The ecology of common heliotrope in a Mediterranean dryland cropping system
James Hunt, Roger Cousens and Sue Knights, Department of Resource Management, Institute of Land and Food Resources, The University of Melbourne, Victoria 3010, Australia.
Common heliotrope (Heliotropium europaeum L.) is a herbaceous Mediterranean summer annual that grows on fallows, stubbles and senesced pastures in the dryland cropping regions in the north of Victoria. It is considered a weed because it transpires moisture that could otherwise be used by ensuing crops and is toxic to livestock.
In this study, laboratory experiments have shown that germination of seeds of common heliotrope is not limited by light, cold treatment, or a leachable inhibitor. Temperature and water potential are the principal environmental factors that limit germination. The optimum conditions for germination are at a temperature of 35°C and a water potential of 0 MPa. Although 100% of seeds will germinate under these conditions, the percentage of seeds that germinate at sub-optimal conditions changes seasonally and between Australian populations. This is the principal mechanism of dormancy in the species, ensuring that seed do not germinate out of season when frost and competition will limit growth, and only after rainfall sufficient to allow successful reproduction. Germination of seeds of common heliotrope do not conform to the assumptions of the hydrothermal time model frequently used to predict field emergence of weed species.
Plant Protection Quarterly (2004) 19 (3) 126.