Potassium, not lepidimoide, is the principal ‘allelochemical’ of cress-seed exudate that promotes amaranth hypocotyl elongation

Background and Aims Imbibed cress (Lepidium sativum L.) seeds exude ‘allelochemicals’ that promote excessive hypocotyl elongation and inhibit root growth in neighbouring competitors, e.g. amaranth (Amaranthus caudatus L.) seedlings. The major hypocotyl promoter has recently been shown not to be the previously suggested acidic disaccharide, lepidimoic acid (LMA), a fragment of the pectic polysaccharide domain rhamnogalacturonan-I. The nature of the hypocotyl promoter has now been re-assessed. Methods Low-molecular weight cress-seed exudate (LCSE) was fractionated by high-voltage electrophoresis, and components with different charge:mass ratios were tested for effects on dark-grown amaranth seedlings. Further samples of LCSE were size-fractionated by gel permeation chromatography, and active fractions were analysed electrophoretically. Key Results The LCSE strongly promoted amaranth hypocotyl elongation. The active principle was hydrophilic and, unlike LMA, stable to hot acid. After electrophoresis at pH 6·5, the only fractions that strongly promoted hypocotyl elongation were those with a very high positive charge:mass ratio, migrating towards the cathode 3–4 times faster than glucosamine. Among numerous naturally occurring cations tested, the only one with such a high mobility was potassium. K(+) was present in LCSE at approx. 4 mm, and pure KCl (1–10 mm) strongly promoted amaranth hypocotyl elongation. No other cation tested (including Na(+), spermidine and putrescine) had this effect. The peak of bioactivity from a gel permeation chromatography column exactly coincided with the peak of K(+). Conclusions The major ‘allelopathic’ substance present in cress-seed exudate that stimulates hypocotyl elongation in neighbouring seedlings is the inorganic cation, K(+), not the oligosaccharin LMA.