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Rainfall variability in Northeast Thailand during 2000-2015 was examined with objectives to determine any changes in rainfall pattern with time, and to determine its effects on duration of rice growing period and grain yield using a simulation model. Variation in mean annual rainfall over 16 years in 93 locations in the region ranged from over 1,600 to less than 1,200 mm, and the locations were grouped into 4 based on the annual rainfall. The change in annual rainfall, and early, mid and late season rainfall was analysed for the rainfall groups. There was a significant reduction in the amount of early season rainfall during the 16-year period in all groups. However, there was no significant change for annual, and mid and late season rainfall. Simulation study showed that the start of rice growing period (SGP) was delayed with reduced early rainfall during the 16 year period and the end of rice growing period (EGP) was also delayed while there was no significant change for the length of rice growing period (LGP). Simulation results showed that grain yield of KDML105, leading variety in Thailand, tended to increase during the 16 year period, as delayed planting time was optimum for achieving maximum yield in all rainfall groups. With general delay in rainfall season, occurrence of late season drought was predicted to be reduced and this helped to increase simulated grain yield. However, adaptation to changing rainfall pattern needs to be planned in advance to maximize its effect.

Khon Kaen Agriculture Journal, 48, 4, 809-822

Poor response of rice to phosphorus (P) fertilization and low phytoavailability of soil P have been reported in sandy rainfed fields in northeast Thailand. In order to evaluate the effects of mild soil drying on the uptake of P by rainfed lowland rice, we carried out nutrient omission trials for nitrogen (N) and P at Ubon Ratchathani Rice Research Center under rainfed and flooded conditions. The surface soil was classified as sandy loam. To avoid severe soil drying and drought stress in the rainfed field, soil water potential at a depth of 20 cm was maintained at the field capacity (> −20 kPa) by flush irrigation. The effects of flooding and drying on the soil properties were also evaluated in the laboratory using soils with diverse textures in and around the center. In the field experiments, the above-ground biomass of rice plants (RD6) did not respond significantly to P fertilization in the rainfed field, although it responded positively to N fertilization. Root length in the surface 10 cm under...

Rainfed lowland rice is the most common crop ecosystem in Northeast Thailand and varieties well adapted to drought-prone environments are required. On-farm multi-location experiments were conducted for 3 years to determine the drought type commonly developed, to identify genotype by environment interaction for rainfed lowland rice and to examine whether genotypic variation in these experiments was related to that in on-station controlled water- environment screening experiments conducted previously ( Monkham et al., 2015 ). Among 32 on-farm experiments, 4 experienced continuous drought with no standing water during the whole growing season, and 3 of them all in high toposequence positions died before maturity. For the remaining experiments, both intermittent and terminal droughts were common, while some experiments developed early season drought and delayed transplanting but resulted in no apparent yield penalty. The first set of multi-location experiments consisted of 14 experiments in 6 provinces in Northeast Thailand across 2 years with 24 common genotypes. Mean grain yield of 13 environments that were harvested was 2300–3900 kg ha −1 and no severe drought developed at any environment. While main effect of genotype was not significant for grain yield, genotype by environment interaction was significant, and the results of clustering analysis identified 3 environmental groups and 3 genotype groups that explained the main source of the genotype by environment interaction. The genotypic variation obtained in the on-farm multi-location experiments in one environmental group of low yielding environments was correlated with that obtained under earlier intermittent drought selection experiments on-station. The results also indicate the usefulness of screening genotypes based on short delays in the intermittent drought conditions on-station. A second set of multi-location experiments conducted throughout Northeast Thailand to test the performance of four genotypes selected from the first set of multi-location experiments confirmed that these genotypes outperformed the popular variety KDML105. The present study confirmed the usefulness of initial screening under intermittent drought conditions conducted on research stations followed by further on-farm selection. It is suggested that on-station selection experiments should target yield levels representative of on-farm yield levels.

Many of the economically important rice cultivars including ‘Khao Dawk Mali 105’ (KDML105) or jasmine rice, one of the world’s famous rice exported from Thailand suffers from drought due to erratic rainfalls and limited irrigation. To improve drought tolerance and reserve genetic background of KDML105, chromosome segment substitution lines (CSSL) containing drought tolerant quantitative trait loci (DT-QTL) has been previously developed by backcrossing between KDML105 and drought tolerant donor, IR58586-F2-CA-143 (DH212). To understand the physiological responses related to drought tolerance in CSSL lines compared to parents, two CSSLs namely CSSL1-16 and CSSL1-18, respectively were used in this study. Twenty-one-d-old hydroponically grown plants were subjected to 20% PEG for 0, 7, 14 d and then recovered from stress for 3 d. The results indicated that CSSL lines especially, CSSL1-16 showed better performance under drought stress compared to their recurrent parent. Drought tolerance superior CSSL1-16 line was indicated by high water status (high relative water content and leaf water potential), good osmotic adjustment, high proline and greater membrane stability. Moreover, this line was able to resume growth after stress recovery whereas other lines/cultivar could not recover. Similarly, drought tolerant donor showed high water status suggesting that well-maintained plant water status was associated with drought tolerant trait. It could be concluded that the highest drought tolerant line was CSSL1-16 followed by DH212, CSSL1-18 and KDML105. It would be interesting to go further into introgressed section in CSSL1-16 to identify potential candidate genes in DT-QTL for breeding drought tolerant rice in the future.

Rainfed lowland rice often grows in unsubmerged soil conditions during dry spells. Even if the drought effect is negligible, nutrient uptake and rice yield may decrease because of chronic unsubmerged soil stress on coarse-textured soil. The objective of this study was to evaluate the effect of unsubmerged soil conditions on N and P uptake, biomass accumulation and grain yield of 20 diverse rice genotypes. Unsubmerged and submerged field trials were conducted at the Ubon Ratchathani Rice Research Center in northeast Thailand in the wet seasons of 2010 and 2011. In the unsubmerged trial, rice was grown aerobically by draining the perched water; soil moisture at 20-cm depth fluctuated between −10 and −30 kPa. On average, the yield decline in unsubmerged soil conditions was 47% compared with submerged soil conditions (3.75 vs. 1.99 t ha −1 ), which was attributable to reduced biomass accumulation and N and P uptake. Unsubmerged soil stress had minimal effect on harvest index, days to heading and N concentration. In unsubmerged soil conditions, N and P uptake and N-use efficiency (biomass/N uptake) positively correlated with biomass accumulation at heading stage. Rice plants showed severe P deficiency because of unsubmerged soil stress, which limited the contribution of P-use efficiency to biomass accumulation. Our results showed that the physiological traits improving P uptake and plant P nutrition under unsubmerged soil stress would be important targets for future research. We suggest that genotype screening under unsubmerged soil stress on coarse-textured soil may further improve rainfed lowland rice for the drought-prone plains.

To assess the extent of improvement of rainfed rice production by using a participatory approach, we compared research project participants and non-participants (total of 206 rice-growing households) with regard to yield variability and their perspective on climate change at seven sites in Northeast Thailand. The participants were characterized by membership in local groups, an active learning attitude, and confidence in their farming. Compared to non-participants, the participants produced crops with higher yield and had more knowledge about the Intergovernmental Panel on Climate Change (IPCC) message and advanced farming technologies. Both groups had similar reactions to past climatic damage experiences, but the participants tended to have a more positive attitude about adaptation to climate change and mitigation by refraining from residual straw burning than the non-participants. The farmers’ attitude about adaptation to climate change was positively associated with their active learning and close relationship with researchers. There was a large yield gap between the bottom 10 percentile farmers (0.63 t/ha) and the top 10 percentile farmers (4.05 t/ha), with an average yield of 2.18 t/ha. Yield was associated with the level of market orientation, with the market-oriented farmers attaining higher yield, including yield from broadcast seeding (2.71 t/ha), than the subsistence farmers (1.66 t/ha). Our findings suggest that technical improvement of rice production in the region by using the participatory approach could be enhanced by selecting participants who are linked with local groups, tend to be market orientated, and are willing to learn with researchers.

Rainfed lowland rice in the Mekong region is mostly grown in drought-prone areas, and drought resistant varieties could be selected using either terminal drought (severe water stress after flowering) or intermittent drought (frequent mild stress) screening. Delay in flowering is considered as a putative trait for selection of drought resistant genotypes, and its effectiveness as an indirect selection trait and its association with genotype's flowering duration, i.e. the time taken from the first to last flower in a plot, was evaluated under the two drought screening methods. These screening methods together with flood control were used at two locations, Chum Phae and Ubon in north-east Thailand using a total of 70 genotypes with different growing duration. Delay in flowering was estimated for the two drought screening methods and flowering of individual panicles within a plot was recorded to determine flowering duration of each genotype in the flood and terminal drought conditions. Mean reduction in grain yield was 52–55% under the terminal drought screening while it was 10–19% for the whole population and 23–33% for early flowering genotypes in the intermittent drought. Delay in flowering under intermittent drought condition at both locations was associated with yield reduction; genotypes that exhibited shorter delay in flowering had smaller yield reduction percentage and larger number of panicles. At Chum Phae, where grain yield reduction was associated with flowering time, genotypes with longer flowering duration under terminal drought had smaller yield reduction, and this was found even among genotypes that flowered about the same time under flood condition. However, there was no association between delay in flowering and flowering duration. It is concluded that delay in flowering appears promising as a selection criterion under intermittent drought conditions, provided a higher yield reduction can be achieved for example by using an increased irrigation interval.

Oxidative damage occurring in plant cells under drought stress is a known cause of reduced plant primary production. Decreasing oxidative damage through oxidative stress tolerance is expected to confer drought stress tolerance. In this study, we estimated cross-tolerance to oxidative stress and drought stress for breeding populations and analyzed the effects of the cross-tolerance on dry matter production in field experiments. For a total of 91 rice genotypes, including 72 backcross lines (BCLs), cross-tolerance was estimated from the first principal component score (PCS1) derived from a principal component analysis using a data set with a parameter of chlorophyll fluorescence and cell membrane stability index in both the oxidative and the drought stress treatments as the factors. The higher cross-tolerance was represented by the higher PCS1, and generally the values of PCS1 were segregated in the BCLs, suggesting that cross-tolerance is a heritable trait that can be improved by crossbreeding. The effects of positive and negative PCS1 on dry matter production under flooded and aerobic conditions were tested in field experiments. The decrease in dry matter production under aerobic conditions was smaller for the positive PCS1 genotypes. However, these genotypes also showed a lower stomatal conductance and smaller shoot biomass, especially under flooded conditions. We concluded that cross-tolerance is a useful trait for improving dry matter production, especially under severe drought stress. In view of the trade-offs between cross-tolerance and dry matter production, it is important to develop rice varieties with an optimal level of cross-tolerance for a target environment characterized by drought stress.

Drought is a major problem for rainfed lowland rice in Laos as well as for other countries in the Mekong region, and different drought screening methods under managed environments have been used to develop varieties adapted to drought prone environments. This paper evaluates the appropriateness of on-research station screening methods conducted under intermittent and terminal drought conditions, using the yield results of 12 on-farm multi-location trials with 25 genotypes some of which had been selected for drought tolerance. In these multi-location trials drought stress was rather mild, even in the intermittent or terminal drought trials which tended to develop in the higher fields of the rainfed lowlands. The high position generally produced low yielding crops but no significant interaction between position and genotypes were found for grain yield of 25 genotypes grown at 4 pairs of high and low field positions. Genotypes that performed well under higher yielding conditions in the on-(research) station experiments also performed well in on-farm multi-location trials. Genotypes that were identified as drought tolerant using drought response index as a criterion in the on-station experiments generally performed well in the on-farm multi-location trials (r= 0.49**), particularly in high yielding environments, but not under low yielding environments where low soil fertility appeared to be a major limitation for high yield. Similarly, genotypes that were identified to have high potential yield in flooded conditions in the on-station experiments also performed well in on-farm multi-location trials (r= 0.43*). Drought tolerance determined on-station either intermittent or terminal drought was similarly effective and was at least as effective as potential yield determined on-station in predicting on-farm yield. On-farm multi-location trials in combination with on-station experiments identified several promising lines that were well adapted to rainfed lowland conditions. It is concluded that genotypes selected for high yield potential under favorable conditions and for high drought tolerance determined in the on-station experiments would result in the development of well adapted cultivars suitable for drought-prone rainfed lowlands, and that either intermittent or terminal drought conditions developed at a research station is suitable for this purpose. The intermittent drought condition had the added advantage that genotypes selected directly for high yield, performed well in farmer's fields in the rainfed lowlands.

Drought is a major constraint to rice production in the rainfed lowlands of South East Asia. The timing and severity of water stress is highly variable from year to year and the current drought screening method for rainfed lowland rice utilizes a late planting to impose a prolonged terminal drought in the wet season. In this paper, an alternative drought screening method was investigated for areas where photoperiod insensitive genotypes are commonly grown with the aim of identifying promising genotypes exposed to short-duration intermittent drought, a condition that is more frequently experienced in the rainfed lowlands in the wet season. To assess the usefulness of intermittent drought screening the performance of genotypes was compared with that tested under traditional flooded and terminal drought treatments in wet and dry seasons for two years. Mean yield reduction in intermittent and terminal drought was 13 and 35% respectively in the wet season and 34 and 59% respectively in the dry season, indicating that the proposed intermittent drought screening in the dry season and the current terminal drought screening in wet season would provide the magnitude of drought severity that would be appropriate for screening. Genotypes shown to be drought tolerant, based on their drought response index (DRI), were consistent in intermittent and terminal drought screening. There was no significant relationship between potential grain yield obtained under well watered treatments and DRI, indicating that some drought tolerant genotypes can achieve high yield, thus adaptation to a wide range of rainfed lowland conditions is expected. Due to genotypic variation in flowering in relation to rainfall or irrigation events in the terminal and intermittent drought treatments respectively, delay in flowering was not consistent across experiments and thus, is not expected to be a useful trait for selection particularly when using intermittent screening methods. It is therefore concluded that intermittent drought with appropriate level of drought stress could be used with DRI as a criterion for selection of well adapted genotypes suitable for the rainfed lowlands.

Mild intermittent drought is one of the major factors of yield reduction in rainfed rice. To minimize the yield reduction, the mechanisms that maintain photosynthetic activity play a crucial role for sufficient plant growth. The photoinhibition of photosystem II (PSII) depends on the balance between the rate of energy absorption and the rate of energy dissipation, including the energy flux for photosynthesis. Photoinhibition decreases the photosynthetic rate and restricts the primary production of the plant. In contrast, PSII photoinhibition can be regarded as a redox control mechanism that alleviates the excess light energy. In this study, the genotypic variation of photoinhibition in rice was examined in relation to the adaptability of photosynthesis to the fluctuating soil water conditions. 20 rice genotypes were grown under flooded and aerobic conditions, and the midday photoinhibition was evaluated by measuring the maximum quantum yield of PSII (Fv/Fm, a parameter of chlorophyll fluorescence). The larger decrease in Fv/Fm under aerobic condition than flooded condition was thought to be caused by the photosynthetic depletion derived from stomatal closure. The decrease of Fv/Fm occurred concomitantly with the increase of non-photochemical quenching (NPQ). Enhanced photoinhibition and NPQ might oxidize the redox state of the plastoquinone (PQ) pool under the aerobic condition to the same level as that under the flooded condition. The genotypic variation of the Fv/Fm and energy partitioning were affected little by the mild soil water deficiency of the aerobic condition. The genotypic variation of the midday Fv/Fm was dependent on the decreasing rate of Fv/Fm against an increase in midday light intensity. Some genotypes that showed more severe midday photoinhibition tended to have a lower rate of reduction of the shoot dry weight under the aerobic condition. These findings suggest that photoinhibition is a preferable trait for energy control in PSII and for photosynthesis under mild soil water deficiency.

Background and aims Rice plants alternately experience anaerobic and aerobic conditions during their life cycle in rainfed lowlands. Each condition affects root growth differently. Our objective was to clarify the specific rice root response to aerobic conditions in rainfed lowlands.

Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae is one of the most devastating diseases of rainfed lowland rice in Thailand. The gene Xa21 shows broad-spectrum resistance and has been widely utilized to improve BB resistance in rice worldwide. However, Xa21 is not fully expressed in the early stages of development (seedling stage). In this study, we attempted to improve the Thai jasmine rice variety KDML105 to obtain non-age-related broad-spectrum resistance to BB. The Xa21 gene and seedling resistance genes from rice variety IR1188 (a variety with non-age-related broad-spectrum resistance to BB) were introgressed into KDML105 through three rounds of marker-assisted backcrossing (MAB) and phenotypic selection. Sixty KDML105 backcross introgression lines (KBILs) carrying the Xa21 gene were successfully developed. They were used to evaluate seedling resistance against thirteen Xoo strains. Three seedling resistance (SR) loci inherited from IR1188 were identified on rice chromosomes 1 (RM302-RM212), 8 (RM210-RM149) and 11 (RM287-RM224). The agronomic characters of the KBILs were assessed by planting these lines in the paddy field at Kasetsart University in 2003. Phenotypic variation was observed in the agronomic traits of these lines. Seven KBILs carrying the Xa21 and multiple SR loci and also having a similar plant type to the original KDML105 were chosen for testing in multi-location trials at research stations in rainfed lowland environments. The trials were conducted in 10 and 11 locations in the North and Northeast of Thailand, respectively, between 2005 and 2006. The yield, agronomic traits, cooking quality and important diseases were examined and compared with those of the original KML105. All of the 7 KBILs had a cooking quality profile (aroma, amylose content, gel consistency and alkaline spreading value) and agronomic performance similar to the original KDML105. In 2007, four KBILs were planted in farmers’ fields at 5 locations. All BILs and the original KDML105 did not differ significantly in their agronomic performance. In this study, multiple loci for broad-spectrum seedling resistance were identified from the KBIL population developed by the integrated marker-assisted and phenotypic selection procedures (MAS and PS). These results facilitated the successful improvement of non-age-related broad-spectrum BB resistance in KDML105. The established non-age-related broad-spectrum BB-resistant KDML105 is currently recommended by Kasetsart University for planting in farmers’ fields where the crop is vulnerable to BB.

Two quantitative trait loci (QTL) that confer a broad-spectrum resistance to blast disease identified in the rice cultivar Jao Hom Nin (JHN) were transferred to the Thai glutinous jasmine rice cultivar RD6 by marker-assisted backcrossing (MAB). Four backcrosses and one selfing were conducted in which the derived introgression lines (ILs) that carried homozygous JHN alleles at the QTL recovered up to 98% of the RD6 genome. Ninety-eight BC 4 F 2 introgression lines that consisted of four combinations of the two QTL were evaluated for their resistance to eight single spore isolates and natural isolates at three research locations. Introgression lines were examined for their agronomic performance by comparing them to the original RD6 in multi-location trials across five locations. All ILs that carried the QTL (either one or two QTL) showed lower disease scores than ILs without any QTL and lower scores than the recipient cultivar RD6, whereas their agronomic performances were not significantly different from the original RD6. All ILs that carried at least one QTL showed a broad-spectrum blast resistance that was similar to the donor cultivar JHN. The success in improving the blast resistance in RD6 by MAB was shown. In this study, MAB accelerated the development of broad-spectrum blast resistance in the genetic background of the Thai glutinous rice cultivar RD6 within 4 years. The improved RD6 has now been tested in cultivation for several years. Two of the ILs were released in 2008. Currently, farmers in the north and northeast regions of Thailand (areas that are particularly vulnerable to blast disease) benefit from growing these two varieties.

The crop management in direct-seeded rice to promote growth during the reproductive stage was evaluated in weed-controlled toposequentially intermediate fields in a small watershed in Northeast Thailand. In 2004, the effectiveness of topdressing with 21 kg ha-1 of nitrogen at the panicle initiation stage were examined using two genotypes, KDML105 and IR57514-PMI-5-B-1-2 (IR57514) seeded at the rates of 500, 250 and 125 seeds m-2. In 2005 and 2006, the effectiveness of a new management (seeding rate of 125 seeds m-2 and nitrogen application of 90-101 kg ha-1; CM2) was compared with that of conventional management (seeding rate of 500 seeds m-2 and nitrogen application of 50 kg ha-1; CM1) using 3 genotypes (KDML105, IR57514 and HY71) seeded in May and June. In 2004, the number of spikelets on the tertiary pedicel at a low seeding rate in KDML105 was greatly increased by topdressing. In 2005 and 2006, CM2 had higher grain yield than CM1 (346 vs. 235 g m-2), owing to its larger spikelet number per panicle, heavier shoot dry weight and greater nitrogen uptake. May-seeding resulted in longer non-flooded period in the seedling to tillering stage, lower SPAD reading value around heading and less shoot dry weight increase from heading to maturity, and had lower grain yield than June-seeding (253 vs. 328 g m-2). This reduction in grain yield was larger for late-heading KDML105 than in early heading IR57514. These results indicated the effectiveness of the new crop management (CM2) for direct-seeded rice in toposequentially intermediate fields with less weed infestation or weed-controlled conditions.

Large within-field variation in rice growth often causes production loss in broadcast-seeded (BC) rainfed lowland rice. The spatial variability of direct-seeded rainfed lowland rice was evaluated in 2004, 2005, and 2007 in on-station experiments at Ubon Ratchathani, northeast Thailand, in relation to soil water content and weed infestation, by adopting semivariogram and block kriging, including comparisons among BC with harrowing (BCH; no weeding), BC with no harrowing (BCNH; no weeding), and row-seeded (RS; interrow weeding once) fields. BCH and BCNH were also compared in 11 farmers’ fields in 2006 and 2007, to assess the effect of harrowing on rice growth and weed infestation. During most of the rice growing periods, flooded and non-flooded portions existed simultaneously in the fields, with different proportions among years and among seeding methods in the on-station experiment. BCH and BCNH rice had large within-field variation in seedling density, heading date, shoot dry matter, grain yield, harvest index, panicle density, and filled spikelet per panicle, as well as in weed infestation, measured by a quick visual estimation. Many of the measured variables (except mean soil water content in RS in 2007, seedling density in BCH in 2005 and 2007, shoot dry matter in BCH and BCNH in 2007, and panicle density in BCH in 2007) were spatially dependent (i.e., data from nearby locations were most similar) by geostatistical analysis. Analysis of correlations using the 420 data sets of BCH plots in 2005 and BCH, BCNH, and RS plots in 2007 revealed a positive correlation between soil water content and grain yield and negative correlations between weed infestation and soil water content and grain yield. Compared with BCH, in 2007 BCNH had much lower grain yield because of lower soil water content after establishment and more weed infestation. BCH had higher grain yield than BCNH in weedy fields in the farmers’ fields experiment. RS with interrow weeding resulted in a smaller coefficient of variation, smaller sill value, and higher grain yield than BCH, due to less weed infestation and a higher proportion of flooded water. These results indicated that reducing the spatial variability in rice growth requires careful field preparation, such as harrowing to level the soil surface and to reduce the uneven distribution of standing water and the variability in soil water content, combined with effective crop and weed management (i.e., harrowing and row-seeding). This is the first study that examined spatial variability in the growth of direct-seeded rice as a function of soil water content and weed infestation in a rainfed lowland environment.

We evaluated the genotypic differences in grain yield of 14 rice ( Oryza sativa L.) genotypes with different phenology under four growing conditions: transplanting (TP) or direct-seeding (DS) in a toposequentially lower (with favorable water conditions) or upper (drought at around flowering stage) field at Ubon Ratchathani, northeastern Thailand. Thirteen of the genotypes – five early-maturing, four intermediate, and four late – had been bred for rainfed lowlands in northern and northeastern Thailand. IR24, a semi-dwarf, high-yielding, and early-maturing genotype bred for irrigated lowlands, was included for comparison. Genotypic differences in grain yield were significant in a combined analysis of all 4 growing conditions, and both high sink size (spikelet number per area) and high ripened grain percentage were associated with high yield. IR24 did not out-yield the rainfed-lowland genotypes, and its yield was particularly low in DS, owing to poor shoot dry matter production and low spikelet number per panicle. In the lower field, the interaction between cultivation method and genotype was also significant. In the lower field, late maturity was more strongly related to high shoot dry weight at maturity in TP than in DS; some of the early- to intermediate-maturing genotypes in DS produced shoot dry weights at maturity that were comparable to those of the late-maturing genotypes. High shoot dry matter production and large spikelet number per panicle were associated with high grain yield in DS genotypes in the lower field, whereas in TP genotypes with large numbers of panicles were required for high grain yield. Although the field location–genotype interaction and the field location–cultivation method–genotype interaction were not significant, regression analysis showed that late-maturing genotypes yielded less than earlier maturing genotypes, owing to the smaller ripened grain percentage resulting from late-season drought, in the upper field but not in the lower field. The presence of a trade-off between number of ripened grains and grain size in the lower field indicated the possibility of increasing the yield in rainfed-lowland genotypes by increasing assimilation capacity during grain filling. Phenology is important in the development of higher-yielding genotypes for different cultivation methods and different toposequential positions.

A large portion of the world’s poor farm in rainfed systems where the water supply is unpredictable and droughts are common. In Thailand there are approximately 6.2 million ha of rain fed lowland rice which account for 67% of the country’s total rice-growing area. This rice system is often characterised by too much and too little water in the same season. Farmers’ estimates of their annual losses to drought are as high as 45% in the upper parts of the toposequence. In contrast to irrigated rice systems, gains from crop improvement of rainfed rice have been modest, in part because there has been little effort to breed and select for drought tolerance for the target rainfed environments. The crop improvement strategy being used in Thailand considers three mechanisms that influence yield in the drought prone targets: yield potential as an important mechanism for mild drought (where yield loss is less than 50%), drought escape (appropriate phenology) and drought tolerance traits of leaf water potential, sterility, flower delay and drought response index for more severe drought conditions. Genotypes are exposed to managed drought environments for selection of drought tolerant genotypes. A marker assisted selection (MAS) scheme has been developed and applied for selection of progenies in the backcrossing program. The plant breeding program uses rapid generation advance techniques that enable early yield testing in the target population of environments (TPE) through inter-station (multi-location yield testing) and on-farm trials. A farmer participatory approach has been used to identify the TPE for the breeding program. Four terrace paddy levels have been identified, upper (drought), middle (drought prone to favorable) and lower (flooded). This paper reports the change in the breeding program for the drought prone rainfed lowland rice environments of North and Northeast Thailand by incorporating our knowledge on adaptation and on response of rice to drought.

Drought is a major constraint to rice (Oryza sativa) yield and its stability in rainfed and poorly irrigated environments. Identifying genomic regions influencing the response of yield and its components to water deficits will aid in our understanding of the genetics of drought tolerance and development of more drought tolerant cultivars. Quantitative trait loci (QTL) for grain yield and its components and other agronomic traits were identified using a subset of 154 doubled haploid lines derived from a cross between two rice cultivars, CT9993-510 to 1-M and IR62266-42 to 6-2. Drought stress treatments were managed by use of a line source sprinkler irrigation system, which provided a linearly decreasing level of irrigation coinciding with the sensitive reproductive growth stages. The research was conducted at the Ubon Rice Research Center, Ubon, Thailand. A total of 77 QTL were identified for grain yield and its components under varying levels of water stress. Out of the total of 77 QTL, the number of QTL per trait were: 7-grain yield (GY); 8-biological yield (BY); 6-harvest index (HI); 5-d to flowering after initiation of irrigation gradient (DFAIG); 10-total spikelet number (TSN); 7-percent spikelet sterility (PSS); 23-panicle number (PN); and 11-plant height (PH). The phenotypic variation explained by individual QTL ranged from 7.5% to 55.7%. Under well-watered conditions, we observed a high genetic association for BY, HI, DFAIG, PSS, TSN, PH, and GY. However, only BY and HI were found to be significantly associated with GY under drought treatments. QTL flanked by markers RG104 to RM231, EMP2_2 to RM127, and G2132 to RZ598 on chromosomes 3, 4, and 8 were associated with GY, HI, DFAIG, BY, PSS, and PN under drought treatments. The aggregate effects of these QTL on chromosomes 3, 4, and 8 resulted in higher grain yield. These QTL will be useful for rainfed rice improvement, and will also contribute to our understanding of the genetic control of GY under drought conditions at the sensitive reproductive stage. Close linkage or pleiotropy may be responsible for the coincidence of QTL detected in this experiment. Digenic interactions between QTL main effects for GY, BY, HI, and PSS were observed under irrigation treatments. Most (but not all) DH lines have the same response in measure of productivity when the intensity of water deficit was increased, but no QTL by irrigation treatment interaction was detected. The identification of genomic regions associated with GY and its components under drought stress will be useful for marker-based approaches to improve GY and its stability for farmers in drought-prone rice environments.

A sample of recombinant inbred lines (RILs) was derived from a bi-parental cross between Lemont and BK88-BR6, which contrasted in maintenance of leaf water potential (LWP) and expression of osmotic adjustment (OA). Genotypic variation for LWP and OA, and their associations with yield determination under water deficit, was studied in a series of five field experiments. Genotypic variation in the maintenance of high LWP was consistent across water deficit experiments. In the determination of genotypic variation in the maintenance of LWP, rate of water deficit was not an important factor influencing ranking, but degree of water deficit, and phenological development stage were important, particularly around heading. Genotypic variation in expression of OA was also observed under water deficits during both vegetative and flowering stages but ranking was inconsistent across experiments. This was in part because of large experimental errors associated with its measurement, but also because the expression of OA was associated with extent of decline of LWP. The relationship between OA and LWP was demonstrated when data were combined across experiments for vegetative and flowering stages. Under water-limited conditions around flowering, grain yield reduction was mainly due to a increased spikelet sterility. Variation in OA was not related to grain yield nor yield components. There were however, negative phenotypic and genetic correlations between LWP and percentage spikelet sterility measured at flowering stage on panicles at the same development stage during a water deficit treatment. This suggests that traits contributing to the maintenance of high LWP minimized the effects of water deficit on spikelet sterility and consequently grain yield. (C) 2002 Elsevier Science B.V. All rights reserved.

In this review, reasons for the slow progress in development of drought resistant cultivars for rainfed lowland rice are considered first. Recent advances in screening methods for development of drought resistant cultivars from mostly our research in Thailand in the 1990s, are then described for drought that develops early or late in the wet-season. There now appears to be a good prospect for developing drought resistant cultivars that produce higher yield than existing cultivars when drought develops late in the wet-season. Appropriate phenology to escape late-season drought and high potential yield under well-watered conditions are important characters for cultivars adapted to rainfed lowland conditions. In addition, ability to maintain higher leaf water potential when drought develops late in the season is another desirable character. Maintenance of leaf water potential just prior to flowering is associated with higher panicle water potential, reduced delay in flowering time, and reduced spikelet sterility, and hence contributes to higher yield. Genotypes that are adapted to areas of late-season drought should also have high harvest index, intermediate height and rather small total dry matter compared to existing traditional cultivars, under well-watered conditions. This combination of characters would ensure high potential yield under favourable conditions and also contribute to resistance against late-season drought. Screening against early-season drought that develops during the vegetative stage is more difficult, as the genotype’s ability to recover from the stress appears more important than drought tolerance during the stress period. Other than appropriate phenology, high potential yield and ability to maintain high leaf water potential, no specific physiological and morphological characters appear to contribute directly to higher yield under drought conditions in rainfed lowland rice in Thailand, where drought develops rather rapidly due to the prevailing coarse textured soils. It is thus appropriate to develop a breeding program that is primarily based on selection for grain yield. There are large genotype by environment interactions for yield in rainfed lowland rice and hence it is important that genotypes are selected for yield under appropriate target environments. Addition of a drought screening program that is conducted in the field in the wet-season to the overall breeding program would enhance the opportunity to select for drought resistance within the breeding materials and increase the chance of developing high yielding cultivars adapted to the drought-prone rainfed lowland environments.

This paper is a section of the book "Drought phenotyping in crops: from theory to practice" (Monneveux Philippe and Ribaut Jean-Marcel eds, published by CGIAR Generation Challenge Programme. Texcoco, Mexico). The section describes recent experience in drought phenotyping in rice which is one of the most drought-susceptible crops. The section contains genetic and genomic resources for drought adaptation and methods for selection of drought-resistant varieties in rice. In appendix, there is experience from Thailand on integration of direct selection for grain yield and physiological traits to confer drought resistance.