Rice Science, 2014, 21(3): 162−169 Copyright © 2014, China National Rice Research Institute Published by Elsevier BV. All rights reserved DOI: 10.1016/S1672-6308(13)60180-8
Germplasm Innovation of Heat Tolerance in Rice for Irrigated Lowland Conditions in the Philippines Norvie L. MANIGBAS1, Leslie Angela F. LAMBIO2, Luvina B. MADRID1, Corazon C. CARDENAS3 (1Plant Breeding and Biotechnology Division, DA-Philippine Rice Research Institute, Science City of Munoz, Nueva Ecija 3119, the Philippines; 2Crop Science Cluster, Department of Agronomy, U.P. Los Banos, College, Laguna 4031, the Philippines; 3Department of Agriculture-Southern Cagayan Research Center, Iguig, Cagayan, 3504, the Philippines)
Abstract: Heat-tolerant varieties, such as N22 and Dular, which were used in this study, usually have low yield potential and undesirable plant characteristics but combining them with high yielding and improved rice varieties, new heat-tolerant rice genotypes with high yield potential can be achieved. In this study, phenotyping and selecting desirable materials from various crosses were performed under high temperature conditions during the reproductive stage. Screening was performed in the field and glasshouse to select individuals with heat tolerance and high yield potential. Several advanced breeding lines from Gayabyeo/N22 cross produced desirable individuals with heat tolerance, resistance to pests and diseases, and high yield potential. The genetic variation in percent sterility among the selected backcross populations grown in high temperature environments showed that large number of plants can be identified and selected with lower percent sterility. Key words: heat tolerance; high temperature; irrigated lowland; rice
Rice is grown mainly in tropical and sub-tropical zones, where environmental temperatures exceeding 35 °C at flowering can induce floret sterility and reduce grain yield (Osada et al, 1973; Satake and Yoshida, 1978; Matsushima et al, 1982). Since the 1980s, the increase in the atmospheric concentration of greenhouse gases, such as carbon dioxide, is believed to cause the increase in air temperature (Hansen et al, 1984). Global warming results in high temperature- induced floret sterility in rice. Jagadish et al (2012) reported that high temperature stress negatively affects rice production, especially in vulnerable regions in South and Southeast Asia. High temperature stress is a major constraint in rice production in tropical and subtropical regions. Crop scientists have attempted to assess the effects of increasing temperature and high carbon dioxide concentration in the atmosphere on the growth and yield of rice using simulation models (Boote et al, 1994; Horie et al, 1996, 1997; Matthews et al, 1997). Many reports confirmed that high temperature affects all rice growth stages, from emergence to ripening. However, the flowering stage and, to a lesser extent, the booting stage are the most sensitive to temperature Received: 24 April 2013; Accepted: 9 October 2013 Corresponding author: Norvie L. MANIGBAS (
[email protected])
(Imaki et al, 1982; Shah et al, 2011). Horie et al (1996) suggested that the anticipated high temperature will induce floret sterility and increase the instability of rice yield even in temperate regions. The main cause of floret sterility, which is induced by high temperature at the flowering stage, is anther indehiscence (Satake and Yoshida, 1978; Mackill et al, 1982; Matsui et al, 1997a, b, 2001). High temperatures at the flowering stage inhibit pollen grain swelling (Matsui et al, 2000), which triggers anther dehiscence in rice (Matsui et al, 1999a, b). Anthers of heattolerant cultivars dehisce more easily than those of heat-susceptible cultivars, and contribute to pollination under high temperature conditions (Satake and Yoshida, 1978; Mackill et al, 1982; Matsui et al, 2000, 2001). Rice yields are estimated to be reduced by 41% due to high temperatures by the end of the 21st century (Ceccarelli et al, 2010). Increasing severity of the problem in rice-growing areas in Asia is due to rising temperatures (Catherine et al, 2012). Global temperatures are estimated to rise by 1.1 °C to 6.4 °C during the next century (IPCC, 2012), thereby threatening rice production. The development of rice varieties for high temperature tolerance has received little attention in the past. With climate change, breeding for heat tolerance is one of the key research areas that may address problems related to temperature increase (Manigbas and
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Sebastian, 2007; Redona et al, 2009). In the Philippines, high temperatures are usually observed in the central and northern parts of Luzon, where the majority of rice is grown. High temperatures usually occur from mid-April to mid-May when temperatures can reach 38.0 °C to 39.9 °C (Manigbas and Sebastian, 2007). At this temperature range, empty grains are produced in the panicles, leading to a reduction in yield. To prevent heat stress in rice at the reproductive stage, farmers normally plant during December and January, and harvest during high temperature months. In rainfed areas, late planting is inevitable because of unpredictable rains and shortage of water supply. Rice becomes vulnerable to heat stress during the reproductive stage, reducing yield by as much as 10% for every 1 °C increase in temperature (Peng et al, 2004). Breeding for heat tolerance in rice was started in 2010 at the Philippine Rice Research Institute, the Philippines, when a research proposal titled ‘Breeding Heat-Tolerant Rice’ was approved by the ASEANKorea Economic Cooperation Working Group, which sponsored the funding together with the Rural Development Administration, South Korea. Five other institutions from five countries were involved in the project, namely, Indonesian Center for Rice Research, Sukamandi, Indonesia; Cambodia Agricultural Research and Development Institute, Phnom Phen, Cambodia; Ubon Ratchathani Rice Research Center, Ubon, Thailand; Institute of Agricultural Sciences, Ho Chi Minh, Vietnam; and Rural Development Administration, South Korea. The project aimed to develop heattolerant rice, establish a protocol for screening heattolerant genotypes, and identify location-specific varieties that can adapt and tolerate heat stress.
MATERIALS AND METHODS Germplasm for heat tolerance Three heat-tolerant donors (N22, Dular and Nipponbare) and widely grown high-yielding rice varieties from Korea, Vietnam, Cambodia, Thailand, and the Philippines were used for hybridization in 2010 (Table 1). F1 seeds developed in different breeding institutions were shared among the country partners. Several breeding methods, such as pedigree, rapid generation advance (RGA) using modified single seed descent method, and backcrossing, were used to develop the populations. Days to 50% flowering were recorded each year, and the entries
were grouped according to flowering time. The heattolerant donor parents, namely, Dular, N22 (Acc. 03911), N22 (Acc. 46458), N22 (Acc. 46459), Nipponbare, and WAB56-125 were mutated using gamma irradiation at the Philippine Nuclear Research Institute, Quezon City, the Philippines. All genotypes in this experiment were exposed to high temperatures during the entire flowering time of panicles. Generation advance Field planting was performed on a staggered basis so that the flowering time of the breeding materials can coincide with the hottest period of the year, which is from mid-April to mid-May. The breeding materials were subjected to field screening and selected under heat stress conditions. The donor parents were used in crosses with released varieties from each collaborating country to incorporate the heat tolerance. In most countries, screening for heat tolerance was performed in glasshouses and field nurseries using selected advanced breeding lines because of the absence of a controlled growth chamber facility. The pedigree method of selection (Fig. 1) was employed, in which an individual plant was obtained during the selection
Table 1. Crosses of different varieties from collaborating countries with heat-tolerant donor parents N22, Dular and Nipponbare. Cross combination
Origin
Dular N22 Nipponbare AS996-9/Dular AS996-9/N22 Dongjin 1/Dular Dongjin 1/N22 Gayabyeo/Dular Gayabyeo/N22 Hanareumbyeo/Dular IR66/Dular IR66/N22 Jinmibyeo/Dular Jinmibyeo/N22 Jinmibyeo/Nipponbare Junambyeo/Dular Junambyeo/N22 NSIC Rc134/WAB56-125 NSIC Rc150/Dular NSIC Rc150/N22 NSIC Rc160/Dular NSIC Rc160/N22 NSIC Rc148/WAB56-125 OM5930 (OM3536-12)/Dular OM5930/N22 RD6/Dular
India India Japan Vietnam Vietnam South Korea South Korea South Korea South Korea South Korea Cambodia Cambodia South Korea South Korea South Korea South Korea South Korea Philippines Philippines Philippines Philippines Philippines Philippines Vietnam Vietnam Thailand
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2010WS
F1
2011DS
F2
2011WS
F3
BC1F2
2012DS
F4
BC1F3
BC1F1 BC2F1 BC2F2
BC3F1 BC4F1
2013DS
F5
BC1F4
BC2F3
BC3F2
BC4F2
Fig. 1. Composition of the pedigree nursery of the breeding heattolerant rice project in the Philippines. WS, Wet season; DS, Dry season.
process starting from F2. Backcrosses were performed to improve heat tolerance of the popular cultivars. In subsequent generations, desirable families were initially identified, and desirable plants from the selected families were selected and harvested individually. Selection was based on phenotypic acceptability and high spikelet fertility. F1 hybrids were also subjected to backcross, using the country-recommended rice varieties as recurrent parents and heat-tolerant varieties as donors. Desirable BC1s were subsequently backcrossed until BC4s, in which 96.87% of the recurrent parents could be recovered. At BC3:4, we employed marker-assisted selection to determine which plants to backcross. For mutated heat-tolerant parental lines, bulk selection was performed. In 2011 wet season, individual plant selection from families was performed, in which one individual selection was planted in one row in the following season. Recombinant inbred lines (RILs) were developed using the RGA facility by applying the modified single seed descent method from F2 to F6 generations. Plants were exposed to artificial short days, from 30 d after sowing (DAS) to flowering. With this method, three to four generations are possible in a year. Plants were subjected to a constant 8 h light / 16 h dark photoperiod to induce flowering. Table 2 shows the summary of the total number of selected plants and populations during 2012 dry and wet seasons. These populations were exposed to high temperatures (37 °C to 38 °C) in the field, and selected under these conditions. Heat tolerance screening Glasshouse A study was conducted in the dry season of 2012 in a glasshouse facility to test the 20 selected breeding
Table 2. Summary of the total number of selected plants and populations during 2012 dry and wet seasons at the PhilRice-Central Experiment Station. No. of lines No. of lines No. of plants Population planted selected selected BC1F3 946 58 112 BC2F2 116 48 185 48 14 15 BC3F1 18 15 172 BC3F3 BC4F2 124 93 909 13 bulk harvest BC4F2 46 1 3 F3 F4 229 29 61 16 11 32 F5 RILs 56 15 15 Mutant 575 59 59 OYT from IRRI 15 0 0 Seeds from Vietnam 37 18 51 RILs, Recombinant inbred lines; OYT, Observational yield trial; IRRI, International Rice Research Institute.
lines (including checks and parents) developed in the project. In the absence of a controlled growth facility chamber, a glasshouse was used to screen for heat tolerance. The experiment was laid out in a randomized complete block design with three replications. The temperature and relative humidity at 1 m above the canopy and within-canopy layer of rice plants were measured every 2 min (24 h for 70 days) using an automated Micrometeorological Instrument for Near Canopy Environment of Rice (MINCER) from 1 d after transplanting to maturity stage. From the daily temperature and relative humidity data, three data points at 900 h, 1100 h, and 1400 h were considered because they were the critical periods when temperatures were high during flowering inside the glasshouse. The mean relative humidity inside the glasshouse during the study was 81%. The following materials were used in the experiment. Three checks: IR52 (susceptible), IR64 (intermediate), and N22 (Acc. 03911) (tolerant); Five parental genotypes: AS996-9, NSICRc150, OM5930, Jinmibyeo, and Hanareumbyeo; Three RILs (F5): Jinmibyeo/N22, Hanareumbyeo/N22, and AS996-9/N22; Three BC2F2 generations (segregated): PR42217 (OM5930//OM5930/ Dular), PR42218 (NSIC Rc150//NSIC Rc150/Dular), and PR42226 (AS996-9/AS996-9/N22); Three F4 generations: PR42029-23-1 (Hanareumbyeo/N22), PR42100-9-1 (Gayabyeo/N22), and PR40788-18-1 (AS996-9/N22); One F5: PR40330-4-2-4 (NSIC Rc134/ WAB56-125); Two advanced mutant lines: N22 and Nipponbare mutants. Twelve plant boxes were used for the experiment. A wooden plant box measures 267 cm × 77 cm (2.1 m2), and can accommodate 75 plants at 20 cm × 20
Norvie L. MANIGBAS, et al. Germplasm Innovation of Heat Tolerance in Rice
cm spacing (5 rows with 15 hills per row). The recommended rate for fertilizer application at PhilRice during the dry season was 120N-60P2O5-60K2O (120 kg/hm2 nitrogen, 60 kg/hm2 phosphorus and 60 kg/hm2 potassium). Basal fertilizer application was performed at final harrowing using complete fertilizer (14%N-14%P-14%K) 1 d before transplanting. Complete requirements for P and K and half of the N requirement were applied at basal. The remaining N requirement was split into two subsequent applications. Urea (46%N-0%P-0%K) was applied as top dressing during the mid-tillering stage and panicle initiation (35 and 45 DAS, respectively). Faucets installed in the glasshouse provided irrigation for the plants. A water depth of 3 cm to 5 cm was maintained until the hard dough stage. Pests and diseases were monitored, and control measures were applied if needed. At the physiological maturity (i.e., > 90% of grains are ripe), agronomic characteristics, such as plant height and tiller number of five random hills, were recorded. Five panicles per entry were sampled for the determination of sterility percentage. Yield per hectare was determined. Growth chamber Selected mutants of heat-tolerant parents and their non-mutant counterparts were subjected to high temperature stress in a growth chamber at the International Rice Research Institute, the Philippines. One panicle of each entry was exposed to 38 °C during the entire flowering period (3 to 4 d) from 8:00 am to 3:00 pm at 70% relative humidity. The plants were exposed to normal or ambient conditions after the treatment. Sterility percentage was determined during harvest.
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(Fig. 2). Maturity of these mutants was similar to that of popular varieties. Generation advance Table 1 indicates the number of breeding lines produced and selected in each selfing generation under high temperature conditions at the flowering stage. Selection was based on satisfactory crop stand, high fertility (90% to 100%), good panicle exertion, erect leaves, resistance to pests and diseases, and intermediate to high tillering habits. The variability of spikelet sterility percentage among 19 sampled backcross populations under high temperature conditions in the northern and central parts of Luzon, Philippines is shown in Fig. 3. It indicates that large number of plants can be selected from the backcross populations with low percent sterility. Heat tolerance screening under glasshouse conditions Table 3 shows the performance of the three checks, namely, IR52 (susceptible), IR64 (intermediate tolerant), and N22 (tolerant); 10 selected populations obtained by crossing heat-tolerant donors with five popular varieties; and two mutant lines. The tolerant check N22 (Acc. 03911) expressed low spikelet sterility (6.8%) because of early flowering (47 DAS).
RESULTS Germplasm for heat tolerance Twenty-three cross combinations were performed using heat-tolerant donor parents N22, Dular, Nipponbare, and WAB56-125 and popular varieties. These breeding materials were advanced, selected and screened for heat tolerance under field conditions. Twenty selected populations and the check varieties underwent heat tolerance screening in the glasshouse during the 2012 dry season. Selected mutants of heattolerant donor parents N22 and Nipponbare were also evaluated. These mutants had short plant height, high tiller number, erect culm and leaves, dense panicles, and flesh-colored grains compared with the wild types
Fig. 2. Phenotypes of N22 (Acc. 03911) (left) and mutated N22 (Acc. 03911) (PR42129) (right). N22 is tall, with slightly open culm habit, lodge-prone, early maturity; panicle with small, bold grains with purple tip, occasionally with awns. Mutated N22 is short in height, with erect and sturdier culm, medium maturity, denser panicle with larger, awnless, semi-bold grains with flesh apiculus.
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B
Fig. 3. Genetic variation in spikelet sterility percentage among the selected BC1F1 populations grown in high temperature areas in Nueva Ecija (A) and Cagayan (B), the Philippines.
Nevertheless, N22 was still exposed to 35.8 °C during flowering. According to the Rice Descriptors (Bioversity International et al, 2007) standard, N22 (Acc. 03911)
produces highly fertile spikelets and is highly tolerant to high temperature (Jagadish et al, 2010). The Hanareumbyeo/N22 cross consistently produced desirable plant types with low sterility. The days to flowering of its progeny were similar to that of its donor parent N22 (Acc. 03911), which showed early flowering. Crosses of NSIC Rc150/NSIC Rc150/Dular and AS996-9/N22 and the Nipponbare mutant also showed low sterility. IR64, with reported intermediate heat tolerance response, had 14.2% sterility. IR52, the susceptible check, showed relatively high spikelet sterility (22.6%) and low yield. These two cultivars were subjected to within- and above-canopy temperatures of 32.8 °C and 42.9 °C, respectively, during the time of flowering. Jinmibyeo/N22 (RIL-F5) and PR40330-4-2-4 were exposed to a relatively low temperature, but had high spikelet sterility. The performances of PR42218 and PR40788-18-1 were comparable with that of the intermediate heat-tolerant IR64. PR42217, which showed similar days to 50% flowering with the heattolerant genotype, had significantly high spikelet sterility (26.8%) and was partly sterile. The percent sterility of the parental cultivar Jinmibyeo and breeding line PR42217 were significantly different from N22 (Acc. 03911) at the 0.05 level. Hanareumbyeo was also found to be significantly different from N22 (Acc. 03911) at the 0.01 level. PR40330-4-2-4 was significantly different
Table 3. Plant yield, spikelet sterility, within-canopy and above-canopy (in parenthesis) temperatures at 900 h, 1100 h and 1400 h inside the glasshouse during 50% flowering of selected lines and check varieties. Days to 50% Within- and above-canopy temperature during flowering (°C) Sterility Yield Designation Cross combination or source flowering (%) (t/hm2) 9:00 am 11:00 am 2:00 pm IR52 (Check) IRRI 69 30.6 (38.3) 32.4 (39.7) 32.8 (42.9) 22.6 3.0 IR64 (Check) IRRI 70 29.4 (35.3) 31.8 (37.8) 32.8 (42.6) 14.2 4.0 N22 (Check) India 47 25.3 (28.0) 28.7 (33.0) 30.1 (35.8) 6.8 1.7 AS996-9 Vietnam 68 29.5 (35.5) 32.7 (38.6) 30.0 (33.8) 16.8 3.6 NSIC Rc150 Philippines 69 30.6 (38.3) 32.4 (39.7) 32.8 (42.9) 21.0 1.8 OM5930 Vietnam 68 29.5 (35.5) 32.7 (38.6) 30.0 (33.8) 21.6 1.7 Jinmibyeo Korea 70 29.4 (35.3) 31.8 (37.8) 32.8 (42.6) 26.5 0.5 Hanareumbyeo Korea 71 30.3 (35.1) 30.1 (37.3) 33.4 (41.6) 23.9 2.3 Jinmibyeo/N22 65 29.0 (34.2) 31.1 (37.7) 32.9 (42.8) 20.6 1.1 RILs F5 Hanareumbyeo/N22 56 29.6 (36.6) 32.6 (33.9) 33.5 (42.8) 12.9 1.8 RILs F5 AS996-9/N22 60 29.0 (35.5) 31.3 (38.4) 32.9 (41.3) 15.9 1.9 RILs F5 PR42217 OM5930/OM5930/Dular 68 29.5 (35.5) 32.7 (38.6) 30.0 (33.8) 26.8 2.0 PR42218 NSIC Rc150/NSIC Rc150/Dular 68 29.5 (35.5) 32.7 (38.6) 30.0 (33.8) 13.8 2.2 PR42226 AS996-9/AS996-9/N22 69 30.6 (38.3) 32.4 (39.7) 32.8 (42.9) 16.3 2.3 PR42029-23-1 Hanareumbyeo/N22 54 30.1 (35.6) 32.6 (39.2) 33.4 (41.6) 11.5 2.2 PR42100-9-1 Gayabyeo/N22 68 29.5 (35.5) 32.7 (38.6) 30.0 (33.8) 18.2 4.3 PR40788-18-1 AS996-9/N22 69 30.6 (38.3) 32.4 (39.7) 32.8 (42.9) 14.1 2.7 PR40330-4-2-4 NSIC RC134/WAB56-125 79 30.6 (37.4) 32.4 (39.0) 33.3 (40.5) 21.5 3.4 N22 mutant 80 29.3 (35.0) 31.0 (37.7) 33.3 (39.4) 17.0 2.5 Nipponbare mutant 80 29.3 (35.0) 31.0 (37.7) 33.3 (39.4) 14.1 2.9 Above and within-canopy temperature were recorded using MINCER. Values of days to 50% flowering and sterility are average of three replications.
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from the tolerant check N22. The sterility value of PR40330-4-2-4 was high, but the variety had relatively high yield. The temperature conditions for Hanareumbyeo/N22 (F5) and PR42029-23-1 were generally similar (reaching the maximum of 33.5 °C and 42.8 °C within- and above-canopy temperatures, respectively) during flowering. Results showed that Hanareumbyeo/ N22 (F5) and PR42029-23-1 had intermediate sterility values compared with their parents, and the sterility values were close to that of heat-tolerant N22. Hanareumbyeo had 10% higher spikelet sterility than its progeny. Heat tolerance screening under growth chamber Among the mutated heat-tolerant donors, N22 (Acc. 03911) had higher mean percentage of spikelet fertility under heat stress than the control treatment (Table 4). The mutant line N22 had the characteristics of a modern rice cultivar compared with its wild type. N22 was tall, matured early, and showed less tillering, but still maintained the heat tolerance trait (Fig. 2).
DISCUSSION Germplasm and generation advance Popular varieties in Southeast Asia, particularly in the Philippines, Vietnam, Thailand, Indonesia, and Cambodia, have high yields, good grain quality, and resistance to pests and diseases. However, they lack heat tolerance. Due to the advent of climate change caused by global warming, breeding for heat-tolerant varieties has become important. New rice varieties should possess adaptability to rising temperatures in addition to the desirable traits that a variety should have. The breeding populations created through a regional collaboration project need to adapt to increasing temperatures in specific locations. The 23 cross combinations from heat-tolerant donor parents (N22, Dular, Nipponbare, and WAB56-125) and popular varieties were planted and tested in different locations across the aforementioned countries. The N22 and Dular crosses produced progeny with heat tolerance, and most of the developed and selected populations (Table 2) came from the crosses involving N22 and Dular parents. The Nipponbare crosses were not selected because of poor recombinants and susceptibility to pests and diseases. The remaining populations are currently undergoing further field analysis and evaluation for yield traits.
Table 4. Mean performance of selected mutant lines and their wild type evaluated at 38 °C during flowering in a controlled growth chamber at IRRI, 2012 dry season. Material
Days to Mean spikelet fertility (%) Spikelet 50% fertility based No heat Heat stress flowering on control stress
Mutant Dular 95 N22 (Acc. 03911) 90 N22 (Acc. 46458) 90 N22 (Acc. 46459) 83 WAB56-125 91 Non-mutant Dular 67 N22 (Acc. 03911) 64 N22 (Acc. 46458) 74 N22 (Acc. 46459) 70 WAB56-125 76 Means followed by different different at the 5% level.
80.4 76.5 70.9 77.9 66.2 80.7 94.0 93.4 92.1 57.7 letters in a
3.4 8.4 4.1 6.0 5.1 11.3 4.3 34.3 63.0 7.7 column are
4.2 a 10.9 b 5.7 a 7.7 a 7.7 a 14.0 b 4.5 a 36.7 c 68.4 d 14.6 b significantly
Heat tolerance screening Most studies were conducted in the glasshouse or controlled growth chamber, and the time of flowering is usually 1 to 2 h later than those grown in fields (Imaki et al, 1982). In this experiment, the abovecanopy temperature was > 35 °C at 9:00 am and continued to increase to 42 °C at 2:00 pm. At 42 °C, flowering was already affected, as shown by the increased sterility percentage of the susceptible check IR52 (Table 3). Low sterility percentage was observed in N22 because of early flowering at the maximum above-canopy temperature of 35.8 °C. However, some of the spikelets were exposed to high temperatures during the ripening and maturity stages. The cross combinations of NSIC Rc150/NSIC Rc150/Dular, AS996-9/N22, and Hanareumbyeo/N22 (NSIC Rc150 is a Philippine variety, AS996-9 is a Vietnamese variety, and Hanareumbyeo is a Korean variety) had the lowest sterility percentages compared with the susceptible check IR52 (Table 3). This result showed that varietal differences in heat tolerance were present. Yield data were obtained from a glasshouse experiment, but genotypic differences were observed. The highest yield was produced by the check IR64, which had intermediate heat tolerance similar to PR42100-9-1. However, breeding line PR42100-9-1 showed higher sterility percentage than IR64. An increase in number of grains per panicle in genotypes was observed, and this increase resulted in high yields even with high sterility percentage compared with genotypes that had lower sterility percentage. This case was also observed in PR40330-4-2-4. The heat-
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tolerant N22 had lower sterility percentage, but it showed the lowest plot yield because of its inherent low yield potential and undesirable morphological characteristics. By combining a heat-tolerant donor parent, such as N22, with high yielding, pests and diseases resistant, and good quality rice cultivars, selecting new genotypes with better adaptation to new emerging climate conditions is possible. Variation in maturity was observed among mutant lines, which were similar to modern varieties in terms of maturity compared with very early maturing nonmutants. Improved plant type of mutant lines was prominent compared with its wild type, but higher spikelet fertility could still be observed in the other non-mutants. Mutation is another method for increasing genetic variation in rice, and produced numerous desirable traits for heat tolerance. A large number of mutant lines could be evaluated under high temperature conditions in a growth chamber for selecting desirable plants with high heat tolerance.
CONCLUSIONS Breeding heat-tolerant rice is one of the strategies used to mitigate the effects of climate change, particularly in high temperature regions where the majority of rice is grown. The screening and selection strategies that we developed (in the absence of a controlled growth chamber) for breeding under heatprone conditions could differentiate the genotypes and plant populations (new cross combinations) according to heat tolerance traits. The donor parents, N22 and Dular, are excellent sources of genes for heat tolerance, but may be effective only in particular cross combinations. Further studies on combining ability traits should be performed to address this issue.
ACKNOWLEDGEMENTS The project was supported by the ASEAN-Korea Economic Cooperation Fund, the Ministry of Foreign Affairs and Trade, South Korea and the Rural Development Administration, Republic of Korea. We are grateful to our country collaborators and their staff: Untung SUSANTO, Indonesian Center for Rice Research, Indonesia; Ouk MAKARA, Cambodia Agricultural Research and Development Institute, Cambodia; Young-Chan CHO, Rural Development Administration, Korea; Varapong CHAMARERK, Ubon Ratchathani Rice Research Center, Thailand; Bui Chi BUU, Institute of Agricultural Sciences, Vietnam; Misses
Alilia MAGHIRANG and Zenaida VILLEGAS, Project Development Service, Department of AgriculturePhilippines; Wilhelmina V. BARROGA of DA-PhilRice, Science City of Munoz, Nueva Ecija; Ernesto D. GUZMAN and Ferdinand B. ENRIQUEZ of DASouthern Cagayan Research Center, Iguig, Cagayan. Appreciation also goes to Krishna S. V. JAGADISH and Lovely LAWAS of the International Rice Research Institute for providing the use of the controlled growth chamber facility.
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