The total phenol pool (kg ha−1) of the aerial parts and roots of sorghum (Sorghum bicolor L. cv. CE145−66) crops and their relationships to dry matter and total phenol concentrations (g kg−1) were studied in four different data sets (2 sites × 2 yr for a total of 52 smallholder fields) in Senegal. The total phenol pool size varied from 4 to 156 kg ha−1 in the aerial parts and from 1 to 16 kg ha−1 in the roots. The phenol pool size was closely correlated with the amount of dry matter in both the aerial parts (r = 0.95, P<0.001) and the roots (r = 0.91, P<0.001). In contrast, the phenol concentrations varied less between fields than the dry matter content, and therefore had less impact on the phenol pool size. Using a N nutrition index (NNI) to assess the N nutrition of the sorghum crops, both phenol the pool and concentration were higher when N nutrition was better. Grain yield and the phenol pool of aerial parts were also positively correlated. The data indicate that the environmental factors that promote growth and grain yield also enhance the total phenol synthesis in sorghum vegetative parts.

CE145-66 is an improved early-maturing grain sorghum genotype, increasingly grown by farmers in the Sahelian part of Senegal. This genotype is known to have negative effects on the following groundnut crop, because of the release of allelopathic phenolic compounds into the soil. We have assessed the synthesis of phenolics in sorghum vegetative parts and the variations in synthesis between years and sites. Total phenols and phenolic acids in the aerial parts and roots of flowering sorghum plants from 52 farmers' fields at two sites (Sagnanème and Médina) in Senegal in 1996 and 1997 were measured. Thirty-eight soil samples, collected after the sorghum harvest, from sorghum rows and interrows also were analyzed for their phenolic content. Total phenols reached 1.1-1.5% of root dry weight and 1.1-2.2% of aerial parts dry weight, with little variation between sites, and large variability between years, presumably due to climatic conditions. Eight phenolic acids and three associated aldehydes were identified by HPLC, with p-hydroxybenzoic, p-coumaric, and ferulic acids the most abundant. Their totals reached 2.9-3.2 mg/g in 1996 and 2.6-2.8 mg/g in 1997 for the aerial part; and 3.3-3.6 mg/g in 1996 and 2.8-3.3 mg/g in 1997 for roots. In soils under sorghum rows, the mean water-soluble total phenols increased from 4.6 in 1997 to 6.7 micrograms/g in 1998 in Sagnanème, and from 3.8 in 1997 to 5 micrograms/g in 1998 in Médina. The concentrations of total phenols and phenolic acids were higher in rows than in interrows. All the phenolic monomers identified in vegetative parts were recovered in associated soil samples, with vanillic and p-hydroxybenzoic acids the most abundant. Finally, variability in plant phenolic content seemed more due to climatic than to cropping or soil factors, as differences between years appear more important than differences between or within sites.

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