Identification of L-tryptophan as an allelochemical in wheat bran extract

To evaluate the allelopathic potential of ferulic acid, L-tryptophan, and syringoylglycerol 9-O-β-D-glucopyranoside in wheat (Triticum aestivum L.) bran extract, their contents were determined in wheat bran extract. In bioassays, the effects of wheat bran extract of these compounds were studied on the root and shoot growth of lettuce and cockscomb. When seeds of lettuce (Lactuca sativa L. cv. Great lakes 366) and cockscomb (Celosia cristata L.) were incubated with wheat bran extract dissolved in water (100 g eq./l), lettuce and cockscomb root growth were inhibited by 92% and 85%, respectively, and lettuce shoot growth by 86%, but not cockscomb shoot growth. The concentrations of ferulic acid and L-tryptophan in the wheat bran extract (100 g eq./l) were estimated as 34 and 282 μM by measuring the peak area of HPLC, respectively; syringoylglycerol 9-O-β-D-glucopyranoside was not detected by a photodiode array detector. Ferulic acid alone at 34 μM did not inhibit root or shoot growth of lettuce or cockscomb. However, L-tryptophan alone at 282 μM inhibited the lettuce and cockscomb root growth by 62 and 26%, respectively, but not lettuce and cockscomb shoot growth. When seeds of cockscomb were incubated with wheat bran extract dissolved in water (10 g eq./l), cockscomb shoot growth were promoted by 53%. The concentrations of ferulic acid and L-tryptophan in the wheat bran extract (10 g eq./l) were estimated as 3 and 28 μM by measuring the peak area of HPLC, respectively. Neither compound alone at these concentrations promoted the cockscomb shoot growth. These results suggest that L-tryptophan contributes to the allelopathic nature of wheat bran extract for root growth inhibition.