaraj et al., 1990), and this may be due to thorough puddling and the low soil strengths reported here for surface layers, or different cultivars. Root diameter of rice lines in the experiments was also greater than those reported for both upland and lowland rice grown in aeroponic and hydroponic culture (Armenta-Soto et al., 1983; Ekanayake et al., 1985a,b; Zuno et al., 1990). In contrast, root diameters from the experiments corresponded well with those reported by Yu et al. (1995) for upland and lowland rice cultivars grown in pots. Yu et al. (1995) were testing a system using wax±petrolatum layers to simulate soil hard pans to screen cultivars for ability of roots to penetrate soil hard pans. The similarity of soil physical constraints in Yu et al. (1995) and this study may account for the strong correspondence with their root diameter data. Support for the plasticity of RLD with regard to moisture regime is provided by an examination of the pattern of soil depths where marked changes in RLD occurred in the experiments. Under the irrigated regime, marked changes in RLD occurred in the surface layer, whereas in rainfed experiments such changes were observed in the deeper layers as drought progressed. Because water becomes less available in surface layers as drought progresses, exploration of deeper layers may be essential to complete grain®lling. 4.3. Root parameters and hardpan penetration Adequate nutrients were provided, so no nutrient problems were observed. As rainfall was adequate until late September, soil moisture was suf®cient during germination, seedling establishment and vegetative growth in all three experiments, but little rainfall was received from late September onwards. In the rainfed experiments, a compacted soil layer was observed at about 20 cm depth, that increased greatly in soil strength from the onset of soil drying. The experimental sites were similar in soil properties,

except for the intended variation in late-season drought and its consequences for soil strength. The results for RLD showed that IR20 produced many additional roots in the soil surface layer after heading, especially in irrigated early, while IR58821 and Mahsuri produced many additional roots at depth, especially in rainfed early in response to late-season drought (Table 4). These patterns are in accord with greenhouse data reported by Azhiri-Sigari et al. (2000). Further, Lilley and Fukai (1994a) and Kamoshita et al. (2000) found that increased RLD at depth during the drought period was directly related to water extraction from each soil layer. RLD was large in the surface layer (12.9± 29.3 cm cm 3), dropping to 0.60±2.10 cm cm 3 below 15 cm soil depth. Reports for many crops suggest that a value of about 1.0 cm cm 3 is critical for effective water extraction (De Willigan et al., 2000), although Lilley and Fukai (1994a) suggested a value of 1.6 cm cm 3 may be needed for upland rice. On that basis, adequate RLDs at soil depths below 15 cm were only present for IR58821 and Mahsuri in rainfed early (Table 4). For the remaining lines in rainfed early, and for all lines in rainfed late, RLD was probably below the critical value required for effective water extraction from depth. This remains to be adequately tested with ®eld data on water extraction for rainfed lowland rice. In relation to hardpan penetration, IR58821 and Mahsuri were consistently able to signi®cantly increase RLD after heading as soil strength increased, over a wide range of soil depths in both rainfed early and rainfed late. In contrast, there was little change in RLD in the third late maturing line, KDML105, which was similar in ¯owering time to IR58821 and similar in biomass to Mahsuri in rainfed early. Associated with this smaller increase in RLD at depth in KDML105 was a smaller increase in soil penetration resistance. The higher values of soil penetration resistance for IR58821 and Mahsuri presumably re¯ect ability to extract additional soil water by greater RLD as the soil dried and soil strength increased. Root thickness was not able to explain differences in hardpan penetration ability. Since obtaining statistically signi®cant differences for root traits is notoriously dif®cult in ®eld experiments (O'Toole and Bland, 1987; Pantuwan et al., 1997), we draw attention to the consistent but

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non-signi®cant trends evident in rainfed late, whilst recognizing a need for their independent veri®cation. In rainfed late, IR62266 and IR52561 tended to increase RLD at intermediate soil depths, and these changes were accompanied by increases in soil penetration resistance to about 3.0 MPa. Interestingly, soil penetration resistance also tended to increase to comparable values in IR20, but without the concomitant increase in RLD in those soil layers. Since the increase in soil strength must be associated with soil drying, this result implies that IR20 was able to extract additional soil water without relying on large RLD alone. Other studies have reported a strong capacity for osmotic adjustment in IR20 (Lilley and Ludlow, 1996; Kamoshita et al., 2000), and this trait may have assisted green leaf retention and water demand from the soil. IR52561 and IR62266 have also been reported to possess some capacity for osmotic adjustment (Kamoshita et al., 2000), which may help to explain why values of soil penetration resistance at dough stage in IR20, IR52561 and IR62266 were similar to IR58821 and Mahsuri in rainfed late. Presumably the cool conditions and low evaporative demand during grain-®ll improved the opportunity for expression of osmotic adjustment in rainfed late than in rainfed early. 5. Conclusions This study has quanti®ed growth and yield of rainfed lowland rice in the drought-prone Barind tract in northwest Bangladesh. In particular, it has demonstrated how lines must adapt to changing soil conditions as drought progresses, especially to increase RLD at depth as soil penetration resistance increases. Lines differed in their capacity to extend and proliferate roots into deeper soil layers as drought progressed and as soil strength increased, with Mahsuri, and especially IR58821, being most effective in hardpan penetration. When the situation could permit its expression, there was an implication that osmotic adjustment could assist water extraction from deeper soil layers in the ®eld. This must be tested, as must the expected relationship between a capacity to increase RLD at depth and to extract more water from deeper layers as drought progresses in rainfed lowland rice.

187

Acknowledgements This research was conducted in conjunction with the regional experiment station at Rajshahi of the Bangladesh Rice Research Institute (BRRI), under the auspices of the Rainfed Lowland Rice Research Consortium that is coordinated by IRRI. We thank Dr. Aminul Haque for permission to use the facilities at BRRI, Rajshahi, Mr. Ruhul Amin for experimental support, and Mr. Salim Ahmed and Ms. Jamila Khandekhar from the IRRI Dhaka of®ce for logistic support.

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