Opportunity in change: key crops rice and sorghum

Climate change (CC) is perceived either as a menace to our livelihood, peace and environment, or as a collective hype that will probably go away. But it can also be seen as change per se, real or imagined, in the conditions that set the frame for all human activities. The perception of such conditions has driven technological development ever since. Change in the conditions, or change in their perception as societal needs evolve, is thus a driving force of innovation. This is particularly true in agriculture, which a priori is not a struggle against nature, but rather an intelligent collaboration with nature to produce food and other things. Of course CC is also a threat. Regional changes in precipitation will, for example, bring hardship to many poor regions (Fig. 1) and will require profound local changes of systems and geographical migration of crops. Such changes necessarily meant humanitarian and civilizational crises in pre-scientific and pre-globalization times. They will still mean that today in many parts. But as researchers we know that change, if it is gradual and not disruptive, also means: Mobilization of economic and technical creativity, Readiness for adoption of new solutions by stake holders including farmers¿ In short: When frame conditions change there is a chance for renewal, and readiness to question habitual dogmas. Rice and sorghum systems together cover a large spectrum of situations with very different vulnerabilities and opportunities. The problematic of rice hinges on this crop's high water requirements and good adaptation to excess water, providing remarkable sustainability and productivity under flooded conditions: easier weed control, stability of soil pH and fertility, salinity control, transpirational cooling and more. The combination of CC and increasing competition for water conspire against the flooded rice crop. Water-saving management and breeding objectives are likely to have negative trade-offs because they aim at taking the crop out of its natural niche of adaptation. In fact, upland rice already represents such a compromise, and it is among the most vulnerable crops in terms of CC impacts. Should we "leave rice in the swamp" where it came from? No: (1) because biotechnology is moving fast towards molecular breeding for "new crops", requiring the development of ideotype and ecosystem management concepts now; and (2) because rice as a food has such a specific appeal and importance that it cannot be easily substituted. (Texte intégral

Extrapolating crops to new climatic environments: grey zones of knowledge and research needs for modelling

Any evaluation of climate change (CC) impacts on crop yields is based on quantitative extrapolation of knowledge and thus uses crop modelling as central tool. However, the validity, or robustness, of available models is limited even for currently observed ranges of environments, as parameter values still tend to be quite environment specific. Simulations for new environments thus constitute a major challenge. This is particularly true for rice, a species known for its great diversity of adaptations but also high level of vulnerability to environmental stresses. As point of entry, 3 recent papers are discussed that each highlight a particular grey zone in our knowledge on crop response to climate in the field, and in particular thermal factors. One detects long term yield trends in rice experiments and struggles to explain them with climate, one questions the stability of cardinal temperatures governing plant development, and the last raises questions on the accuracy of our notion of maintenance respiration (Rrn). The author then identifies major potential sources of error in extrapolative simulation of phenology and yield, (1) by failing to consider the conditions locally experienced by the plant organ concerned (micro climate) and (2) by making "established" but possibly wrong assumptions on process responses. Examples are given. The paper terminates by asking what is "vigour" and "general adaptation" in terms of physiological plant-environment interaction, and if some of this is accessible to crop modelling. The question is particularly relevant in the CC context because breeding efforts and agronomic adaptation strategies increasingly consider shifts in ecosystem management (e.g., aerobic rice or water saving irrigation) and geographic/zonal shifts of cultivation. This involves new ideotype concepts, use of exotic germplasm sources and genetically engineered, modified plant behaviour (e.g., C4 rice project). Are models conceivable that not only extrapolate existing genotypes to changing environments, but also explore such adaptation for virtual varieties envisaged by research? (Texte intégral

Rice panicle temperature and crop microclimate in stressful thermal environments: toward a model of spikelet sterility

Rice inflorescences are sensitive to chilling and heat, resulting in spikelet sterility. It is not the air temperature itself, however, that causes the stress but the temperature of the sensitive tissues during specific developmental stages. Chilling affects sterility mainly through (1) disruption of meiosis during microspore stage (tissues located at bottom of canopy exposed to floodwater temperature at the beginning of booting) and (2) failure of panicle exertion (temperature of elongating internodes at mid height of canopy). Heat affects mainly pollination and fertilization processes at anthesis at the top of the canopy. The organs concerned can have markedly different temperature from air by up to 6°, depending on microclimate generated by the architecture, roughness and transpiration rate of the canopy. Quantifying and predicting these complex thermal relationships is essential to evaluate the impact of climate change scenarios and the adaptation of cultivars to them. So far, no crop model is available to simulate crop microclimate dynamics and to link them to physiological processes. This study, conducted in the context of the GTZ-funded RISOCAS project, aims as a first step to observe experimentally on 7 contrasting varieties of irrigated rice at climatically contrasting sites (Senegal, Philippines, Camargue in France) the thermal relationships and components of the crop heat balance. This includes diurnal soil, water, canopy, panicle and air temperature patterns and their relationship with canopy structure and weather. First results are presented from the 3 environments, notably vertical temperature gradients in the canopy and temperature distribution with the panicle, using infrared photography and thermocouples, as well as heat balance measurements and recordings of standard agrometeorology. As a 2nd step, these observations will be related to spikelet sterility and yield losses. Finally, the results will be used to develop a micrometeorological module for the cereal crop models SARRAH and EcoMeristem. This paper presents first results, discusses methodological issues and provides an outlook on the planned modelling approaches. (Texte intégral

Adapting lowland rice cultivation to climate change - thermal stress tolerance breeding in the Sahel region of West Africa

The Sahel region of West Africa is characterized by extreme diurnal and seasonal temperature variation subjecting the rice crop to thermal stress at different growth stages. The Africa Rice Center in collaboration with partners is aims to identify rice genotypes and associated traits for use in breeding varieties adapted to the Sahel climate. In one set of field trials established at Ndiaye, Senegal, 244 diverse rice genotypes, including four checks, were sown in February, March, April and July subjecting the rice plants to cold and heat stress at different growth stages. Daily minimum temperatures fell below 20 °C in the months of February and March whilst maximum temperatures regularly rose above 40 °C in April, May and June. The rice crop is thus subjected to cold stress in February and March and to heat stress in April to June. Across the planting dates, total biomass production was highest on average for the February planting date (293.8g/plant) followed by the April planting date (281.0g/plant) and lowest for the July planting date (215.4g/plant). However, spikelet sterility was highest for the April planting date on average relative to other planting dates and lowest for the July planting date. On average plantings in July were earliest (100 days from sowing to maturity) relative to other planting dates whilst plantings in February which corresponded to the sowing date for the dry season crop had the longest crop durations (137 days from sowing to maturity). With regards to crop duration across the planting dates, Chromrong a cold tolerant check from Nepal had the shortest duration across all dates whilst N22 the international heat tolerant check had the longest duration. IR64 an international irrigated lowland check variety and Sahel 108 a local check variety had crop durations generally intermediate between these two checks. Large genotypic variations detected in these traits will be exploited in selecting parents to develop new varieties better adapted to the seasonal Sahelian climate. (Texte intégral

Modelling rice phenotypic plasticity in diverse climates using EcoMeristem: Model evolution and applications to rice improvement

Climate change and variability (CCV) exposes tropical crops as rice to heat and drought. Increasing atmospheric CO2 is expected to improve plant transpiration efficiency but benefits may be more than offset by reduced transpirational cooling and accelerated phenology. Yield is generally affected by environment effects on morphogenesis, particularly to stress during reproductive sink capacity determination. Exposure of reproductive processes to heat and drought, in turn, depends on plant structura1 development and resulting microclimate and water availability. These highly genotype-dependent interactions make it difficulty to predict CCV impacts and design adaptations. An important step is to model this system, particularly interactions between plant morphogenetic and phenological processes with climate and resources, and resulting microclimate within the crop stand. Models are needed that consider crop structura1 development at organ level, while providing sufficient phenological and physiological detail to situate stress sensitive processes within time and canopy. Such a model must be coupled with a heat balance providing accurate information on soil, floodwater, leaf and panicle temperature. Key physiological processes would thus become predictable, including: tillering and tiller maintenance/abortion, leaf area dynamics including senescence, spike number dimensioning and adjustments, stem carbohydrate accumulation and mobilization to grains, thermal and drought induced spike sterility determined at the sensitive microspore and anthesis stages or by panicle exertion limitation, and finally grain filling process. A new type of functional-structura1 plant models is needed that integrates environment dynamics within soil-water-plant-atmosphere continuum. EcoMeristem model was designed to simulate environment and genotype driven phenotypic plasticity for rice and other cereals. It simulates rice plant morphogenesis at organ, plant and canopy levels in response to drought and climatic (excluding C02) factors. The key concept is the feedback of trophic status (source-sink and competition among sinks) on organ initiation and (pre-)dimensioning processes, through signals to the meristem. A state variable quantifying internal competition for assimilates constitutes this signal (Ic: supply-demand ratio), which also governs resource and stress feedbacks on senescence processes. Water deficit is described by Fraction of Transpirable Soil Water (FTSW) and derived physiological coefficients. The sensitivity of development vs. trophic feedbacks is set by genotypic parameters (threshold and slope parameters, e. g. for tillering response to Ic or leaf expansion, assimilation and transpiration rates to FTSW). EcoMeristem was developed to - explore phenotypic plasticity concepts as affected by abiotic factors (drought, T ... ), - explore ideotype concepts for specific environments, and - measure heuristically hidden (process based) traits/parameters within phenotyping context. A new objective is to study rice varietal response to CCV (CIRAD, NIAES, IRRI, WUR collaboration). EcoMeristem was therefore recently linked with 3D visualization tool (OpenAJea), opening the way to spatial micrometeorological computations within the canopy. Further model developments are needed: - Completing the model for all developmental stages, - Extending the water balance to flooded and mixed flooded/aerobic systems, - Introducing C02 effects on plant gas exchange, - Introducing a stratified heat balance for the soil-water-plant-atmosphere continuum. This paper presents the current state of model development and applications, and future improvements for research on rice crop adaptation to CCV. (Texte intégral

Phenological responses of irrigated rice in the Sahel

Worldwide rising temperatures are already being observed and are expected to increase within the next decades. In the Sahel cool periods cause yield losses due to spikelet sterility in late sown rice and thus Sahelian rice production systems might benefit from increasing temperatures. Higher temperatures during the vegetative phase will lead to shortened crop duration and during the reproductive phase higher temperatures during cool periods might reduce sterility. For the hot periods negative effects on biomass production and increased sterility due to heat stress are expected. The complexity of those phenomena requires well validated crop models able to precisely assess development and yield according to genotype and climate for predictive conclusions and adaptive decisions (choice of genotype, sowing date) under changing climatic conditions. In the early 90s for a wide range of germplasm phenology was observed and yield components were determined in staggered planting dates at AfriceRice's Sahel station in Ndiaye, Senegal. Based on this, a model (RIDEV) was developed by Dingkuhn et al. (1995) to estimate duration and sterility for multiple rice varieties in the Sahel as a function of sowing date. Until now, it has been used by the operational services. However, differences between crop cycles observed in farmers' fields and assessed by RIDEV have been reported. This could be explained either by model deficiencies, varietal evolution and/or climatic changes. Presently in Ndiaye (coastal-semi-arid) and Fanaye (continental-semi-arid), 10 strongly contrasting rice varieties are grown year-around in monthly-staggered planting dates in order to determine duration, leaf appearance rate and sterility under current climatic conditions. Those varieties include some of the formerly observed genotypes as well as heat- and cold-tolerant reference varieties. Results will be used to improve RIDEV thus allowing for predictions of crop responses to climate change. Preliminary results with a focus on derivation of photo-thermal constants will be presented for the first completed year and compared to results from former years. (Texte intégral