Relationships between Biomass and Phenolic Production in Grain Sorghum Grown under Different Conditions

as sorgoleone (Einhellig and Souza, 1992; Einhellig et al., 1993; Weston et al., 1997) and phenolic acids (BenThe total phenol pool (kg ha21) of the aerial parts and roots of Hammouda et al., 1995; De Raissac et al., 1998) to the sorghum (Sorghum bicolor L. cv. CE145266) crops and their relationships to dry matter and total phenol concentrations (g kg21) were soil by decomposing crop residues (Patrick et al., 1963; studied in four different data sets (2 sites 3 2 yr for a total of 52 Chou et al., 1981; Paul et al., 1994; Siqueira et al., 1991; smallholder fields) in Senegal. The total phenol pool size varied from Hoffman et al., 1996) and root exudates. Some of these 4 to 156 kg ha21 in the aerial parts and from 1 to 16 kg ha21 in the compounds are shown to have negative impact on soil roots. The phenol pool size was closely correlated with the amount fertility and crops. An assessment of their potential toxic of dry matter in both the aerial parts (r 5 0.95, P,0.001) and the effects requires an estimation of the amount of polypheroots (r 5 0.91, P,0.001). In contrast, the phenol concentrations nolic compounds in the soil. This is related not only to varied less between fields than the dry matter content, and therefore the phenolic concentration in living tissue, but also to the had less impact on the phenol pool size. Using a N nutrition index absolute biomass production of the plant. Consequently, (NNI) to assess the N nutrition of the sorghum crops, both phenol the pool and concentration were higher when N nutrition was better. the total amounts of phenolic compounds accumulated Grain yield and the phenol pool of aerial parts were also positively in plants may be as important as their concentrations correlated. The data indicate that the environmental factors that prowhen determining the ability of plants to release allelomote growth and grain yield also enhance the total phenol synthesis chemicals. in sorghum vegetative parts. In order to improve our understanding of the allelopathic potential of sorghum, we studied the variability of the phenolic production in its shoots and roots over a wide range of agricultural conditions in the Sahel zone T production of plant phenolic compounds, of Senegal. An estimation of the total phenol concentrawhich are thought to be part of the chemical defense tion (g kg21) and pool size (kg ha21) was made based of the plant, is under the control of both genetic and on dry matter data. The relationships between phenolic environmental factors. For instance, herbivore attacks production, grain production, and N content (for the (Tempel, 1981), insect damage (Woodhead and Beraerial parts) were established. In the areas investigated, nays, 1978; Woodhead and Cooper-Driver, 1979; Guinn the differences among fields in soil fertility, rainfall, and and Eindenbock, 1982), or reduced soil fertility (Coocropping systems are assumed to be large enough to per-Driver et al., 1977; Coley et al., 1985; Dustin and result in differences in the available N and water for Cooper-Driver, 1992; Einhellig, 1996) will generally insorghum. This could lead to differences in both sorghum crease the synthesis of these compounds by the plants. growth (Heron et al., 1963; Langlet, 1973; Ramu et al., This has been partly explained by the balance between 1991; Donatelli et al., 1992; Rego et al., 1998; Singh et C and nutrient availability (Bryant et al., 1983). In paral., 1998) and phenol accumulation. ticular, N deficiency has been shown to strongly affect the synthesis of polyphenols (Koricheva et al., 1998) because it will affect growth more than photosynthesis, MATERIALS AND METHODS and thus allows more carbohydrates to be available for Measurements of the Plants phenolic synthesis. These results have usually been obFour sets of smallholder fields cropped with sorghum tained by determining the concentrations of total pheCE145266 in the central peanut basin region of the Sahel area nols or of different classes of compounds (e.g., tannins of Senegal were studied—two in 1996 and the other two in and phenolic acids) in plant tissues. 1997. In both years, one set was in Sagnanème village (148009 However, phenolic compounds are also known to N, 168159 W) and the other in Médina village (148209 N, 158309 have important effects once they are released from the W). The soils are ferralitic or ferruginous tropical soils with living plant by directly or indirectly influencing soil qual61–78% sand in the 0to 10-cm layer. The soil bulk density ity and living organisms, including microorganisms and was not significantly different among fields and ranged from other plants. For instance, some agricultural plants like 0.147 to 0.160 kg m2. The pH ranged from 5.2 to 7.2 in the sorghum are known to release phenolic compounds such 0to 40-cm layer. In 1996, 12 fields were studied in Sagnanème and 15 were studied in Médina. In 1997, eight other fields were studied in Sagnanème and 17 other fields were studied M. Sène, Inst. Sénégalais de Recherches Agric. (ISRA) CNRA, BP in Médina. No fertilizer, plant protection products, tilling, 53 Bambey, Sénégal; T. Doré, Inst. Natl. Agron. Paris-Grignon (INA or irrigation were used, following the usual practice of local P-G), Lab. d’Agron. Inst. Natl. de la Recherche Agron. and INA farmers. Weeds were removed by hand. P-G, 16 rue C. Bernard, 75231 Paris Cedex 05, France; and C. Gallet, Univ. of Savoie, Lab. sur la Dynamique des Ecosystèmes d’Altitude, The sorghum was sown by the farmers at the beginning of 73376 Le Bourget-du-Lac Cedex, France. Received 29 Nov. 1999. the rainy season, which lasts from July to October. A 25by *Corresponding author ([email protected]). Abbreviations: DW, dry weight; NNI, N nutrition index. Published in Agron. J. 93:49–54 (2001).