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Gene flow between domesticated plants and their wild relatives is one of the major evolutionary processes acting to shape their structure of genetic diversity. Earlier literature, in the 1970s, reported on the interfertility and the sympatry of wild, weedy and cultivated sorghum belonging to the species Sorghum bicolor in most regions of sub-Saharan Africa. However, only a few recent surveys have addressed the geographical and ecological distribution of sorghum wild relatives and their genetic structure. These features are poorly documented, especially in western Africa, a centre of diversity for this crop. We report here on an exhaustive in situ collection of wild, weedy and cultivated sorghum assembled in Mali and in Guinea. The extent and pattern of genetic diversity were assessed with 15 SSRs within the cultivated pool (455 accessions), the wild pool (91 wild and weedy forms) and between them. F (ST) and R (ST) statistics, distance-based trees, Bayesian clustering methods, as well as isolation by distance models, were used to infer evolutionary relationships within the wild-weedy-crop complex. Firstly, our analyses highlighted a strong racial structure of genetic diversity within cultivated sorghum (F (ST) = 0.40). Secondly, clustering analyses highlighted the introgressed nature of most of the wild and weedy sorghum and grouped them into two eco-geographical groups. Such closeness between wild and crop sorghum could be the result of both sorghum's domestication history and preferential post-domestication crop-to-wild gene flow enhanced by farmers' practices. Finally, isolation by distance analyses showed strong spatial genetic structure within each pool, due to spatially limited dispersal, and suggested consequent gene flow between the wild and the crop pools, also supported by R (ST) analyses. Our findings thus revealed important features for the collection, conservation and biosafety of domesticated and wild sorghum in their centre of diversity.

High photoperiod sensitivity is a singular trait for adaptation of sorghum to environmental constraints in sudano-sahelian West Africa. Difficulties encountered by selected models such as CERES-sorghum and STICS to simulate crop development may result from the representation of sorghum response to daylength during the photoperiod inductive phase. Four modeling approaches combining two temperature and photoperiod responses (linear, hyperbolic) and two calculation methods for development rates (cumulative, threshold) were evaluated to simulate time to panicle initiation (PI) in highly photoperiod sensitive Guinea sorghum variety CSM388. In the cumulative method, development rates were computed as summations of daily photothermal ratios, whereas in the threshold method accumulated degree days were tested against thermal time requirement to PI modulated by current photoperiod. Each model was calibrated based on observations from a Sotuba, Mali (12839 0 N) planting date experiment spanning a 2-month period in 1996. Observed time from emergence to PI decreased from 54 to 22 days for a 20 min variation in daylength. Apparent higher performance by threshold methods was further tested against a 1994 independent dataset featuring three latitudes and a much wider range of sowing dates extending from February to September. Results validate the superiority of threshold over cumulative methods and confirm the better fit of a hyperbolic temperature and photoperiod response. A threshold-hyperbolic modeling approach is believed to be more consistent with crop physiology as it associates cumulative (temperature) processes and trigger (photoperiod) events that better reflect the concepts of quantitative plant growth and qualitative plant development. Its mathematical form and computational simplicity should ensure wide applicability for varietal screening over a large range of photoperiod sensitivities including neutral cultivars, and easy implementation into existing models.

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The Sahel region is known for the high vulnerability of its agriculture to climate variability. Early warning systems that make use of agrometerological forecasts are one of the coping strategies developed by policy makers. However, the predictive quality of the tools and methods used needs improvement. In order to address some of these challenges, we conducted agronomic trials and on-farm surveys to adapt the SARRAH (Système d'Analyse Régionale des Risques Agroclimatiques, version H) crop simulation model, and also evaluated it in farmers' field conditions. The farmers' practices such as sowing dates and densities, fertilizer use and yields potentials of the millet and sorghum crops were characterized under different climatic conditions.

Sorghum is known as a short day plant requiring short photoperiods (PP) to flower. However, in field experiments based on monthly plantings reported elsewhere, panicle initiation took place during long days as well as during short days even for highly photoperiod-sensitive varieties thought to be of the absolute type, or was particularly early for the sowings in September, well before days were shortest. In order to investigate these contradictory results, three tropical sorghum varieties of different photoperiod sensitivity were planted monthly in Mali and the dates of panicle initiation (PI) were recorded. The results indicate that the common concepts of a gradual (linear, or quantitative) response of photoperiod-induced phase (PIP) to PP at PI (PPi) (case of the variety Sariaso 10) or a threshold-type response (hyperbolic, or absolute; cases of CSM 335 and IRAT 174) could be reproduced for the sowing dates falling into the wet season (May-October), but did not describe adequately crop behaviour during the remaining months of the year. The same varieties were tested in controlled environments with four differential PP treatments composed of three different initial PP (12 h 15 min, 12 h 00 min and 11 h 45 min) and two opposite rates of day length change (AE1 min/day). Thus after 15 days, two pairs of treatments with opposite day length changes reached the same average PP (12 h 07 min and 11 h 52 min) and after 30 d, another pair reached a similar average PP (12 h 00 min). In 2001, 2 varieties reached PI earlier under decreasing days and the same average PP, but no such additional effect of the rate of change of PP was observed in 2002. Modelling exercises showed that field variation in the duration to PI was better explained with rate of change of PP than with absolute PP observed during PIP, and best with a combination of both factors in an additive model. It is concluded that in tropical sorghum, floral induction is strongly associated with a negative rate of change of day length in the field, and under certain circumstances under controlled conditions, but more evidence is needed to ascertain the capability of sorghum to sense the rate of change of PP before a definite model can be formulated. #

Grain and sweet sorghum (Sorghum bicolor (L.) Moench) differ in their ability to produce either high grain yield or high sugar concentration in the stems. Some cultivars of sorghum may yield both grains and sugar. This paper investigates the trade-offs among biomass, grain and sugar production. Fourteen tropical sorghum genotypes with contrasted sweetness and PP sensitivity were evaluated in the field near Bamako (Mali) at three sowing dates under favourable rainfed conditions. Plant phenology, morphology, dry matter of different organs and stem sugar content were measured at anthesis and grain maturity. A panicle pruning treatment was implemented after anthesis. Late sowing (shorter days) led to a decrease in total leaf number, dry mass and sugar yield even in PP-insensitive genotypes because of an increased phyllochron. Dry matter production and soluble sugar accumulation were strongly correlated with leaf number. Sugar concentration varied little among sowing dates or between anthesis and maturity. This indicates that sugar accumulation happened mainly before anthesis, thus largely escaping from competition with grain filling. This was confirmed by the low impact of panicle pruning on sugar concentration. Changes in sugar concentration from anthesis to maturity were negatively correlated with harvest index but not with grain yield. Physiological trade-offs among sugar, biomass and grain production under favourable rainfall are small in late-maturing and PP-sensitive sweet sorghums cultivated under sudano-sahelian conditions. The results differ from earlier reports that focussed on early maturing, PP-insensitive germplasm. Further research is needed on the interactions of these traits with agricultural practices and drought. (Résumé d'auteur

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