Abstract
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.
Highlights
Cress (Lepidium sativum L.) seeds adversely affect the growth of neighbouring, potentially competing, ‘receiver seedlings’ such as amaranth (Amaranthus caudatus L.) – an effect that has been described as allelopathic (Hasegawa et al, 1992; Yamada et al, 2007; Iqbal and Fry, 2012)
Considerable interest has centred on the report that lepidimoic acid (LMA), an unsaturated acidic disaccharide presumed to be derived from rhamnogalacturonan-I by the action of a lyase, is an allelochemical exuded by seed(ling)s of cress and many other species, and capable of detrimentally influencing the growth of neighbouring, potentially competing, seedlings of other species (Hasegawa et al, 1992; Yamada et al, 1995, 1996, 2007)
The existence was confirmed of heatstable, hydrophilic, low-molecular weight material, exuded by cress seeds during imbibition, capable of overstimulating amaranth seedling hypocotyl elongation and inhibiting amaranth root growth
Summary
Cress (Lepidium sativum L.) seeds adversely affect the growth of neighbouring, potentially competing, ‘receiver seedlings’ such as amaranth (Amaranthus caudatus L.) – an effect that has been described as allelopathic (Hasegawa et al, 1992; Yamada et al, 2007; Iqbal and Fry, 2012). It is possible that such biological effects raise the chances of cress seedling establishment by weakening potentially competing neighbours. Whether or not this interpretation is correct, it is of interest to characterize further the nature and production of growth-regulating active principle(s) exuded by seeds. 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. Further samples of LCSE were size-fractionated by gel permeation chromatography, and active fractions were analysed electrophoretically
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