Persistent and Transient Inhibition of Seed Germination by Fractions of Methanol-Water Extract of Wheat Straw

The objectives of this work were to investigate the effects of wheat straw constituents on total seed germination and on time needed for seed germination to start and to assess the usefulness of evaluating the recovery of germination to help guide the screening for a sequential search of bioactive natural phytochemicals. Dose-response bioassays of lettuce seed germination to increasing concentrations of six fractions of methanol-water extract were conducted and whenever inhibition of germination exceeded 90% non-germinated seeds were transferred to distilled water and recovery of germination investigated. Only the fractions essentially composed by neutral polar compounds and by acids failed to inhibit total germination. Fractions composed by strong acids, basic compounds and fatty acids plus steroids completely inhibited total germination at 5mg mL−1, the former also inhibiting germination by more than 90% at 1mg mL−1. In these three fractions the concentration necessary for 100% inhibition was estimated to range between 4.1 mg mL−1 and 4.4mg mL−1. Time needed for germination to start was always significantly increased even by fractions that failed to affect total germination. Basic compounds were the most effective in delaying the start of germination increasing it more than 4-fold at 2mg mL−1 from 1.0 to 4.3 days. When non-germinated seeds were transferred to distilled water persistence of complete inhibition was only found with strong acids, almost full persistence with basic compounds. Thus the consideration of recovery of germination makes possible a more informed choice of the fraction to select for further bioassay guided search for phytochemical with herbicide activity. The fraction is the one composed by strong acids, mainly phenolic acids, all of them identified, provided that persistent seed inhibition is desired and long term weed control through depletion of weed seed banks in soil is aimed.