Evaluation of allelopathic potential of Helianthus tuberosus L. root exudates

Allelopathy has been defined as the ability of a plant to inhibit or stimulate the growth of another plant through the release of chemicals into the environment by volatilisation, leaching, plant residues decomposition, and root exudation. Several studies have shown that Helianthus tuberosus L. (Jerusalem artichoke), a species belonging to the Asteraceae family can exhibit allelopathic activity. Experiments conducted with leaf extracts and degradation of plant residues into the soil have pointed out that this species affects germination of Amaranthus retroflexus and Echinochloa crus-galli by about 50%. The aim of this study, conducted in the laboratory in 2008-2009, was to evaluate in agar medium the allelopathic potential of H. tuberosus towards lettuce a species sensitive to allelopathic compounds. Tubers of H. tuberosus were sterilized and allowed to produce roots in plastic Magenta boxes on the agar medium (0.8% w/w), while shoot sprouts were periodically cut, to avoid competition. After seed sterilization, lettuce was directly seeded on the agar medium at the following timings: immediately after tuber tissue transplanting, 1, 2, 3, and 4 weeks after tuber transplanting in boxes with the presence or absence of tuber tissues (as control reference). At all timings, starting from 1 week after tuber transplanting, lettuce was also seeded in boxes where tuber tissues of H. tuberosus were removed just before seeding. Boxes seeded were kept for 14 days at room temperature and lettuce germination, root length and fresh weight of the whole lettuce plant were assessed at the end of the experiment. A complete randomized block design with 5 replicates was adopted, and the whole experiment was carried out three times. Lettuce germination was inhibited by 37%, 35%, 29%, 15% and 30% when seeds were placed in the boxes after 1, 2, 3, and 4 weeks of tuber presence. The germination recorded in boxes in which the tuber tissues were removed immediately before lettuce seeding, showed to be inhibited by 22%, 14% and 39% at 2, 3 and 4 weeks from transplanting, respectively, while no inhibition was observed during the first week of tuber presence. Lettuce root length was reduced by 50% and 40% only when seeds were placed in the boxes with the presence of the tuber, and at one or four weeks after H. tuberosus tuber transplanting, respectively. The lettuce whole plant weight was similar to that of the control, with the exception of the plants grown in the presence of tuber tissues at one week from transplanting, in which about 69% of reduction was observed. H. tuberosus tubers showed a significant inhibitory effect on germination and root growth of lettuce, in particular during the first week following the transplanting. Germination and growth inhibition were lower if the tuber tissues were removed before lettuce seeding, likely because inhibitory compounds produced by H. tuberosus roots rapidly diminished their activity after release. These results may have an important implication in the choice of the species to be included in a cultural system in order to avoid inhibitory effects on cultivated the crop grown after H. tuberosus.