PHYTOTOXIC EFFECTS OF THE HIGH-MOLECULAR-WEIGHT FRACTION OF AN AQUEOUS LEAF-LITTER EXTRACT ON BARLEY ROOT DEVELOPMENT

The root development of barley seedlings grown for one week in an aera ted nutrient solution was studied in the presence of dissolved organic matter from an aqueous chestnut leaf litter extract. In particular, t he different effects of low and high molecular weight fractions (small molecules: molecular weight <1000; large molecules: >10,000) of the l eaf litter extract were examined. In the presence of large molecules r oot growth was inhibited, an irregular root tip morphology was observe d, and Ca and Mg concentrations in the shoots were lower than in contr ol plants. These phytotoxic effects were not caused by the formation o f an impermeable layer of large molecules on the root surfaces that lo wer accessibility for nutrient cations as inferred from voltammetric e xperiments. A germination assay using spruce seeds, however, indicated allelochemical effects of large molecules, which exhibit a higher aro maticity than the small molecules as indicated by spectroscopic charac terisation. In the growth experiments with small molecules, no influen ce on the root development of barley was evident, but an increase of C a and Mg in the shoots was detected. During these growth experiments, a large amount of the small molecules, mainly simple phenols and amino acids, disappeared from the nutrient solution. The loss of small mole cules was most likely the effect of mineralisation.