Heat-induced Spikelet Sterility Research Articles

Mitigation of heat-induced spikelet sterility at flowering by transferring a QTL for the early-morning flowering trait, <i>qEMF3</i>, into the genetic background of <i>japonica</i> cultivars

Mitigation of heat-induced spikelet sterility at flowering by transferring a QTL for the early-morning flowering trait, <i>qEMF3</i>, into the genetic background of <i>japonica</i> cultivars

An early-morning flowering trait in rice can enhance grain yield under heat stress field conditions at flowering stage

An early-morning flowering (EMF) trait is supposed to be effective in enhancing grain yield due to mitigation of heat-induced spikelet sterility at flowering in rice. This study evaluated (i) phenotypic differences between a near-isogenic line carrying a QTL for EMF trait (designated as IR64 +qEMF3) and a recurrent parent, IR64, under wide variation in climates and (ii) whether the EMF trait can enhance grain yield under heat stress at flowering. IR64 +qEMF3 had significant earlier flower opening time (FOT) in diverse environmental conditions including temperate, subtropical, and tropical regions. FOT differentially responded to maximum and minimum air temperatures between genotypes. Under non-heat stress temperatures at flowering, IR64 +qEMF3 had similar grain yield to IR64 with minimal changes in yield components. Seven field trials in heat-vulnerable regions of Myanmar for multiple years showed that higher percentage of grain set contributed to the significantly higher grain yield in IR64 +qEMF3 when plants were exposed to daily maximum air temperatures around 36.1 °C or higher. Lower spikelet sterility in IR64 +qEMF3 was attributed to the earlier FOT during cooler early morning hours. This is the first field study that clearly demonstrates the enhancement of grain yield due to EMF trait under diverse heat stress field conditions at flowering.

Monitoring canopy micrometeorology in diverse climates to improve the prediction of heat-induced spikelet sterility in rice under climate change

Rice is well adapted to a wide range of climates, but is highly susceptible to heat during flowering. However, there are uncertainties in assessing the occurrence of heat-induced spikelet sterility (HISS) and the impact of climate change. One reason is the gap between the ambient air temperature and the panicle temperature, which determines the magnitude of HISS in field studies. To improve our understanding of this gap, we established a multi-site monitoring network (MINCERnet) to measure canopy micrometeorology and heat stress in the major rice growing regions (Sub-Saharan Africa; South, Southeast, and East Asia; and the USA). MINCERnet assessed the processes that determine panicle temperature and the resulting HISS in open fields using the same cultivars (‘IR64’, ‘N22’, and ‘IR52’) and a standard system (MINCER) for micrometeorological monitoring under diverse climates. By using the MINCERnet data in the canopy heat-balance model (IM2PACT), we confirmed that the canopy and panicle transpiration and the resulting evaporative cooling strongly affected the gap between the ambient air temperature and the panicle temperature, and that the HISS rate in open fields could be predicted accurately in diverse climates by using the mean panicle temperature during the flowering hours. The “oasis effect” in the broad sense, that is, evaporative cooling and the increase of relative humidity, which is nested at the various levels along the continuum from the landscape to the panicle, formed temperature and relative humidity gradients along the continuum in response to different climatic conditions.The heat-balance characteristics (i.e., a stronger evaporative cooling under drier climate conditions) suggested that the risk of HISS caused by global warming will increase more in wetter climates, where panicle temperatures tended to increase. Thus, accurate relative humidity data as well as air temperature will be required, along with spatial downscaling, to permit accurate prediction of rice heat stress and yield. HISS prediction using an approach based on the panicle temperature as input for models and monitoring of canopy micrometeorology will reduce uncertainties in rice yield prediction and the response of yield to various climate change adaptation measures.

Different effects of water-saving management on canopy microclimate, spikelet sterility, and rice yield in the dry and wet seasons of the sub-humid tropics in northern Ghana

• Drainage at flowering (DF) reduced dry (DS) but not wet season (WS) rice yield. • DF had small impact on inner-canopy temp (T c ) at flowering time and grain sterility. • DF greatly increased T c from late afternoon to midnight in DS but not in WS. • Negative effect of DF is ascribed to late-afternoon-to-midnight T c increase in DS. Increasing temperatures and water scarcity are concomitant threats to sustainable rice production in future climates. Although both aspects have been widely studied, little is understood about how water-saving management might affect heat-induced stress and grain yield of rice under open-field conditions. The objective was to clarify how water management practices affect daily inner-canopy temperature (T c ) dynamics, flowering time, heat-induced spikelet sterility, and rice grain yield. Field experiments were conducted in four consecutive wet and dry seasons in northern Ghana. Two rice varieties, IR64 and Jasmine85, were grown under two water regimes: 1) continuous flooding (FL), and 2) continuous flooding except for an approximately 20-day drainage treatment at the flowering period (DF). Although the DF treatment was non-flooded, a high moisture content was maintained (>60 % of saturated volumetric water) to avoid any significant drought stress. The effect of water regimes on grain yield and T c differed significantly between the dry season (DS) and wet season (WS). DF significantly reduced yields by 13–26 % in the DS but not in the WS. However, the effect of DF on T c at flowering time (0.2–0.3 °C increase) and spikelet sterility (3–5% increase) was relatively small even in the DS. In contrast, DF greatly increased T c from solar noon to midnight, in particular during the period of ±3 h from sunset in the DS. At this time of the day, DF had a higher T c than FL by 0.7–0.9 °C on average. The T c did not differ between DF and FL either at flowering time or nighttime in the WS. The T c changes over the course of the day imply that water-saving management may have a higher risk of yield reduction in the DS of the sub-humid tropics not only by increasing heat-induced sterility but also by increasing late-afternoon-to-nighttime temperatures, which can cause physiological stress and respiration loss. These results can help improve water-saving management practices under contrasting climatic conditions in the sub-humid tropics and predict the combined effect of increasing temperatures and water scarcity on rice production.

On-site single pollen metabolomics reveals varietal differences in phosphatidylinositol synthesis under heat stress conditions in rice

Although a loss of healthy pollen grains induced by metabolic heat responses has been indicated to be a major cause of heat-induced spikelet sterility under global climate change, to date detailed information at pollen level has been lacking due to the technical limitations. In this study, we used picolitre pressure-probe-electrospray-ionization mass spectrometry (picoPPESI-MS) to directly determine the metabolites in heat-treated single mature pollen grains in two cultivars, heat-tolerant cultivar, N22 and heat-sensitive cultivar, Koshihikari. Heat-induced spikelet fertility in N22 and Koshihikari was 90.0% and 46.8%, respectively. While no treatment difference in in vitro pollen viability was observed in each cultivar, contrasting varietal differences in phosphatidylinositol (PI)(34:3) have been detected in mature pollen, together with other 106 metabolites. Greater PI content was detected in N22 pollen regardless of the treatment, but not for Koshihikari pollen. In contrast, there was little detection for phosphoinositide in the single mature pollen grains in both cultivars. Our findings indicate that picoPPESI-MS analysis can efficiently identify the metabolites in intact single pollen. Since PI is a precursor of phosphoinositide that induces multiple signaling for pollen germination and tube growth, the active synthesis of PI(34:3) prior to germination may be closely associated with sustaining spikelet fertility even at high temperatures.

Physiological Characterization of Diverse Landraces of Rice to Identify Donors for High Temperature Stress Tolerance Based on First Spikelet Opening Time

Rice is susceptible to heat stress at flowering, where temperatures above 33 °C during anthesis causes spikelet sterility. Heat-induced spikelet sterility at flowering is associated with reduction in grain yield. So far only N22 has been used as universal donor for heat stress. One of the major traits employed in N22 in mitigating the impact of high temperature is the early morning flowering (EMF). Hence, the present study was designed to identify novel donors from the indigenous landraces of rice available at TNAU. The physiological basis of flower opening time (FOT) showed early flowering lines had higher transpiration rate coupled with low photosynthetic rate and stomatal conductance during anthesis. The transpiration rate also led to a reduction in panicle temperatures in these lines. It was concluded that at the time of anthesis, the panicle temperatures were 24 °C and less which led to advanced FOT. The results revealed that among the 21 lines, seven landraces, namely Thatan samba, Panamara samba, Thooyamali, Norungan, Mattikar, Sornavari and Seakar, flowering (9.30 to 10.00 IST). Hence, the lines Thatan samba, Panamara samba which flowered before 9.40 IST can be used as potential donors of EMF trait to develop heat-tolerant varieties.

Global warming and rice production in Asia: Modeling, impact prediction and adaptation.

Since the projection of global warming emerged in 1980s with the potential of laying enormous impacts on agriculture and food security of the world, we have conducted experimental and modeling studies for clarifying its effects on rice production in Asia and for developing adaptive rice production technologies. On the basis of measurement of rice responses to climate and carbon dioxide concentration ([CO2]), the dynamic process model named SIMRIW was developed to predict global warming effects on irrigated rice. The model predicted differential regional effects of the projected global warming by doubling [CO2] on the rice yield over Asia, and indicated that high tolerance to heat-induced spikelet sterility and high yield potential under elevated [CO2] are the two important characteristics required for rice genotypes adaptive to global warming environment. Further, genetic traits associated with these characteristics and their genetic resources for breeding adaptive genotypes were identified from diverse rice germplasms. This article reviews our initiative studies in the light of the recent studies, and points out further research that is needed for better understanding and overcoming of this unprecedentedly large problem.

Stage-dependent temperature sensitivity function predicts seed-setting rates under short-term extreme heat stress in rice

Seed-setting rate of rice (Oryza sativa L.) significantly decreases under high temperatures during the reproductive period largely due to heat-induced spikelet sterility. The effects depend largely on the timing and severity of heat stress, but the variation in the sensitivity of seed-setting rate in response to heat stress at different growth stages is not yet well simulated by most of the present crop models. To investigate the quantitative relationships between seed-setting rates of rice and various exposures of heat stress, we conducted four-year phytotron experiments with four different temperature regimes (the target maximum/minimum temperatures, 32 °C/22 °C, 35 °C/25 °C, 38 °C/28 °C and 41 °C/31 °C in 2011 and 2012; 32 °C/22 °C, 36 °C/26 °C, 40 °C/30 °C and 44 °C/34 °C in 2014 and 2015) with factorial combinations of durations (2, 4 and 6 days in 2014 and 2015; 3, 5 and 7 days in 2011; 3, 6 and 9 days in 2012) and the timings (0, 6 or 12 days after flowering), using three japonica cultivars. The duration and intensity of heat stress were quantified by the heat degree-days (HDD), defined as the temperature sum above a critical temperature value, which varied from 35 to 36 °C, depending on the cultivars. The observed seed-setting rates were well expressed as a logistic function of HDD, but the temperature sensitivity parameter varied with the timing of heat stress and the spikelet positions on the panicle. The variation in timing of flowering was apparent among upper, middle and lower parts of the panicles: 1 and 3-day delay of flowering for the spikelets on the middle and lower parts of the panicles, respectively, compared with upper ones. We therefore developed a simulation model that reflects the changes in the sensitivity of seed-setting rate to HDD to estimate the effects of heat stress with different intensities or durations at any time from flowering onward. The model with the stage-dependent parameters improved substantially the root mean square error (RMSE) and mean bias error (MBE) of seed-setting % from 20.1 and 6.0 to 7.6 and 0.2, respectively, compared to that with the stage-independent parameters. The proposed model needs to be tested under field conditions, but will be an important basis for accurate prediction of seed-setting rates in rice, which is critical for reliable estimates of crop production under climate change.

Spatial Assessment of Heat Stress Impact on Rice Production in Two Districts of Andhra Pradesh, India

Physiological studies have provided clear evidence of heat-induced spikelet sterility in rice, which can cause significant yield reductions. However, it is rather difficult to evaluate such yield losses in real world situations in farmers’ fields, especially across large areas. State-of-the-art technologies, such as remote sensing and crop modeling, can offer solutions for evaluating the heat-induced yield penalty in rice across a spatial area. Remote-sensing technologies, especially Synthetic Aperture Radar (SAR), can provide spatial detection of the start of the rice-growing season. This information, combined with time-series temperature data, can be assimilated into a crop simulation model, which can provide a thorough assessment of any heat-induced yield penalty over a specific geographic region. In this paper, we demonstrate how SAR can be used to assess the effects of heat stress in rice in two districts of Andhra Pradesh, India, during the 2017 rabi (dry) season. The accumulated data suggest that 53,623 and 21,436 ha of rice fields in Nellore and West Godavari districts, respectively, had yield losses due to heat stress. Rice fields in Nellore suffered higher yield and production losses due to heat stress because of steeper trend of increasing temperatures during the rice plants’ reproductive stage.

Development of a new heat tolerance assay system for rice spikelet sterility

Background Reduction in rice yield caused by high temperature-induced spikelet sterility has been a serious concern in rice production. To date, several screening methods have been used, although their reproducibility is sometimes poor due to artifacts mainly caused by varietal differences in heading dates and panicle heights (i.e., the distance from the lamps).MethodsWe have developed a novel assay system for heat-induced spikelet sterility by using artificial rice paddies in phytotrons to conduct a highly reproducible assay throughout a year. Plants restricted to the main culm were treated under a series of heat conditions, and height uniformity of each plant was ensured by using height-adjustable pots.ResultsResults suggested that a 3-day heat treatment of 35 °C-day/29 °C-night cycles was the most suitable condition. Under the treatment, two distinct groups were identified among nine heat tolerant cultivars, with no varietal difference in panicle temperature, indicating that the system is capable of eliminating the varietal difference in panicle temperature.ConclusionsIt is concluded that the assay system would be a powerful tool for selecting heat tolerant varieties, as well as the analysis of genetic factors from various cultivars, eliminating potential artifacts.

Surfing parameter hyperspaces under climate change scenarios to design future rice ideotypes.

Growing food crops to meet global demand and the search for more sustainable cropping systems are increasing the need for new cultivars in key production areas. This study presents the identification of rice traits putatively producing the largest yield benefits in five areas that markedly differ in terms of environmental conditions in the Philippines, India, China, Japan and Italy. The ecophysiological model WARM and sensitivity analysis techniques were used to evaluate phenotypic traits involved with light interception, photosynthetic efficiency, tolerance to abiotic stressors, resistance to fungal pathogens and grain quality. The analysis involved only model parameters that have a close relationship with phenotypic traits breeders are working on, to increase the invivo feasibility of selected ideotypes. Current climate and future projections were considered, in the light of the resources required by breeding programs and of the role of weather variables in the identification of promising traits. Results suggest that breeding for traits involved with disease resistance, and tolerance to cold- and heat-induced spikelet sterility could provide benefits similar to those obtained from the improvement of traits involved with canopy structure and photosynthetic efficiency. In contrast, potential benefits deriving from improved grain quality traits are restricted by weather variability and markedly affected by G×E interactions. For this reason, district-specific ideotypes were identified using a new index accounting for both their productivity and feasibility.

Temperature thresholds for spikelet sterility and associated warming impacts for sub-tropical rice

Abstract Identifying a reliable and robust temperature threshold inducing rice spikelet sterility under field conditions involving cultivars with highly varying phenology has been a major limitation. This challenge has been a consistent bottleneck in devising a phenotyping strategy for rice breeders and to accurately estimate heat stress impacts by the climate and crop modeling communities. In an attempt to address this challenge, we used 292 diverse indica cultivars, recorded 15 min air temperature data during the key flowering period across wet season and two consecutive dry (summer) seasons. We identified 33 °C as the critical threshold beyond which large sterility increases were observed. This is the first report that demonstrates the need to consider cultivar and seasonal fixed effects in a regression model framework to derive meaningful temperature thresholds. We find that an additional growing degree hour (GDH) above 33 °C is associated with a 0.26 percentage point increase in sterility. The average and maximum exposures above this threshold in the sample data are 13.0 and 46.5 h, respectively, corresponding to a 3.4 and 12.0 percentage point increase in sterility. We simulate warming impacts between 0.5–2.0 °C increase in daily temperatures, which increases sterility between 2.9–13.1 percentage points for the dry summer season and 0.2–3.2 percentage points for the wet season. Evidence for threshold heterogeneity across different sterility classes – low, medium and high sterility cultivar groups is tested and we find limited evidence for this extension of the model. The identified threshold can be utilized by breeders to test contrasting cultivars or segregating mapping populations in an unbiased set up and for precise heat stress impact modeling exercises. Heat sensitivity analysis indicated substantial genetic variability for heat induced spikelet sterility, providing ample evidence for producer adaptation to a warming climate by developing and switching to more heat stress resilient cultivars.

Quantifying rice spikelet sterility in potential heat-vulnerable regions: Field surveys in Laos and southern India

Global warming is projected to cause yield losses due to heat-induced spikelet sterility (HISS) in hot, vulnerable rice-growing regions, but reports documenting HISS at a regional level in the tropics are very limited. As a case study, HISS at flowering stage was surveyed under field conditions using local popular varieties in Savannakhet, Laos, and Tamil Nadu, India, during the dry season. At the field site in Savannakhet, sterility in variety Thadokkham1 (TDK1) was 10.8% when the maximum average temperature at heading was 32.9°C, which increased to 23.3% at 37.3°C. When plots were covered with a shading net, solar radiation was reduced by 60% at heading, which significantly reduced sterility to 10.0%. Panicle temperature estimates produced with a micrometeorology model revealed that shading during the hours when flowering occurs decreased panicle temperature by 2.2–2.6°C. At the field sites in Tamil Nadu, sterility of variety Coimbatore 51 (CO51) was highly correlated with the maximum temperature at heading; sterility was around 5–10% when maximum average temperature was below 36.5°C, which increased to 33.4% at 38.0°C. Marking of opened spikelets at hourly intervals allowed us to detect increases in sterility as temperature increased from early morning until noon. We conclude that HISS at flowering stage can occur in local popular varieties grown in Savannakhet and Tamil Nadu during the hot, dry season. Effective protocols such as using a shading net and hourly marking of open spikelets are useful approaches to quantifying HISS during flowering stage across hot and potential heat-vulnerable rice-growing regions.

District specific, in silico evaluation of rice ideotypes improved for resistance/tolerance traits to biotic and abiotic stressors under climate change scenarios

Using crop models as supporting tools for analyzing the interaction between genotype and environment represents an opportunity to identify priorities within breeding programs. This study represents the first attempt to use simulation models to define rice ideotypes improved for their resistance to biotic stressors (i.e., diseases); moreover, it extends approaches for evaluating the impact of changes in traits for tolerance to abiotic constraints (temperature shocks inducing sterility). The analysis—targeting the improvement of 34 varieties in six Italian rice districts—was focused on the impact of blast disease, and of pre-flowering cold- and heat-induced spikelet sterility. In silico ideotypes were tested at 5-km spatial resolution under current conditions and climate change scenarios centered on 2020, 2050, and 2085, derived according to the projections of two general circulation models–Hadley and NCAR–for two IPCC emission scenarios–A1B and B1. The study was performed using a dedicated simulation platform, i.e., ISIde, explicitly developed for ideotyping studies. The ideotypes improved for blast resistance obtained clear yield increases for all the combinations GCM × emission scenario × time horizon, i.e., 12.1 % average yield increase under current climate, although slightly decreasing for time windows approaching the end of the century and with a marked spatial heterogeneity in responses across districts. Concerning abiotic stressors, increasing tolerance to cold-induced sterility would lead to a substantial yield increase (+9.8 %) only for Indica-type varieties under current climate, whereas no increases are expected under future conditions and, in general, for Japonica-type varieties. Given the process-based logic behind the models used—supporting coherence of model responses under future scenarios—this study provides useful information for rice breeding programs to be realized in the medium-long term.

QEMF3, a novel QTL for the early-morning flowering trait from wild rice, Oryza officinalis, to mitigate heat stress damage at flowering in rice, O. sativa

A decline in rice (Oryza sativa L.) production caused by heat stress is one of the biggest concerns resulting from future climate change. Rice spikelets are most susceptible to heat stress at flowering. The early-morning flowering (EMF) trait mitigates heat-induced spikelet sterility at the flowering stage by escaping heat stress during the daytime. We attempted to develop near-isogenic lines (NILs) for EMF in the indica-type genetic background by exploiting the EMF locus from wild rice, O. officinalis (CC genome). A stable quantitative trait locus (QTL) for flower opening time (FOT) was detected on chromosome 3. A QTL was designated as qEMF3 and it shifted FOT by 1.5-2.0 h earlier for cv. Nanjing 11 in temperate Japan and cv. IR64 in the Philippine tropics. NILs for EMF mitigated heat-induced spikelet sterility under elevated temperature conditions completing flower opening before reaching 35°C, a general threshold value leading to spikelet sterility. Quantification of FOT of cultivars popular in the tropics and subtropics did not reveal the EMF trait in any of the cultivars tested, suggesting that qEMF3 has the potential to advance FOT of currently popular cultivars to escape heat stress at flowering under future hotter climates. This is the first report to examine rice with the EMF trait through marker-assisted breeding using wild rice as a genetic resource.

The Early-Morning Flowering Trait of Rice Reduces Spikelet Sterility under Windy and Elevated Temperature Conditions at Anthesis

We previously demonstrated that heat-induced spikelet sterility at anthesis could be mitigated by using an early-morning flowering (EMF) line of Oryza sativa L. cv. Koshihikari crossed with wild rice, O. officinalis. Another microclimate factor, high wind velocity, is known to increase the sterility induced by heat. In this study, we evaluated whether EMF rice could mitigate sterility under the combined stresses of heat and wind. Rice plants were exposed to three levels of wind velocity (1.1, 2.2, 3.4 m s−1) from early-morning until 1500 in a glasshouse, where air temperature reached 30°C at 0800, 34°C at 1000 and 38°C around noon. Under these conditions, sterility steadily increased in Koshihikari, ranging from 28.4 to 86.9% as wind velocity increased. However, in the EMF line, low levels of sterility were observed since most spikelets flowered before 1000 when air temperature reached 35°C, the critical value for causing sterility. These results indicated that the increase in heat-induced spikelet sterility by wind can be potentially mitigated by using EMF rice.

Integrated micrometeorology model for panicle and canopy temperature (IM2PACT) for rice heat stress studies under climate change

Projected global warming is expected to increase the occurrence of heat-induced spikelet sterility (HISS) of rice (Oryza sativa L.). Previous chamber experiments have shown that HISS can occur where temperature at flowering time exceeds the threshold temperature of around 35°C. The occurrence of HISS is, however, difficult to predict because the thermal conditions of rice canopy can be different from the air temperature under field condition. To cope with this, we developed a simple micrometeorology model focusing canopy and panicle temperatures; IM2PACT (Integrated Micrometeorology Model for Panicle And Canopy Temperature) as a tool to be incorporated into general meteorology databases. The IM2PACT was validated by leaf transpiration retarding (RT) experiment. During their flowering stage, leaf temperature elevated by RT treatment, which caused warming and drying of the air inside the canopy, resulting in the elevation of the panicle temperature (Tp). The IM2PACT well simulated the RT experiment, and was proved to simulate not only the Tp magnitude but also the effects on Tp of leaf transpiration characteristics via changes in micrometeorology inside the canopy. The IM2PACT was applied to the meteorology dataset based on ANEMOS in order to analyse the Tp at Kanto and Tokai regions of extremely hot summer in 2007. There was a great gap in spatial distributions between the Tp and the daily maximum air temperature which is commonly used as a measure of HISS, because the difference of meteorology, especially relative humidity, among areas altered the panicle-air temperature difference. This strongly suggests that we must refer to the Tp instead of the air temperature in daily maximum, as a measure variable for HISS. The IM2PACT is a powerful tool to elucidate the Tp in the climate change impact study to bridge between the responses of crop susceptible to heat and the meteorological data.

Spikelet sterility of rice observed in the record hot summer of 2007 and the factors associated with its variation

Heat-induced spikelet sterility is one of the major threats to rice production from global warming. Previous chamber experiments have shown that even a short exposure to heat can induce spikelet sterility, but actual damage to a crop in the open field has rarely been reported. In the mid-summer of 2007, the Kanto and Tokai regions in Japan experienced record high temperatures above 40°C. We examined whether this extreme heat event induced sterility in open fields. We collected panicle samples from 132 fields in 5 prefectures in the Kanto and Tokai regions. Maximum air temperature averaged over a 5-day period around the heading stage exceeded the threshold for sterility (35°C) in more than 40% of the crops surveyed. Spikelet sterility ranged from 2% to 23%, and was more than 10% in 20% of the crops studied. Spikelet sterility in 2007 was apparently higher than that in the normal summer of 2008 and peaked with the heat wave, which confirms for the first time that extreme heat in midsummer induced sterility under open field conditions in Japan, but to a lesser extent than what was predicted from previous chamber experiments. The lesson from this survey is that air temperature per se is not sufficient to predict the occurrence of heat-induced sterility and that factors influencing the heat budget of the panicles are needed to account for the crop damage. Crop management practices such as selection of tolerant cultivars and optimizing the timing and amount of nitrogen application could moderate the occurrence of heat induced spikelet sterility which can help to develop adaptation measures to climate change.

Climatic determinants of irrigated rice performance in the Sahel — III. Characterizing environments by simulating crop phenology

Agricultural development programs in most countries of the Sahel seek to increase the intensity of irrigated rice production. Variable crop duration and temperature stresses, however, limit yields and calendrial options. In an effort to characterize climatic constraints to rice double-cropping in the region, a field-based model simulating flowering was combined with models predicting maturity and chilling- and heat-induced spikelet sterility. Simulations were performed for key sowing dates, 38 sites and three genotypes based on 10–33 years of weather records. Sowing between mid-September and mid-November was associated with near-total spikelet sterility due to chilling in all Sahelian environments. Crop duration was longest in November, increasing by 5 days per degree latitude in the continental Sahel. Greatest annual variation in duration was observed in the coastal west and extreme north of the Sahel. Climatic gradients along the courses of the Senegal and Niger rivers were associated with different annual patterns of duration and sterility. Based on the genotypes available to farmers, local rice-rice cropping calendars left little room for alternative calendars. Achieving a greater flexibility for cropping calendars would require the introduction of alternative short-duration varieties. The model permits the zonation of the Sahel in terms of calendrial options and varietal requirements. For a greater spatial resolution of such studies in the future, however, the pooling and digitalization of more weather data is recommended.