Factors modulating the levels of the allelochemical sorgoleone in Sorghum bicolor.

Sorgoleone is the major component of the hydrophobic root exudate of sorghum [Sorghum bicolor (L.) Moench]. The presence of this allelochemical is intrinsically linked to root growth and the development of mature root hairs. However, factors modulating root formation and the biosynthesis of sorgoleone are not well known. Sorgoleone production was independent of early stages of plant development. The optimum temperature for root growth and sorgoleone production was 30 degrees C. Seedling development and sorgoleone levels were greatly reduced at temperatures below 25 degrees C and above 35 degrees C. The level of sorgoleone was also sensitive to light, being reduced by nearly 50% upon exposure to blue light (470 nm) and by 23% with red light (670 nm). Applying mechanical pressure over developing seedlings stimulated root formation but did not affect the biosynthesis of this lipid benzoquinone. Sorgoleone production did not change in seedlings exposed to plant defense elicitors. On the other hand, sorgoleone levels increased in plants treated with a crude extract of velvetleaf (Abutilon theophrasti Medik.) root. This stimulation was not associated with increased osmotic stress, since decreases in water potential (Psi(w)) by increasing solute concentrations with sorbitol reduces sorgoleone production. Sorgoleone production appears to be constitutively expressed in young developing sorghum plants. Other than with temperature, changes in the environmental factors had either no effect or caused a reduction in sorgoleone levels. However, the stimulation observed with velvetleaf root crude extract suggests that sorghum seedlings may respond to the presence of other plants by releasing more of this allelochemical.