Résumé
Studying seasonal adaptation is of great importance for understanding crop diversification. The biological process of seasonal adaptation of sorghum (Sorghum bicolor (L.) Moench.) allowed the development of flood recession agrosystems in the Sahel (West and Central Africa). To analyze the patterns of trait variation linked to this poorly documented process, we evaluated and compared four eco-seasonal groups of sorghum originating from seasonally diversified agrosystems in the same cropping conditions. The muskuwaari and samme groups are composed of landraces adapted to flood recession agrosystems in the Lake Chad Basin (43 accessions) and the Senegal River Valley (14 accessions), respectively. The other two groups are composed of varieties commonly grown in the rainy season (19 accessions from the Lake Chad Basin and 24 accessions from the Senegal River Valley and and surrounding regions). Multivariate analyses based on ten agro-morphological traits structured the sample in two main clusters subdivided into four subclusters. This structure was significantly associated with the eco-seasonal groups, highlighting muskuwaari differentiation. Five traits (head appearance, leaf length, leaf number, aboveground biomass, and the root mass to aboveground biomass ratio) supported the differentiation of eco- seasonal groups and backed the hypothesis of seasonal adaptive divergence of sorghum in the Lake Chad Basin. No trait differentiation was detected in the samme group, suggesting that seasonal adaptation in the Senegal River Valley might probably be associated with population plasticity existing in the rainy season groups. These experimental evidences provide new insights for understanding scenarios of seasonal adaptation of sorghum.