Michel Vaksmann

Grain, sugar and biomass accumulation in tropical sorghums. I. Trade-offs and effects of phenological plasticity

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.

La question de la vulnérabilité et de l'adaptation de l'agriculture sahélienne au climat au sein du programme AMMA

Rural populations in Soudano-Sahelian Africa are known to be particularly vulnerable to climate change due to a combination of naturally high levels of climate variability, high reliance on climate sensitive activities such as rain-fed agriculture and limited economic and institutional capacity to cope with and adapt to climate variability and change. Urgent actions are required to tackle the issues raised by climate change in Soudano-Sahelian Africa and these actions need to be supported by the best knowledge available. The AMMA program is attempting to revitalize research in Africa in this field through an integrated interdisciplinary framework that aims to increase our understanding of the problem and support decision making for the future.The first phase of AMMA tackled three major issues: (i) increase our understanding of the relationships between climate and agriculture (ii) quantify the vulnerability of agriculture productivity to climate change and (iii) explore pathways to cope with climate variability and change. This synthesis will show results from the first phase of the AMMA program to illustrate progress in each of these three issues.